18/Oct/2019

Binocular amblyopia treatment with contrast-rebalanced movies

 

 

BACKGROUND

Binocular amblyopia treatments promote visual acuity recovery and binocularity by reba- lancing the signal strength of dichoptic images. Most require active participation by the amblyopic child to play a game or perform a repetitive visual task. The purpose of this study was to investigate a passive form of binocular treatment with contrast-rebalanced dichoptic movies.

METHODS

A total of 27 amblyopic children, 4-10 years of age, wore polarized glasses to watch 6 contrast-rebalanced dichoptic movies on a passive 3D display during a 2-week period. Amblyopic eye contrast was 100%; fellow eye contrast was initially set to a lower level (20%-60%), which allowed the child to overcome suppression and use binocular vision. Fellow eye contrast was incremented by 10% for each subsequent movie. Best-corrected visual acuity, random dot stereoacuity, and interocular suppression were measured at base- line and at 2 weeks.

RESULTS

Amblyopic eye best-corrected visual acuity improved from 0.57 T 0.22 at baseline to 0.42 T 0.23 logMAR (t26 5 8.09; P \ 0.0001; 95% CI for improvement, 0.11–0.19 log- MAR). Children aged 3-6 years had more improvement (0.21 T 0.11 logMAR) than chil- dren aged 7-10 years (0.11 T 0.06 logMAR; t25 5 3.05; P 5 0.005). Children with severe amblyopia ($0.7 logMAR) at baseline experienced greater improvement (0.24 T 0.12 log- MAR) than children with moderate amblyopia at baseline (0.12 0.06 logMAR; t25 5 3.49; P 5 0.002).

CONCLUSIONS

In this cohort, passive viewing of contrast-rebalanced dichoptic movies effectively improved visual acuity in amblyopic subjects. The degree of improvement observed was similar to that previously reported for 2 weeks of binocular games treatment and with 3- 4 months of occlusion therapy.     ( J AAPOS 2019;-:1.e1-5)

Patching improves visual acuity in amblyopic chil- dren; however, there is substantial variability in response to monocular treatment, with only 50%-85% achieving normal visual acuity.1-4  Residual amblyopia is associated with lifelong limitations in visuomotor tasks,5,6 slow reading,7,8 fixation instability,9-11 and altered self-perception.12-14

Ourevolving understanding of the role of interocular sup- pression as the primary factor interfering with normal visual development in amblyopia15,16 has led to the recent Author affiliations: aRetina Foundation of the Southwest, Dallas, Texas; bUT Southwestern Medical Center, Dallas, Texas; cABC Eyes, Dallas, Texas; dPediatric Ophthalmology & Adult Strabismus, Plano, Texas; eChildren’s Eye Care of North Texas, Plano, Texas This research was supported in part by a grant from the National Eye Institute (EY022313).

development of clinical therapies that aim to alleviate interocular suppression, restore binocular combination, and rehabilitate visual acuity. Binocular amblyopia treatments promote simultaneous use of both eyes by rebalancing  the  strength  of  each  eye’s  image  with  high- contrast or high-luminance input to the amblyopic eye and low-contrast or low-luminance input to the fellow eye.17-22 Currently, most binocular treatments require active participation by the amblyopic child: playing a game or performing a repetitive psychophysical task. We recently reported our results of a passive binocular treatment, where the subject watches contrast-rebalanced dichoptic movies in which reciprocal blob-shaped parts of the image are pre- sented to each eye to promote binocular combination.23 After 2 weeks (6 movies; approximately 9 hours), mean amblyopic eye best-corrected visual acuity (with standard error) improved from 0.72       0.08 logMAR at baseline to 0.52 0.09 logMAR (P 5 0.003), that is, 2 logMAR lines of improvement at the 2-week outcome visit. These results suggested that passive viewing of dichoptic animated feature films is a feasible and effective amblyopia treat- ment. However, the sample size was small (n 5 8), limiting

generalizability and our ability to assess whether treatment effectiveness was associated with baseline factors. In the current study, a larger cohort of 27 amblyopic children participated in a 2-week intervention with contrast- rebalanced dichoptic movies. We investigated  whether any baseline factors were associated with response to this passive binocular intervention.

Subjects and Methods

The study was approved by the Institutional Review Board of University of Texas Southwestern Medical Center and complied with regulations of the US Health Insurance Portability and Accountability Act of 1996. Written informed consent was ob- tained from a parent of each participant and the child’s written assent was obtained in accordance with the Institutional Review Board’s regulations. A total of 27 amblyopic children, 4-10 years of age, were enrolled. Eligibility criteria included a diagnosis of strabismic, anisometropic,  or  combined  mechanism  amblyopia by the referring pediatric ophthalmologist, and best-corrected vi- sual acuity in the amblyopic eye of $0.3 logMAR and in the fellow eye of #0.2 logMAR, with an interocular difference of $0.2 log- MAR. Strabismic children were eligible to participate only after correction of strabismus with glasses or surgery to \5D residual strabismus. Eligible children had to have been wearing their cur- rent spectacle correction for at least 3 months prior to the baseline visit, and the child’s referring pediatric ophthalmologist had to be willing to forgo other amblyopia treatment during the  study  period. Exclusion  criteria  were  gestational  age   at   birth   of #32 weeks, developmental delay, and coexisting ocular or sys- temic diseases. Medical records were obtained from the referring pediatric ophthalmologist to extract diagnosis, cycloplegic refrac- tion, and treatment history.

The movies and protocol were the same as previously reported in our pilot study of 8 amblyopic children.23 Briefly, children wore glasses fitted with polarized film over their habitual glasses to watch 6 dichoptic movies shown on a passive 3D display (LG Elec- tronics USA, Englewood, NJ) in our laboratory. Odd lines on the 3D display were visible to one eye, and the even lines were visible to the other eye. Dichoptic versions of 18 popular animated feature films were created.23 Using a customized MatLab pro- gram, a patterned image mask composed of irregularly shaped blobs was multiplied with the images seen by the amblyopic eye, and the inverse patterned mask was multiplied with the images seen by the fellow eye, so that different parts of the display were seen by each eye. Blobs of the movie seen by the amblyopic eye al- ways had high contrast (100%), whereas the complementary blobs were seen by the fellow eye with reduced contrast. Because the blobs had Gaussian edges, the edges of the blobs overlapped and were seen by both eyes with differing contrasts. The shape and location of the blobs were varied dynamically every 10 seconds.

Children watched 6 movies during the 2-week period. A 2-week study duration was chosen as adequate to evaluate whether di- choptic movies were effective in improving visual acuity.18,20,23 Previous binocular amblyopia treatments have been shown to improve visual acuity with 8-10 hours of treatment, and we needed to minimize the demand on the family for study-required visits to the laboratory to view each movie. Fellow-eye contrast was initially set at a reduced level that allowed binocular vision, based on the child’s dichoptic motion coherence contrast ratio (CR) minus 0.10, with a minimum of 0.20 and a maximum setting of 0.60.19,23,24 Fellow  eye  contrast  was  incremented  by 10% of the  previous  contrast  for  each  subsequent  movie.  With a maximum initial fellow eye contrast of 0.60 and a 10% increment, we ensured that a contrast  imbalance  would  be  present for all 6 movies. A parent  accompanied  their  child  during the movie sessions to  ensure  compliance  (polarized glasses wear and attention to the movie). Compliance was also confirmed by study personnel at 15- to 30-minute intervals.

Best-corrected visual acuity, random dot stereoacuity, and in- terocular suppression were measured at baseline and outcome visits. Best-corrected visual acuity was obtained for each eye with the ATS-HOTV for children \7 years old or E-ETDRS for children $7 years. Retrospective visual acuity data from visits 6 months, 3 months, and 1 month prior to the baseline visit were obtained from medical records for 20, 23, and 27 of the 27 partic- ipants, respectively. Random dot stereoacuity was evaluated using the Randot Preschool Stereoacuity Test (Stereo Optical Co Inc, Chicago, IL), the Stereo Butterfly Test (Stereo Optical Co Inc), and  the  Lang-  Stereotest  I  (Lang-Stereotest  AG;  Ku€snacht, Switzerland). Nil stereoacuity was arbitrarily assigned a value of

4.0 log arcsec. Severity of interocular suppression, measured by CR, was quantified using a dichoptic motion coherence test that determines the maximum contrast of randomly moving dots in the fellow eye that still allows the child to discriminate the direc- tion of coherent motion dots in the amblyopic eye.19,23,24

Sample Size and Data Analysis

The pilot study reported a mean (T standard deviation) improve- ment of 0.23 T 0.14 logMAR.23 However, the inclusion criteria for the pilot study restricted baseline best-corrected visual acuity to $0.5 logMAR and, likely because of a ceiling effect, worse base- line best-corrected visual acuity is associated with more improve- ment with amblyopia treatment.25 In the current study  we included visual acuity of $0.3 logMARandestimateda moreconser- vative mean effect of 0.1 line improvement, to be evaluated by paired t test, with a 5 0.025 and 1-b 5 0.90, requiring a sample size of 24.26 The primary outcome, amblyopic eye best-corrected visual acuity at 2 weeks, was compared with best-corrected visual acuity at baseline using a paired t test. Stereoacuity and suppression at the 2-week visit were also compared from baseline using a paired t test. Secondary group analyses of amblyopic visual acuity improvement were conducted on 6 dichotomized baseline factors using t tests: 3-6 years versus 7-10 years old, moderate versus se- vere amblyopia, history of patching treatment present versus ab- sent, history of binocular amblyopia treatment present versus absent, random dot stereoacuity present versus nil, and initial di- choptic CR 1.0-2.9 (no or mild suppression) versus $3.0 (moder- ate to severe suppression). Because 6 t tests were conducted on the same data set, Bonferroni correction was used to reduce the chance of type 1 error; that is, only P values of #0.008 were considered statistically significant. Pearson r correlations were conducted to determine associations of baseline variables with

Table 1. Baseline characteristics

amblyopic best-corrected visual acuity improvement at the outcome visit.

 

Results

Baseline data of the 27 subjects are provided in Table 1. Overall, 48% were female and 59% were non-Hispanic white.   Mean   age   (with   standard   deviation)   was 7.3 1.8 years. Children had strabismic (7%), anisome- tropic (59%), or combined mechanism (33%) amblyopia. Mean ( standard deviation) best-corrected visual acuity was   0.57    0.22   logMAR   in   the   amblyopic   eye;

0.02 0.12 logMAR, in the fellow eye. Visual acuity data extracted from medical records showed that mean best- corrected visual acuity in the amblyopic eye varied little on multiple visits prior to the baseline visit (mean range, 0.50–0.54 logMAR) and was similar to the mean baseline value (0.57 logMAR; Figure 1A).

At the outcome visit, mean amblyopic eye visual acuity improved from baseline by 0.15 T 0.10 logMAR, from 0.57        0.22   to  0.42        0.23   logMAR  (t26   5 8.09;

P \ 0.0001; 95% CI for improvement, 0.11–0.19 log- MAR). Fellow eye visual acuity was stable throughout all 5 visits, at 0.02 logMAR. Figure 1B shows that the percent- age of children with severe amblyopia ($0.7 logMAR) was reduced from 30% at baseline to 11% and that 19% of chil- dren had mild or no amblyopia after 2 weeks of treatment. Most children (81%) had an improvement of 1-2 lines (0.1–

 

 

 

FIG 1. A, Best-corrected visual acuity (mean with standard deviation) of the amblyopic and fellow eyes for the baseline and 2-week primary outcome visits. Also  shown  are  retrospective  data  at  6 months, 3 months, and 1 month prior to baseline, obtained from medical re- cords for 20, 23, and 27 of the 27 participants, respectively. B, Per- centages of children with severe ($0.7 logMAR), moderate (0.3-0.6 logMAR), and mild or no (#0.2 logMAR) amblyopia at baseline and after treatment. C, Number of lines of best-corrected visual acuity improvement from baseline at the outcome examination.

 

0.2 logMAR) in best-corrected visual acuity, whereas 14% had 3-4 lines improvement (Figure 1C). Only one child failed to show any improvement.

Mean stereoacuity showed no significant improvement between baseline (3.57 T 0.77 log arcsec) and the outcome visit (3.50 0.76 log arcsec; t26 5 1.37; P 5 0.18). Severity of suppression, as indexed by the mean CR was signifi- cantly reduced between baseline (4.1 T 3.2) and the outcome visit (3.0 2.6; t26 5 3.10, P 5 0.01). Reduced suppression (improvement in CR) was correlated with improvement in amblyopic eye visual acuity (r 5 0.39;

FIG 2. Improvement in amblyopic eye best-corrected visual acuity in the younger (3-6 years) and the older (7-10 years) age subgroups and in subgroups with moderate (0.3-0.6 logMAR) or severe ($0.7 log- MAR) amblyopia at baseline.

 

P 5 0.04; 95% CI, 0.01-0.67). Only two baseline factors were associated with amblyopic best-corrected visual acu- ity improvement (Figure 2). Children 3-6 years of age had a mean improvement of 0.21 T 0.11 logMAR, whereas children 7-10 years of age improved by 0.11 T 0.06 log- MAR (t25 5 3.05; P 5 0.005). Also, children with severe amblyopia ($0.7 logMAR) had improvement of

0.24       T 0.12 logMAR, whereas children with moderate

amblyopia improved by 0.12 0.06 logMAR (t25 5 3.49; P 5 0.002). None of the other baseline variables examined (history of prior patching treatment, history of prior binoc- ular treatment, stereoacuity, CR) had a significant associa- tion with amblyopic eye best-corrected visual acuity improvement (t25 \ 1.64; P . 0.1 for all comparisons).

Although we did not have a formal plan to conduct long- term follow-up of participants, visual acuity data were available at 6-11 months (n 5 10) or 12-24 months  (n 5 6) later for children who had no treatment other than spectacles following completion of the study. On average, there was 0.00 0.07 logMAR change between the outcome visit and the follow-up examination. Four children had 0.10 logMAR deterioration, 8 had no change, and 4 had an improvement of 0.1 logMAR. Other partici- pants were lost to follow-up (n 5 4) or were excluded from the follow-up data because they were patching for residual amblyopia following participation in this study (n 5 7).

Discussion

Passive binocular amblyopia treatment of watching 6 contrast-rebalanced dichoptic  movies  (approximately  9 hours total) over a 2-week period resulted in 0.15 log- MAR mean improvement in amblyopic eye best- corrected visual acuity. Retrospective data from medical records showed stable visual acuity was present on multiple visits prior to the baseline visit. Thus, although we did not have a sham movie comparison group, it is unlikely that the observed visual acuity improvement was due simply to repeated testing. A similar improvement in visual acuity has been observed with 2 weeks of active binocular ambly-opia treatment with binocular games20 and with 3-4 months of occlusion therapy in children with stable visual acuity in spectacle correction prior to baseline.27-30

Accompanying the improvement in amblyopic eye visual acuity, there was also a significant reduction in the severity of suppression at the outcome visit, and this reduction was correlated with improved visual acuity. This relationship is consistent with a correlation between amblyopic eye visual acuity and depth of suppression.31,32 Converging evidence implicates interocular suppression in the etiology of amblyopia,15,16 and binocular treatment that reduces or eliminates suppression may be the key to successful amblyopia treatment.

A variety of binocular, dichoptic, and virtual reality perceptual learning tasks and games have been developed as potential treatments for amblyopia.17-22 Some authors have hypothesized that action video games may provide the best approach because they are not only highly engaging, requiring attention to identify and track potential targets, but they also trigger arousal via time constraints, decision making, and task performance and provide immediate feedback on success or failure.22,33 The current study provides evidence for visual acuity improvement as a result of passive exposure to dichoptic contrast-rebalanced video content, in the absence of the requirement to perform any task and without any feedback. Only two baseline variables in the current study were associated with the amount of visual acuity improvement observed at the outcome visit: age and severity of ambly- opia. There was greater improvement in amblyopic eye vi- sual acuity in children 3-6 years of age compared with those 7-10 years of age. Randomized clinical trials conducted by the Pediatric Eye Disease Investigator Group (PEDIG) also show that, although patching treatment is effective in older children, the response tends to be slower, with less gain.34-36 Our finding of greater visual acuity improvement in children with severe amblyopia at baseline is similar to the larger improvement reported for patching treatment by PEDIG.25 The finding of a 0.24 log- MAR improvement in the severe amblyopia group is consistent with our pilot study of 8 amblyopic children, with baseline visual acuity of 0.72   0.24 logMAR who achieved 0.23          0.14 logMAR improvement at the end of 2 weeks.23 The lack of association with other baseline vari- ables (prior treatment, stereoacuity, and severity of sup- pression) suggests that the potential benefit of binocular amblyopia treatment is generally applicable to children with amblyopia.

This study had several limitations. Although 8-10 hours of treatment yielded significant visual acuity improvement, we were not able to assess whether a longer period of treat- ment might result in additional benefit. The short duration of the intervention was chosen as a trade-off based on prior short-term binocular amblyopia treatment studies that demonstrated visual acuity improvement with 8-10 hours of treatment and the demand placed on participating fam- ilies to travel to the laboratory. In addition, there was no randomized comparison to patching or other amblyopia treatments. To address these limitations, we are currently conducting a randomized trial of at-home dichoptic movies versus patching for the treatment of amblyopia (NCT03825107). Lastly, we did not have a formal plan to evaluate long-term stability of visual acuity. Because of the short treatment duration, many participants had resid- ual amblyopia at the outcome visit, and some opted to immediately begin patching treatment. As a result, we were unable to assess the stability of the visual acuity gain achieved with dichoptic movie treatment. Nonetheless, we did find visual acuity stability within 0.1 logMAR among the 16 children who had no additional treatment, other than continued spectacle wear, within the expecta- tion for visual acuity test–retest reliability.37,38

 

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18/Oct/2019

Computer vision syndrome (a.k.a. digital eye strain)

Mark Rosenfield
State University of New York College of Optometry
139 PUBLICATIONS 2,687 CITATIONS

 

Abstract

Computer vision syndrome, also known as digital eye strain, is the combination of eye and vision problems associated with the use of computers (including desktop, laptop and tablets) and other electronic displays (eg smartphones and electronic reading devices). In today’s world, the viewing of digital screens for both vocational and avocational activities is virtually universal. Digital electronic displays differ significantly from printed materials in terms of the within-task symptoms experienced. Many individuals spend 10 or more hours per day viewing these displays, frequently without adequate breaks. In addition, the small size of some portable screens may necessitate reduced font sizes, leading to closer viewing distances, which will increase the demands on both accommodation and vergence. Differences in blink patterns between hard-copy and electronic displays have also been observed. Digital eye strain has been shown to have a significant impact on both visual comfort and occupational productivity, since around 40% of adults and up to 80% of teenagers may experience significant visual symptoms (principally eye strain, tired and dry eyes), both during and immediately after viewing electronic displays. This paper reviews the principal ocular causes for this condition, and discusses how the standard eye examination should be modified to meet today’s visual demands. It is incumbent upon all eye care practitioners to have a good understanding of the symptoms associated with, and the physiology underlying problems while viewing digital displays. As modern society continues to move towards even greater use of electronic devices for both work and leisure activities, an inability to satisfy these visual requirements will present significant lifestyle difficulties for patients.

Introduction

In the modern world, the viewing of electronic displays has become a huge part of daily living at home, at work, during leisure time and on the move. The use of desktop, laptop and tablet computers, smartphones and electronic reading devices has become ubiquitous (Rosenfield et al. 2012a). For example, in 2011 the US Department of Commerce reported that 96% of working Americans use the internet as an integral part of their job (http://2010-2014.commerce. gov/news/fact-sheets/2011/05/13/fact-sheet-digital-literacy), and it is likely that this percentage has increased further since the time of publication. Indeed, while the ‘paperless office’ has been forecast for many years without ever coming to fruition, we may be moving closer to the day when hard-copy printed material will finally be superseded by a digital alternative.

The number of hours that individuals view electronic screens is substantial. For example, it was reported in 2013 that adults in the USA spend an average of 9.7 hours per day looking at digital media (including computers, mobile devices and television: http://adage.com/article/digital/americans-spend- time-digital-devices-tv/243414/). In addition, an investigation of over 2000 American children between 8 and 18 years of age found that, in an average day, they spend approximately

7.5 hours viewing entertainment media (comprising 4.5 hours watching television, 1.5 hours on a computer and over an hour playing computer games; Rideout et al. 2010). Providing further evidence for the omnipresence of technology, on average users may check their smartphones about 1500 times per week or 221 times per day (equivalent to every
4.3 minutes, assuming a 16-hour day: http://www.tecmark. co.uk/smartphone-usage-data-uk-2014). Evidence that the need for instant communication nowadays is so strong comes from the finding that when people first wake up, 35% reach for their phones, ahead of coffee (17%), a toothbrush (13%) or their significant other (10%) (http://newsroom. bankofamerica.com/files/doc_library/additional/2015_BAC_ Trends_in_Consumer_Mobility_Report.pdf)! This dependence may even have an impact on systemic and ocular health. In children, increased screen time, when combined with a reduction in physical activity, has been shown to produce a significant decrease in the calibre of retinal arterioles (Gopinath et al. 2011).

It should also be noted that viewing digital electronic screens is not confined to adults, teenagers and older children. A literature review by Vanderloo (2014) reported that preschoolers spend up to 2.4 hours per day watching electronic screens. As a result, the American Academy of

Date of acceptance: 17 September 2015. Address for correspondence: Prof. M Rosenfield, SUNY College of Optometry, 33 West 42nd Street, New York NY 10036, USA. Rosenfield@sunyopt.edu
© 2016 The College of Optometrists 1

Pediatrics (2013) recommended that children under 2 years should not spend any time watching electronic screens.

Given the substantial number of hours being devoted to viewing screens, it is of significant concern to optometrists that the magnitude of ocular and visual symptoms is significantly higher when viewing these digital displays when compared with hard-copy printed materials (Chu et al. 2011). Although it is difficult to estimate accurately the prevalence of symptoms associated with electronic screens, as both working conditions and the methods used to quantify symptoms vary widely, an investigation of computer users in New York City noted that 40% of subjects reported tired eyes ‘at least half the time’, while 32% and 31% reported dry eye and eye discomfort, respectively, with this same frequency (Portello et al. 2012). Symptoms varied significantly with gender (being greater in females), ethnicity (being greater in Hispanics) and the use of rewetting drops. A significant positive correlation was observed between computer-related visual symptoms and the Ocular Surface Disease Index, a measure of dry eye. In addition, a recent survey of 200 children between 10 and 17 years of age by the American Optometric Association indicated that 80% of participants reported that their eyes burned, itched and felt tired or blurry after using a digital electronic device (http://aoa.uberflip.com/i/348635, page 20).

These ocular and visual symptoms have been collectively termed computer vision syndrome (CVS) or digital eye strain (DES). The latter term is preferable, since the public may not consider portable devices such as smartphones and tablets to be computers. However, it is important that the optometrist questions every patient about their use of technology. A comprehensive history at the start of the examination should collect information on the number and type of devices being used and the nature of the task demands. A list of areas that should be included in the case history is shown in Table 1. Simply asking patients whether they use a computer and recording this as a yes or no answer in the patient record is inadequate.

Number and type of devices being used (including desktop, laptop and tablet computers and smartphones)
Viewing distance and gaze angle for each device
Duration of use for each device
Monitor size (for a desktop computer, also ask about the number of monitors being used)
Type of task being performed on each device
The size of the critical detail being observed during the task

As noted in Table 1, there are a number of areas that must be discussed, since new technologies are used very differently from traditional printed materials. These differences are discussed in greater detail below.

Gaze angle

A pertinent issue is the specific gaze angle being adopted when viewing digital devices. This can present a significant problem during the eye examination, as it may be difficult to replicate in the examination room, particularly when a phoropter is being used. Long et al. (2014) noted that, while desktop and laptop computers are most commonly viewed in primary and down gaze, respectively (although this may vary with a desktop computer if multiple monitors are being used), hand-held devices such as tablet computers and smartphones may be positioned in almost any direction, sometimes even being held to the side, thereby requiring head and/or neck turn. Given that the magnitude of both heterophoria (Von Noorden 1985) and the amplitude of accommodation (Rosenfield 1997) can vary significantly with the angle of gaze, it is important that testing be conducted using conditions that replicate the habitual working conditions as closely as possible.
Text size
In addition, the size of the text being observed, particularly on hand-held devices, may be very small. For example, Bababekova et al. (2011) reported a range of visual acuity demands when viewing a webpage on a smartphone from 6/5.9 to 6/28.5 (with a mean of 6/15.1). While this may not seem overly demanding, it should also be noted that an acuity reserve is required to allow comfortable reading for a sustained period of time. Attempting to read text of a size at or close to the threshold of resolution for an extended interval may produce significant discomfort (Ko et al. 2014). Kochurova et al. (2015) demonstrated that a two-times reserve was appropriate for young, visually normal subjects when reading from a laptop computer, ie for sustained comfortable reading, the text size should be at least twice the individual’s visual acuity. However, higher values may be necessary for older patients, or individuals with visual abnormalities. Therefore, the smallest-sized text recorded by Bababekova et al. (2011) (around 6/6) would necessitate near visual acuity of 6/3. Few, if any, practitioners record near visual acuity to this degree during a standard eye examination.

Glare

Some patients may report significant discomfort from glare while viewing digital screens. Accordingly, it is important that optometrists discuss both appropriate lighting and the use of window shades, as well as proper screen and operator positioning. Any reflections on the computer display, desktop equipment and/or input devices from windows and luminaires are likely to result in both symptoms and a loss of work efficiency. Relatively simple advice regarding the placement of desktop screens perpendicular to fluorescent tubes, and not directly in front of or behind an unshaded window may be extremely beneficial to the patient. For older patients with less transparent ocular media, the effects of glare may be more disabling. For these individuals, a valuable clinical test is to measure visual resolution in the presence of a glare source, such as the Marco brightness acuity tester (Marco Ophthalmic, Jacksonville, FL, USA). In order to provide useful advice on the placement of localised lighting (such as a desk lamp for an individual who needs to be able to view both a desktop or laptop monitor and hard-copy printed materials simultaneously), careful questioning by the optometrist as to the precise task requirements is critical.

Correcting refractive errors

Determining the appropriate refractive correction for the digital user also presents challenges for the optometrist. Required working distances may vary from 70cm (for a desktop monitor) to 17.5cm for a smartphone (Bababekova et al. 2011; Long et al. 2014). These distances correspond to dioptric demands from 1.4D to 5.7D. For the presbyopic patient, it is unlikely that a single pair of correcting lenses will provide clear vision across this dioptric range. Given the previously mentioned variation in gaze angle for different devices, bifocal and progressive addition lenses, with the near addition positioned in the lower part of the lens, may also be unsuccessful. Accordingly, it may be necessary to prescribe multiple pairs of spectacles, of different formats (eg single-vision, bifocals, trifocals) for the various working distances and gaze angles required by the patient. Occupational prescriptions, perhaps combining an intermediate and near correction, are frequently useful. Progressive addition lenses may be unsuccessful due to the narrow width of the reading area. Care should be taken to ensure that the near addition lens prescribed for a presbyopic patient is appropriate for the preferred (or, in some cases, required) viewing distance(s). As noted above, viewing distances that differ markedly from 40cm (2.50D) are frequently adopted.

Additionally, the correction of small amounts of astigmatism may be important. In two similar experiments, Wiggins and Daum (1991) and Wiggins et al. (1992) examined the effect of uncorrected astigmatism while reading material from a computer screen. In both studies, the authors observed that the presence of 0.50–1.00D of uncorrected astigmatism produced a significant increase in symptoms. While astigmatism is typically corrected in spectacle wearers, it is not unusual in contact lens patients to leave small to moderate amounts of astigmatism uncorrected. Given that the physical presence of a contact lens on the cornea may also exacerbate the symptoms associated with DES (Rosenfield 2011), it may be particularly important in these patients that visual discomfort is not aggravated further by the presence of uncorrected astigmatism. Additionally, patients with less than 1D of simple myopic or simple hyperopic astigmatism, where one meridian is emmetropic, may on occasions be left uncorrected. Further, patients purchasing ready-made (spherical), over-the-counter reading glasses may also experience uncorrected astigmatism. Therefore, it may be necessary to correct astigmatism in those patients whose visual demands require them to view information on an electronic screen.

In addition to the discomfort experienced during computer operation, symptoms of DES may also have a significant economic impact. Ocular and visual discomfort can increase

the number of errors made during a computer task as well as necessitating more frequent breaks. Musculoskeletal injuries associated with computer use may account for at least half of all reported work-related injuries in the USA (Bohr, 2000). Indeed, Speklé et al. (2010) noted that conservative estimates of the cost of musculoskeletal disorders to the USA economy as reported in 2001, when measured by compensation costs, lost wages and reduced productivity, were between 45 and 54 billion dollars annually or 0.8% of gross domestic product. Further, the prevalence of neck, shoulder and arm symptoms in computer workers may be as high as 62% (Wahlstrom 2005). In addition to productivity costs, it was estimated in 2002 that employers in the USA paid approximately $20 billion annually in workers’ compensation resulting from work-related musculoskeletal disorders (Chindlea 2008).

When considering DES specifically, Daum et al. (2004) estimated that provision of an appropriate refractive correction alone could produce at least a 2.5% increase in productivity. This would result in a highly favourable cost–benefit ratio to an employer who provided computer-specific eyewear to employees. Accordingly, it is clear that the economic impact of DES is extremely high, and minimising symptoms that reduce occupational efficiency will result in substantial financial benefits (Rosenfield et al. 2012b).

Accommodation and convergence

Given the significant near-vision demands associated with viewing digital screens, a comprehensive assessment of the accommodation and vergence system should be included for all users of digital screens. Parameters to be quantified are listed in Table 2. The use of Cross–Nott retinoscopy (Rosenfield 1997) and associated phoria (ie prism to eliminate fixation disparity) to assess the actual accommodative and vergence response for the specific task demands is particularly important. Failure to maintain an appropriate oculomotor response will result in symptoms and/or loss of clear and single binocular vision. While the assessment of the maximum accommodation (ie amplitude) and vergence (near point) responses is useful, these measures may not provide an indication of the actual response that is maintained during a sustained task. Tests that assess the ability of the patient to make rapid and accurate changes in the oculomotor responses, such as accommodative and vergence facility using lens and prism flippers, respectively, are especially useful for individuals whose task may require them to change fixation from a distant stimulus (perhaps viewing across an office) to an intermediate (such as a desktop computer) or near target (viewing hard-copy printed materials or a smartphone). The Hart chart test, whereby patients have to switch from one target distance to another, and to report when they have clear and single vision at each distance, is an alternative, and possibly superior, method of testing the flexibility of accommodation and vergence, compared with the use of lens or prism flippers. This more naturalistic method, where a patient fixates fine detail at different viewing distances, involves all of the cues to the oculomotor system, including tonic, proximal, retinal disparity and defocus, as well as testing the interaction between

Accommodation testing
Subjective amplitude of accommodation (push-up or minus lens)
Accommodative response at preferred working distance (Cross–Nott retinoscopy)
Monocular and binocular accommodative facility (±2.00 lenses or Hart chart)
Negative and positive relative accommodation
Vergence testing
Near point of convergence
Distance and near heterophoria (near to be performed at the preferred and/or required working distance)
Presence of A- and V-patterns
Presence of A- and V-patterns
Horizontal fixation disparity/associated phoria at preferred and/or required working distance
Vergence facility (using 12Δ base-out/3Δ base-in prisms or Hart chart)
Base-in and base-out vergence ranges
Stereopsis

accommodation and vergence. It should be noted that the Hart chart test does not require the practitioner to purchase any specialised equipment. Simply having the patient change fixation from a standard distance visual acuity chart to a near acuity chart held at an intermediate or near distance will work just as well. The patient is instructed to report when the fine detail on each chart appears both clear and single. The number of cycles (ie the number of times the patient is able to report clear and single vision at both distance and near) that the patient is able to complete in a 60-second period should be recorded, as well as any difficulty in clearing one of the targets quickly.

Dry eye

Dry eye has previously been cited as a major contributor to DES. For example, Uchino et al. (2008) observed symptoms of dry eye in 10.1% of male and 21.5% of female Japanese office workers using visual display terminals. Furthermore, longer periods of computer work were also associated with a higher prevalence of dry eye (Rossignol et al. 1987). In an extensive review, Blehm et al. (2005) noted that computer users often report eye dryness, burning and grittiness after an extended period of work. Rosenfield (2011) suggested that these ocular surface-related symptoms may result from one or more of the following factors:
1. Environmental factors producing corneal drying. These could include low ambient humidity, high forced-air heating or air-conditioning settings or the use of ventilation fans, excess static electricity or airborne contaminants.
2. Increased corneal exposure. Desktop computers are commonly used with the eyes in the primary position, whereas hard-copy text is more commonly read with the eyes depressed. The increased corneal exposure associated with the higher gaze angle could also result in an increased rate of tear evaporation. It should also be noted that laptop computers are more typically used in downward gaze, while both tablet computers and smartphones can be held in either primary or downward gaze
3. Age and gender. The prevalence of dry eye increases with age and is higher in women than men (Gayton 2009; Salibello and Nilsen 1995; Schaumberg et al. 2003).
4. Systemic diseases and medications. Moss et al. (2000, 2008) reported that the incidence of dry eye was greater in subjects with arthritis, allergy or thyroid disease not treated with hormones. Additionally, the incidence was higher in individuals taking antihistamines, antianxiety medications, antidepressants, oral steroids or vitamins, as well as those with poorer self-rated health. Perhaps surprisingly, a lower incidence of dry eye was found with higher levels of alcohol consumption.

Blink rate

Another explanation for the higher prevalence of dry-eye symptoms when viewing digital screens may be due to changes in blink patterns. Several investigations have reported that the blink rate is reduced during computer operation (Patel et al. 1991; Schlote et al. 2004; Tsubota and Nakamori 1993; Wong et al. 2002). For example, Tsubota and Nakamori (1993) compared the rate of blinking in 104 office workers when they were relaxed, reading a book or viewing text on an electronic screen. Mean blink rates were 22/minute while relaxed, but only 10/minute and 7/minute when viewing the book or screen, respectively. However, these three testing conditions varied not only in the method of presentation, but also in task format. It has been noted that blink rate decreases as font size and contrast are reduced (Gowrisankaran et al. 2007), or the cognitive demand of the task increases

(Cardona et al. 2011; Himebaugh et al. 2009; Jansen et al. 2010). Therefore, the differences observed by Tsubota and Nakamori may be related to changes in task difficulty, rather than being a consequence of changing from printed material to an electronic display. Indeed, a recent study in our laboratory compared blink rates when reading identical text from a desktop computer screen versus hard-copy printed materials (Chu et al. 2014). No significant difference in the mean blink rates was found, leading to the conclusion that previously observed differences were more likely to be produced by changes in cognitive demand rather than the method of presentation.

While screen use may not alter the overall number of blinks, Chu et al. (2014) observed a significantly higher percentage of incomplete blinks when subjects read from a computer (7.02%) in comparison with reading hard-copy, printed materials (4.33%). However, it is uncertain whether changes in cognitive demand also alter the percentage of incomplete blinks. This may be important, given that a significant correlation was found between post-task symptom scores and the percentage of blinks deemed incomplete (Chu et al. 2014). Interestingly, increasing the overall blink rate (by means of an audible signal) does not produce a significant reduction in symptoms of DES (Rosenfield and Portello 2015). This might imply that it is the presence of incomplete blinks, rather than changes in the overall blink rate, that is responsible for symptoms. McMonnies (2007) reported that incomplete blinking would lead to reduced tear layer thickness over the inferior cornea, resulting in significant evaporation and tear break-up. Current work in our laboratory is examining the effect of blink efficiency exercises to reduce the rate of incomplete blinking on DES symptoms.

Asthenopia

In a review of asthenopia, Sheedy et al. (2003) noted that symptoms commonly associated with this diagnostic term included eye strain, eye fatigue, discomfort, burning, irritation, pain, ache, sore eyes, diplopia, photophobia, blur, itching, tearing, dryness and foreign-body sensation. While investigating the effect of several symptom-inducing conditions on asthenopia, these authors determined that two broad categories of symptoms existed. The first group, termed external symptoms, included burning, irritation, ocular dryness and tearing, and was related to dry eye. The second group, termed internal symptoms, included eye strain, headache, eye ache, diplopia and blur, and is generally caused by refractive, accommodative or vergence anomalies. Accordingly, the authors proposed that the underlying problem could be identified by the location and/or description of symptoms.

It has been suggested that the poorer image quality of the electronic screen, when compared with printed materials, may be responsible for the change in blink rate (Chu et al. 2011). However, Gowrisankaran et al. (2012) observed that degrading the image quality by either inducing 1.00D of uncorrected astigmatism or presenting the target at only 7% contrast did not produce a significant change in blink rate for a given level of cognitive load. Further, Gowrisankaran et al. (2007) reported that induced refractive error, glare,

reduced contrast and accommodative stress (varying the accommodative stimulus by ±1.50D during the course of the task) actually produced an increase in blink rate. Additionally, Miyake-Kashima et al. (2005) found that introduction of an anti-reflection film over a computer monitor to reduce glare produced a significant reduction in blink rate. Therefore, it does not seem that the digital screen itself represents a degraded visual stimulus that is responsible for significant changes in blink rate.

The blue light hypothesis

It has recently been suggested that the blue light emitted from digital displays may be a cause of DES, although there is no published evidence to support this claim. Blue light is generally considered to comprise wavelengths between
380 and approximately 500nm. Fortunately, the human retina is protected from short-wavelength radiation, which is particularly damaging, by the cornea which absorbs wavelengths below 295 nm and the crystalline lens which absorbs below 400nm (Margrain et al. 2004). However, shorter wavelengths have higher energy, and therefore reduced exposure times may still result in photochemical damage. Visible blue light can easily reach the retina and may cause oxidative stress in the outer segments of the photoreceptors as well as the retinal pigment epithelium. These factors have been implicated in the development of age-related macular degeneration (Taylor et al. 1990). Certain groups may be particularly susceptible to blue light damage, such as children (because of the transparency of their crystalline lens) and both aphakic and pseudophakic individuals who either cannot filter out short wavelengths, or fail to do so adequately.

Additionally, exposure to blue light has been widely reported to be involved in the regulation of circadian rhythm and the sleep cycle, and irregular light environments may lead to sleep deprivation, possibly affecting mood and task performance (see LeGates et al. 2014). Indeed, it has been proposed that the use of electronic devices by adolescents, particularly at night time, leads to an increased risk of shorter sleep duration, longer sleep-onset latency and increased sleep deficiency (Hysing et al. 2015). Accordingly, the use of spectacle lenses containing filters to reduce the transmission of blue light has been proposed as a possible treatment modality for DES. However, it must be noted that exposure to sunlight delivers far more illumination when compared with any form of artificial lighting. For example, while sunlight may vary between 6000 and 70000 lux (Wang et al. 2015), its output exceeds typical levels of artificial lighting by a factor of 100 times or more. Further, the amount of short-wavelength radiation being emitted from digital screens is far smaller than from most artificial light sources.

Nevertheless, a recent study by Cheng et al. (2014) suggested that there may be some benefit from wearing blue filters during a computer task. These authors examined the effect of low-, medium- and high-density blue filters (in the form of wraparound goggles) worn during computer work in groups of dry-eye and normal subjects (n = 20 for each group). They observed a significant reduction in DES-related symptoms in the dry-eye group (but not in the normal

subjects). This effect was seen for all filter densities. However, the study did not include a control condition, and so a placebo effect, where the subjects were aware that they were receiving treatment, cannot be ruled out. Further, the wraparound goggles may have reduced tear evaporation in the dry-eye subjects. Given that several blue-filter lenses are now being marketed specifically for the treatment of DES (eg Hoya Blue Control, SeeCoat Blue (Nikon) and Crizal Prevencia (Essilor)), further research is required to determine both the efficacy and mechanism of action of these filters.

Wearable technology

The area of wearable technology seems likely to expand dramatically over the next 5–10 years. At the time of writing, Google Glass (Figure 1), which projected a virtual image into the superior temporal field of the right eye, is no longer being marketed to the general public. However, it seems likely that similar products will become available in the future. These may present significant issues for the optometrist. For example, in the case of Google Glass, the image was only seen by one eye, thereby creating the potential for binocular rivalry and visual interference (where two images are not clearly distinguishable from one another). Interestingly, there were many anecdotal reports of headaches and other visual symptoms when individuals were first using the device. In addition, it produced significant loss of vision field in upper right gaze (Ianchulev et al. 2014). A subject who was driving, operating machinery or in motion could be severely, and dangerously, impacted by this visual field loss.

Whereas this type of head-up display was once only available in military and commercial aviation, they are now found in motor vehicles to assist with navigation (Figure 2). Their advantages are that they reduce the number of eye movements away from the direction of travel (Tangmanee and Teeravarunyou 2012). However, they can also result in multiple, conflicting stimuli if the projected image lies in a different direction or perceived distance away from the real fixation target. Other forms of wearable technology may present different issues. For example, wrist-mounted displays such as the Apple Watch (Apple, Cupertino, CA, USA: Figure 3) may present extremely small-sized

text due to the limited screen area (approximately 3.3cm by 4.2cm).

However, there may be significant value for spectacle- mounted technology in disabled individuals who require a hands-free device, such as to provide facial recognition for the visually impaired and to monitor eye and head

movements in patients with Parkinson’s disease (McNaney et al. 2014). It seems almost certain that the use of wearable technology will increase rapidly over the next few years, and spectacle frame designers are already developing more attractive options to accommodate these types of device.

In many regards, the visual conflicts described with the Google Glass type of device are not dissimilar from those experienced by users of spectacle-mounted biotic telescopes, where the telescopic device is mounted high on the carrier lens, so that the patient is able to move around while wearing the device, but can still use the telescope when required for ‘spotting’ a more detailed distance target. Indeed, the use of spectacle-mounted video cameras may become more common in visually normal individuals. For example, they are already used by a number of police forces for recording officers’ actions. As the technology develops and gets smaller, one could easily imagine a video camera being hidden within a spectacle frame or lens, with its image being transmitted wirelessly to a recorder (perhaps a smartphone in one’s pocket) or a remote location, where it can be viewed in real time by a third party. While this might be valuable for the training of a new employee (it would be an excellent way of recording an examination performed by a student optometrist for later review) or assisting a colleague away from his or her actual location, the security and privacy implications of being recorded by someone wearing an invisible device are also considerable (Rosenfield 2014).

Conclusion

It is possible that the technological revolution through which we are now living may be seen in the future as equivalent to the industrial revolution of the early 19th century. While the latter saw the development of manufacturing capabilities due to improved iron production processes, the harnessing of steam power and the development of the railways, this expansion comes from almost instantaneous communication around the world and access to vast sources of information. Clearly, technology is here to stay. However, the visual demands of today are very different from those encountered in the past. Digital electronic devices differ significantly from printed materials in terms of their viewing distance, required gaze angle, degree of symptoms and blink patterns. Accordingly, the eye examination must be modified to meet these new demands.

A further issue to consider is the increasing number of older individuals in the population in western Europe and North America (Rosenthal 2009). For example, over the period from 1985 to 2010, the median age of the UK population has increased from 35.4 years to 39.7 years. This median age is projected to be over 42 years of age by 2035. Further, by 2035 it is anticipated that approximately 23% of the total UK population will be 65 years of age and older (http://www.ons.gov.uk/ons/dcp171776 _ 258607.pdf). Accordingly, it seems likely that the prevalence of reported eye strain will continue to rise concurrent with this increase in the number of older people, with the associated age-related increases in hyperopia, astigmatism, dry eye

and loss of media transparency, not to mention that all of these individuals will be presbyopic.

Given the remarkably high number of hours per day that many (or perhaps most) individuals now spend viewing small text on electronic screens at close working distances and varying gaze angles, it is incumbent upon all eye care practitioners to have a good understanding of the symptoms associated with, and the physiology underlying, DES. As modern society continues to move towards the greater use of electronic devices for both work and leisure activities, it seems likely that the visual demands that these units require will continue to increase. An inability to satisfy these visual requirements will present significant lifestyle difficulties for patients, as well as sizeable dissatisfaction and frustration.

Summary

Computer vision syndrome, also known as digital eye strain, is the combination of eye and vision problems associated with the use of computers and other electronic displays. Today, many individuals  spend large numbers of hours viewing these screens. However, the visual demands differ significantly from those presented by traditional printed materials, with the result that up to 80% of users report significant symptoms both during and immediately after viewing electronic screens. This paper reviews the principal ocular causes for this condition, and discusses how  the  standard  eye examination should be modified to meet today’s visual demands.

Conflict of interest
The author has no financial interest in any of the products described in this paper.

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CET multiple choice questions
This article has been approved for one non-interactive point under the GOC’s Enhanced CET Scheme. The reference and relevant competencies are stated at the head of the article. To gain your point visit the College’s website www.college-optometrists.org/oip and complete the multiple choice questions online. The deadline for completion is 30 April 2017. Please note that the answers that you will find online are not presented in the same order as in the questions below, to comply with GOC requirements.

1. Which area is most importantly addressed when completing a case history on a patient who uses a digital device?
• The screen resolution for each device
• The software used on each device
• The viewing distance and gaze angle for each device
• Whether it has a retina display

2. Which of the following statements is correct?
• American schoolchildren spend 7.5 hours a day watching television
• Children under 2 years should not spend any time watching electronic screens
• On average, adults in the USA spend 9.7 hours per day looking at their mobile phone
• On average, US smartphone users check their smartphone 221 times per week
3. What visual reserve is recommended by the paper?
• ×1.5
• ×2
• ×3
• ×4
4. Based on the paper’s recommended visual reserve, what visual acuity would be required to read comfortably on screen text which is 6/8 in size?
• 6/3
• 6/4
• 6/5
• 6/6
5. When viewing a digital device at a range of 17.5–70cm, what would be the corresponding dioptric demand?
• 1.40–3.00D
• 1.40–5.70D
• 3.00–5.70D
• 5.70–6.66D
6. What level of uncorrected astigmatism significantly increased digital eye strain?
• 0.25–0.50DC
• 0.50–1.00DC
• 1.00–1.50DC
• Over 1.50DC

CPD exercise

After reading this article, can you identify areas in which your knowledge of computer vision syndrome has been enhanced?

How do you feel you can use this knowledge to offer better patient advice?

Are there any areas you still feel you need to study and how might you do this?

Which areas outlined in this article would you benefit from reading in more depth, and why?


18/Oct/2019

Computer vision syndrome: a review of ocular causes and potential treatments

 

 

Mark Rosenfield

SUNY College of Optometry, New York, USA

Citation information: Rosenfield M. Computer vision syndrome: a review of ocular causes and potential treatments. Ophthalmic Physiol Opt
2011, 31, 502–515. doi: 10.1111/j.1475-1313.2011.00834.x

Keywords: accommodation, computer vision, convergence, dry eye, near vision, reading

Correspondence: Mark Rosenfield
E-mail address: Mrosenfield@sunyopt.edu

Received: 1 December 2010; Accepted: 10
February 2011

Abstract

Computer vision syndrome (CVS) is the combination of eye and vision prob- lems associated with the use of computers. In modern western society the use of computers for both vocational and avocational activities is almost universal. However, CVS may have a significant impact not only on visual comfort but also occupational productivity since between 64% and 90% of computer users expe- rience visual symptoms which may include eyestrain, headaches, ocular discom- fort, dry eye, diplopia and blurred vision either at near or when looking into the distance after prolonged computer use. This paper reviews the principal ocular causes for this condition, namely oculomotor anomalies and dry eye. Accommo- dation and vergence responses to electronic screens appear to be similar to those found when viewing printed materials, whereas the prevalence of dry eye symp- toms is greater during computer operation. The latter is probably due to a decrease in blink rate and blink amplitude, as well as increased corneal exposure resulting from the monitor frequently being positioned in primary gaze. How- ever, the efficacy of proposed treatments to reduce symptoms of CVS is unpro- ven. A better understanding of the physiology underlying CVS is critical to allow more accurate diagnosis and treatment. This will enable practitioners to opti- mize visual comfort and efficiency during computer operation.

Introduction

The use of computers and digital electronic devices for both vocational and non-vocational activities including e-mail, internet access and entertainment is almost universal in modern Western society. A recent estimate of internet usage by continent ranged from 77.4% of the population of North America to 10.9% of Africa, with an estimated 1 966 514 816 users worldwide (or 28.7% of the world’s population) (http://www.internetworldstats. com/stats.htm).
The viewing of digital electronic screens is no longer restricted to desktop computers located in the workplace. Today’s visual requirements may include viewing laptop and tablet computers, electronic book readers, smart- phones and other electronic devices either in the work- place, at home or in the case of portable equipment, in any location. Furthermore, computer use is not restricted

to adults. A recent investigation of over 2000 American children between 8 and 18 years of age reported that in an average day they spend approximately 7.5 h using entertainment media, 4.5 h watching TV, 1.5 h on a com- puter and over an hour playing video games.1 Some screen sizes may necessitate very small text which the observer frequently positions at a closer viewing distance than had previously been adopted for hard copy printed materials. These increased visual demands may give rise to a variety of symptoms which have been termed com- puter vision syndrome (CVS).
The American Optometric Association defines CVS as the combination of eye and vision problems associated with the use of computers. These symptoms result from the individual having insufficient visual capabilities to perform the computer task comfortably (http:// www.aoa.org/x5374.xml). In a review of CVS, Thomson2 indicated that up to 90% of computer users may

experience visual symptoms including eyestrain, head- aches, ocular discomfort, dry eye, diplopia and blurred vision either at near or when looking into the distance after prolonged computer use. It is unclear whether this number has increased, given the increased use of elec- tronic displays today. Further, Rossignol et al.3 reported that the prevalence of visual symptoms increased signifi- cantly in individuals who spent more than 4 h daily working on video display terminals (VDTs).
Asthenopia is a major complaint in subjects with CVS. The results of a 2008 questionnaire returned by over 400 computer operators in India revealed asthenopic symp- toms in 46.3% of subjects.4 Similarly, a survey of 212 bank workers in Italy found asthenopic symptoms in 31.9% of the subjects, though it is worthwhile noting that this percentage was calculated after 87 subjects were excluded due to uncorrected hyperopia, undercorrected astigmatism, or overcorrected myopia, because the inves- tigators wanted to investigate only subjects ‘without organic visual disturbances’.5 A higher prevalence was found in a study of 35 Mexican computer terminal opera- tors where 68.5% of the subjects experienced symptoms.6 An Australian study of over 1000 computer workers found 63.4% reported symptoms with uncontrolled conditions; this number was reduced to 25.2% when an optimized, ergonomic desk and frequent work breaks were provided.7 It is unclear whether asthenopia during computer use is associated with age,4,7–9 although the prevalence does seem to be higher in females.10–14
In a review of asthenopia, Sheedy et al.15 noted that symptoms commonly associated with this diagnostic term included eyestrain, eye fatigue, discomfort, burning, irrita- tion, pain, ache, sore eyes, diplopia, photophobia, blur, itching, tearing, dryness and foreign-body sensation. While investigating the effect of several symptom-induc- ing conditions on asthenopia, the authors determined that two broad categories of symptoms existed. The first group, termed external symptoms, included burning, irri- tation, ocular dryness and tearing, and was related to dry eye. The second group, termed internal symptoms, included eyestrain, headache, eye ache, diplopia and blur, and is generally caused by refractive, accommodative or vergence anomalies. Accordingly, the authors proposed that the underlying problem could be identified by the location and/or description of symptoms.
It is important to identify whether symptoms (both internal and external, see above) are specific to computer operation, or are simply a manifestation of performing a sustained near-vision task for an extended period of time. If no physiological or subjective differences exist when patients view materials either on electronic screens or in printed form, then there would be little justification for special attention being paid to the visual demands

encountered during computer operation. The electronic screen would simply represent another visual target. However, there is evidence that the two forms of target presentation are not equivalent. For example, Sheedy et al.16 compared the performance of an editing task when the material was either presented on a VDT or in hard copy form. They observed that subjects made fewer errors and performed the task quicker with the hard copy presentation. Similar findings of fewer errors when view- ing printed materials have also been reported in other studies.17–19 More recently, Chu et al.20 compared ocular symptoms immediately following a sustained near-task viewed either on a computer monitor or in hard copy format. Identical text was used in the two sessions, which was matched for size and contrast. In addition, target viewing angle and luminance were similar for the two conditions. Significant differences in median symptom scores were found with regard to blurred vision during the task and the mean symptom score. In both cases, symptoms were higher during computer use. Accordingly, it appears that the symptoms associated with CVS do not result from simply performing a near-vision task for a prolonged period of time. Even when viewing a modern flat panel monitor, subjects reported significantly greater blur during the computer task, when compared with a hard-copy printout of the same material.
In addition to the discomfort experienced during com- puter operation, symptoms of CVS may also have a sig- nificant economic impact. As noted above, symptoms can increase the number of errors made during a computer task as well as necessitating more frequent breaks. Muscu- loskeletal injuries associated with computer use may account for at least half of all reported work-related inju- ries in the USA.21 Indeed, Spekle´ et al.22 noted that con- servative estimates of the cost of musculoskeletal disorders to the United States economy as reported in 2001, when measured by compensation costs, lost wages and reduced productivity were between 45 and 54 billion dollars annually or 0.8% of gross domestic product. Fur- ther, the prevalence of neck, shoulder and arm symptoms in computer workers may be as high as 62%.23 In addi- tion to productivity costs, it was estimated in 2002 that employers in the USA paid approximately $20 billion annually in workers compensation resulting from work- related musculoskeletal disorders.24 When considering CVS specifically, Daum et al.25 estimated that provision of an appropriate refractive correction alone could pro- duce at least a 2.5% increase in productivity. This would result in a highly favourable cost-benefit ratio to an employer who provided computer-specific eyewear to their employees. Accordingly, it is clear that the economic impact of CVS is extremely high, and minimizing symp- toms that reduce occupational efficiency will result in

substantial financial benefit. It should also be noted that both national and international regulations have been issued with regard to health and safety requirements for workers using VDTs to minimize these disorders [e.g. European Council directive 90/270/EEC, the United King- dom Health and Safety (Display Screen Equipment) (DSE) regulations and the Australian Occupational Health & Safety Act of 2000].

Effect of uncorrected refractive error

Given the need to achieve and maintain clear and single vision of relatively small targets throughout the com- puter task, it is important that the retinal image be focused appropriately. Thus, spherical hyperopia and high myopia should be corrected to reduce the ocular stimulus to accommodation and minimize blur. Addi- tionally, the correction of small astigmatic errors may also be important to reduce symptoms of CVS. In two similar experiments, Wiggins and Daum26 and Wiggins et al.27 examined the effects of uncorrected astigmatism while reading material from a computer screen. In both studies the authors observed that the presence of 0.50–
1.00 D of uncorrected astigmatism produced a signifi- cant increase in symptoms. Interestingly, Wiggins et al.27 tested subjects with up to 1 D of residual astigmatism who were corrected with spherical soft contact lenses. This is a common clinical practice. The residual uncor- rected astigmatism produced a significant increase in symptoms during the computer task. Accordingly, the authors suggested that symptoms could be reduced either by fitting these individuals with toric contact lenses, or alternatively by using a spectacle overcorrec- tion to correct the residual astigmatism during computer operation.
A recent study in our laboratory (paper in preparation) recorded ocular symptoms (both internal and external) using a written questionnaire immediately after a sus- tained period of reading from a computer monitor either through the habitual distance refractive correction or with a supplementary )1.00 or )2.00 D oblique cylinder added over these lenses. Additionally, the distance correction condition was repeated on two occasions in 12 subjects to assess the repeatability of the symptom questionnaire. The results showed no significant difference between the habitual correction conditions, but the change from 1 to 2 D of induced astigmatism produced a significant increase in post-task symptoms. These results are shown in Figure 1. The presence of uncorrected oblique astigma- tism will reduce visual acuity significantly. The increase in target blur will make performing the task more difficult, thereby leading to an increase in symptoms such as eyestrain and headache. Therefore, the correction of

Figure 1. Total symptom score following a 20 min period of reading from a computer monitor either through the distance refractive cor- rection or with a supplementary )1.00 or )2.00 D oblique cylinder added over these lenses. The distance correction condition was repeated on two occasions in 12 subjects to assess the repeatability of the symptom questionnaire. Error bars indicate 1 S.E.M.

astigmatic refractive errors may be important in minimiz- ing symptoms associated with CVS.

Smartphone working distances and text sizes

As noted earlier, many of the portable devices used today for written communication (e.g. text messaging, e-mail and internet access) have relatively small screens that may necessitate close working distances and small text sizes. These can increase the demands placed upon ocular accommodation and vergence when compared with printed materials. Indeed, Bilton28 proposed the term ‘1, 2, 10’ to describe commonly adopted working dis- tances, with mobile (cell) phones and e-books typically being held approximately one foot (=30 cm) away, desk- top computers being viewed at about 2 feet (=60 cm), while televisions are often viewed at a distance of 10 feet (approximately 3 m). A study in our laboratory measured both font size and viewing distance in 129 individuals using hand-held electronic devices.29 The mean font size of 1.12 m (S.D. = ±0.24), 6/19.2 (S.D. = ±5.25) or ~N9
was comparable with newspaper print, which generally ranges between 0.8 and 1.2 m (N6–N10).30 These results are shown in Table 1. However, Sheedy and Shaw-McM- inn31 suggested that a 3• acuity reserve should be adopted, indicating that prolonged viewing of a 6/19.2 letter would require visual acuity of at least 6/6.4. Further, as noted in Table 1, in some cases, the text size was as small as 6/8.25 equivalent, which based on the 3• reserve would require visual acuity of 6/2.75 for comfortable, sustained viewing.
Additionally, the mean working distance (36.2 cm) was closer than the typical near working distance of 40 cm for adults when viewing hardcopy text,32 and was as close as
17.5 cm for one individual. Indeed, 75% of the subjects examined used viewing distances between 26 and 40 cm while 22.5% adopted viewing distances of <30 cm (see Table 1). These close distances will place increased

Table 1. Mean and range of values for font size and working distance while using a smartphone

 Mean ± 1 S.D.Range
Font size (mm)1.63 ± 0.351.0–3.0
Snellen fraction6/19.2 ± 5.256/8.3–6/35.3
M acuity1.12 ± 0.240.70–2.10
Working distance (cm)36.2 ± 7.117.5–58.0

Font size is expressed either as the height of the letter, as a Snellen fraction or in terms of M acuity (i.e. the distance in meters at which the letter subtends 5 min of arc). Note that the standard deviation for the Snellen fraction refers only to the denominator of the fraction.

demands upon both ocular accommodation and vergence, especially if maintained for an extended period of time, which could exacerbate symptoms when compared with the longer viewing distances more commonly found when viewing printed materials. Practitioners need to consider the closer distances adopted while viewing material on smartphones when examining patients and prescribing refractive corrections for use at near, as well as when treating patients presenting with asthenopia associated with nearwork.

Correction of presbyopia

The correction of presbyopia can be problematic for patients who spend extended periods of time viewing dig- ital screens. These difficulties may be most severe when viewing desktop monitors placed at fixed viewing dis- tances and gaze angles. These screens are generally placed at or just slightly below primary gaze. Accordingly, the use of a standard bifocal spectacle lens, with the segment placed for a target positioned in downward gaze and pro- viding clear vision for a viewing distance around 40 cm may be inappropriate. Wearers of many progressive addi- tion lenses experience similar difficulties. In providing an appropriate form of spectacle correction, practitioners must consider both the viewing distance and gaze angle (both horizontal and vertical). In terms of viewing dis- tance, the United States Occupational Safety and Health Administration (OSHA) state that the preferred viewing distance for a desktop monitor is between 50 and 100 cm (representing an accommodative stimulus in a corrected individual of between 1 and 2 D). Additionally, they rec-
ommend that the centre of the computer monitor should normally be located 15–20° below the horizontal eye level and the entire visual area of the display screen should be located so the downward viewing angle is never >60° (http://63.234.227.130/SLTC/etools/computerworkstations/ components_monitors.html). Other national agencies such as the United Kingdom Health and Safety Executive (http://www.direct.gov.uk/en/Employment/HealthAnd

SafetyAtWork/DG_10026668) and Australian Standards (http://www.gamc.nsw.gov.au/workplace-guidelines/1_Guide lineContent/guidelines_1_07.htm) also include guidelines for computer set-up and operation. Further, the actual viewing distance and gaze angle may depend on the organi- zation of the workstation, the height of the material being viewed and the physical size of the observer.
The use of non-spectacle methods of correcting presby- opia, such as contact lenses and intra-ocular lenses may also be problematic. For example, alternating or translat- ing lens designs where the near portion of the lens moves in front of the pupil during downward gaze33 are unlikely to be successful when viewing a desktop computer screen positioned in primary gaze. A monovision correc- tion, where one eye is corrected for distance vision while the fellow eye is corrected for near may be successful in early presbyopes (although the loss of stereopsis may provide difficulties). However, as the near addition power increases, the loss of clear intermediate vision may become an issue. ‘Simultaneous vision’ type lenses, whereby multiple powers are positioned before the pupil at the same time, are becoming increasingly common. These lenses require the wearer to suppress the blurred images.34 There appears to be little research at the present time as to whether the quality of intermediate vision provided by these lenses is sufficient to avoid CVS symp- toms, given that small residual refractive errors (or the presence of significant amounts of retinal blur) may be challenging to the patient. Other new forms of presbyopic correction, such as multifocal and ‘accommodating’ intra- ocular lenses35 may raise the same issues as multifocal contact lenses, and further studies are required to deter- mine whether they provide sufficiently clear vision for prolonged viewing of electronic screens at a variety of distances and gaze angles.
Laptop computers are typically placed at different dis- tances and gaze angles to desktop models. The fact that the keyboard is attached to the monitor means there is less flexibility in adjusting the workstation while the key- board remains in comfortable reach.36 The smaller screen size (and text height) may also impact upon the viewing distance depending on the observer’s visual resolution. Harris and Straker37 noted that laptop computers may be used in a variety of positions, ranging from sitting at a desk, sitting with the computer on one’s lap or even lying prone. Accordingly, a form of presbyopic spectacle cor- rection prescribed for a desktop computer is often inap- propriate for a laptop. A laptop computer is often viewed in downward gaze at a distance which may approximate the position at which a presbyopic individual would read hand-held printed materials. This may actually make providing a spectacle correction easier for these types of devices. As noted earlier, smartphones are often held at

closer viewing distances than those adopted when viewing printed materials.19,29,38 Practitioners must consider the viewing distance and gaze angle adopted when providing a refractive correction for use when viewing electronic screens. They should ask about the type and number of devices being used. It is not uncommon for an individual to be using both a laptop and desktop computer as well as one or more handheld devices. In addition, the user may need to read printed materials, view multiple screens simultaneously or desire clear distance vision at the same time they are observing the electronic screens. Multiple pairs of glasses may be required and in some cases single vision spectacles may be the only solution to allow clear vision at the particular gaze angle and working distance in use.

Ocular causes of CVS

In considering ocular factors that may lead to CVS, two primary areas have been identified namely: (1) inappropri- ate oculomotor responses and (2) dry eye. It should be noted that non-ocular causes of CVS such as poor design or organization of the workstation, which may also be a significant cause of symptoms such as back, neck, shoulder and wrist pain as well as inappropriate lighting and excess glare are beyond the scope of this paper and will not be discussed here. However, they were reviewed extensively by Sheedy and Shaw-McMinn.31 Furthermore, it seems reasonable to assume that a combination of symptom- inducing factors, such as uncorrected refractive errors and poor illumination could be additive, thereby increasing the magnitude of symptoms.

Oculomotor responses

Viewing any form of near target requires appropriate accommodative and vergence responses to provide clear and single vision of the object of regard. While both of these oculomotor functions have been cited as contribut- ing to the symptoms associated with computer use, there is relatively little objective data detailing how these parameters change during computer work.

Accommodation

Blurred vision, either at near or when looking into the distance after prolonged computer use is a symptom commonly associated with CVS. This could result from an inaccurate accommodative response (AR) during the computer task or a failure to relax the AR fully following the near-vision demands. Patients’ symptoms frequently relate to near-visual activities, and inappropriate responses, whether under or over-accommodation relative

to the object of regard are a common cause of astheno- pia.39 Indeed, amongst a group of symptomatic computer users, accommodative infacility was the most common oculomotor anomaly found.40
However, the evidence for a difference in the AR between computer and hard-copy tasks is not compelling. For example, Wick and Morse41 used an objective infra- red optometer to measure the accommodative response (AR) in five emmetropic subjects when viewing either a VDT or printed copy of the same text displayed on the monitor. They reported that four subjects showed an increased lag of accommodation to the VDT (mean increase = 0.33 D) when compared with the hard copy condition. Later, Penisten et al.42 used dynamic retinos- copy to assess the AR when subjects viewed a printed card, a VDT or a simulated computer display. The exam- iners do not appear to have been masked during this study, i.e. they were aware of the findings for each test condition. Results were presented for two examiners, and the observed differences were relatively small although a significantly reduced lag of accommodation was observed with the simulated computer display when compared with the printed target. For examiner 1, the mean lag of accommodation for the printed card and VDT was 0.63 and 0.72 D, respectively, while for the second observer the mean lags were 0.92 and 0.75 D, respectively. These differences were smaller than the observed levels of inter- and intra-examiner repeatability. Results from our labo- ratory found no significant difference in the AR as a function of symptom score during the course of a 30 min computer task performed at a distance of 50 cm.43 The mean AR for the most and least symptomatic subject groups was 1.04 D (S.D. = ±0.12) and 1.10 D (S.D.
= ±0.14), respectively. The mean ARs during the trial are
shown in Figure 2.
In a recent paper, Tosha et al.44 examined the relation- ship between visual discomfort and the AR. They observed an increased lag of accommodation in subjects reporting higher discomfort, which became manifest with extended viewing (typically after at least 30 s of sustained fixation). This was attributed to accommodative fatigue. However, these differences were apparent for the 4 and 5 D accom- modative stimulus conditions, but were not significant for the 2 or 3 D stimulus levels. Accordingly, for the lower stimulus demands typically found with desktop and laptop computers, these differences may not be relevant. Given the closer viewing distances commonly adopted with hand held smartphone-type instruments as noted earlier, accommo- dative fatigue may become significant, and future studies should examine whether any change in ARs result when viewing these devices for sustained periods of time.
Accommodative facility is a standard clinical test that stimulates rapid changes in the accommodative stimulus.

The accommodative facility test is a standard clinical procedure which produces rapid changes in the accom- modative stimulus. While different results might have been found had more rapid shifts in the accommodative stimulus been created in the laboratory, and more reliable results might have been obtained if these changes in AR were measured objectively, the facility test has the advan- tage of being a simple and inexpensive test that can easily be performed in the clinical environment.
There is little experimental evidence to support the notion that CVS is associated specifically with accommo- dative abnormalities in young healthy patients, since no

 

 

Figure 2. Mean values of accommodative response during the course of a 30 min computer task performed at a viewing distance of 50 cm for the 10 subjects reporting the most and fewest symptoms, respec- tively. No significant difference in responses was observed between these two subgroups. Error bars have been removed for clarity. Data from Collier and Rosenfield.43

significant relationship was found between symptoms, and either the AR or accommodative facility findings. However, it is of interest that several recent investigations have reported an association between contraction of the ciliary muscle and either musculoskeletal symptoms10,1246 or specifically trapezius muscle activity. Accordingly, the

Indeed, Sheedy and Parsons40 reported that in a retro- spective review of clinical records from CVS patients, accommodative infacility, i.e. an inability to complete 20 cycles in 90 s using a ± 1.50 D flipper was the most com- mon diagnosis. One might predict that computer use would produce a decline in the ability to make dynamic oculomotor changes, possibly due to fatigue. In addition, a reduced facility finding could be predictive of subjects with CVS. Accordingly, Rosenfield et al.45 measured mon- ocular and binocular accommodative facility with
±2.00 D flippers before and immediately after a 25 min computer task performed at a viewing distance of 50 cm. No significant change in either monocular or binocular accommodative facility was observed following the task. These results are shown in Table 2. Furthermore, no significant correlation was found between the mean symptom score during the task and any of the pre- or post-task accommodative facility findings. These results are consistent with the findings of Tosha et al.44 who also reported no significant difference in either monocular or binocular accommodative facility in groups of subjects reporting high or low visual discomfort during nearwork.

Table 2. Mean values of monocular (RE and LE) and binocular (BE) accommodative facility (cycles per minute) measured before and immediately after a 25 min computer reading task performed at a viewing distance of 50 cm

 Accommodative facility (RE or OD)Accommodative facility (LE or OS)Accommodative facility (BE or OU)
Pre-task11.00 (0.81)10.54 (0.90)8.25 (0.86)
Post-task11.54 (0.73)10.50 (0.80)9.04 (0.97)
Change0.54 (0.79))0.04 (0.67)1.22 (0.56)
p0.510.950.16

 

discomfort reported by VDT operators in the shoulder- neck region could be related to oculomotor function. For example, Lie and Watten47 noted that altering the accom- modative and vergence demands produced changes in electromyographic responses from muscles in the head, neck and shoulder region. Similarly, Richter et al.46 used plus and minus lenses and changes in target position to vary the accommodative stimulus, and observed that an increase in the accommodative response was coupled with a positive shift in trapezius muscle activity in a dose- response manner. Further, in an investigation of symp- toms in 1183 call-centre operators, Wiholm et al.10 found a significant positive association between eyestrain and neck-shoulder symptoms. The authors conjectured that either these complaints are physiologically inter-related, or alternatively, the visual demands of the workstation may result in a change in posture leading to musculoskel- etal difficulties. Alternatively, oculomotor fatigue may lead to a secondary change in innervation to the postural muscles in the neck, shoulder and upper back, resulting in discomfort in these areas. However, these findings do not explain why differences in symptoms are reported when viewing materials on either electronic screens or printed hardcopy.
An alternative hypothesis was proposed by Wilkins and co-workers,48,49 who suggested that visual discomfort could result from certain patterns of striped lines giving rise to symptoms of eyestrain and headaches. They also proposed that ‘visual hypersensitivity’ could be amelio- rated by the use of coloured lenses and/or overlays. How- ever, the mechanism whereby these coloured filters could reduce symptoms is unclear. Both Ciuffreda et al.50 and Simmers et al.51 reported that the filters did not produce a significant change in the accommodative stimulus-

Figures in parentheses indicate 1 S.E.M.

response function (although Simmers et al.51 observed a

reduction in low frequency accommodative microfluctu- ations with the tinted lenses). Nevertheless, an increase in reading performance has been reported with the coloured filters,52 and changes in the colour of the text and/or back- ground of the computer display may reduce symptoms of CVS. The latter is worthy of further investigation.
In addition, patients with accommodative anomalies (especially accommodative insufficiency and infacility53) would be expected to exhibit similar symptoms to those experienced when viewing hard copy materials, and prac- titioners examining patients presenting with CVS should perform a full assessment of the accommodative system. The clinical parameters which should be examined are listed in the later section on treatment.

Vergence

While few studies have examined the vergence response during the course of VDT work, several investigators have measured vergence parameters before and after periods of computer usage. For example, Watten et al.54 measured positive and negative relative vergence (or vergence ranges)55 at near both at the beginning and end of an 8-h workday. They observed significant decreases in both parameters, implying that computer use decreased one’s ability to converge and diverge appropriately. In contrast, Nyman et al.56 found no significant change in positive or negative relative vergence at near after 5 h of VDT work. They also reported no significant change in either dis- tance and near heterophoria or the near point of conver- gence (NPC) following the work period. Similarly, Yeow and Taylor57 also observed no significant change in NPC after short term VDT use (up to 2.35 h of continuous use or an average of 4 h intermittent use in a normal working situation). In a subsequent longitudinal study, Yeow and Taylor58 monitored NPC, near horizontal heterophoria and associated phoria (AP), i.e. the prism to eliminate fixation disparity, over a 2-year period in both VDT and non-VDT workers in the same office environment. While both the VDT and control groups exhibited a decline in NPC with age, no significant difference was observed between these groups. Similarly, no significant change in either near heterophoria or AP was found.
Jaschinski-Kruza59 measured both accommodation and fixation disparity during the course of a 30 min computer task at viewing distances ranging from 25 to 85 cm. No significant change in either of these parameters was observed over time. However, no assessment of visual symptoms was made during the task, and he noted that ‘subtle oculomotor effects’ could contribute to difficulties in performance or visual fatigue in the workplace. Subse- quently, Jaschinski60 used fixation disparity as a measure- ment of near vision fatigue following work at a computer

workstation. Near vision fatigue was associated with greater exo (or less eso) fixation disparity as the target was brought closer to the observer. In order to examine the within-task vergence response and its relationship to CVS symptoms, Collier and Rosenfield43 measured AP during a period of sustained VDT fixation. The mean AP for the subjects who reported the least and greatest
discomfort during the task was 1.55D exo and ortho, respectively (p = 0.02). These findings are illustrated in Figure 3. CVS was significantly worse in subjects exhibit- ing zero fixation disparity when compared with those
subjects having exo AP.
The increased vergence response in those subjects who converged accurately on the monitor (as shown by zero AP) may be responsible for the greater symptoms when compared with those individuals who had a lower symp- tom score and small amounts of exo AP. The notion that having exo fixation disparity at near may be more com- fortable than accurate vergence differs from earlier work indicating a positive relationship between AP and symp- toms.39,61–63 It should be noted that the range of exo AP found in the low symptom group was relatively small (mean = 1.55D; range = 0.78–2.33D). Interestingly, the Optometric Extension Program (OEP) system of case analysis regards exophoria at near as desirable, since it provides a ‘buffer’ to overconvergence.64 The minimum vergence response necessary to place the retinal images within Panum’s fusional area (thereby allowing binocular single vision) may provide a more comfortable oculo- motor posture than precise ocular alignment.

CVS and dry eye

Dry eye has previously been cited as a major contributor to CVS. For example, Uchino et al.65 observed symptoms

Figure 3. Mean values of associated phoria in prism dioptres (PD) during the course of the 30 min computer task performed at a view- ing distance of 50 cm for the 10 subjects reporting the most and few- est symptoms, respectively. A significant difference in vergence response between the two groups was observed. Error bars indicate 1 S.E.M. Data from Collier and Rosenfield.43

of dry eye in 10.1% of male and 21.5% of female Japanese office workers using VDTs. Furthermore, longer periods of computer work were also associated with a higher prevalence of dry eye.3 In an extensive review, Blehm et al.66 noted that computer users often report eye dryness, burning and grittiness after an extended period of work. They suggested that these ocular surface related symptoms may result from one or more of the following factors:
(1) Environmental factors producing corneal drying. These could include low ambient humidity, high forced-air heating or air conditioning settings or the
use of ventilation fans, excess static electricity or air- borne contaminants.
(2) Reduced blink rate. Several investigations have shown that blink rate is reduced during computer operation. For example, Tsubota and Nakamori67 compared the
rate of blinking in 104 office workers either when they were relaxed, reading a book or viewing text on a VDT. Mean blink rates were 22 per min while relaxed, but only 10 and 7 per min when viewing the book or VDT, respectively. Additionally, Patel et al.68 observed mean blink rates prior to and during VDT operation of 18.4 and 3.6 per min, respectively. In addition, they noted a significant relationship between the stability of the precorneal tear film and the interval between blinks. Schlote et al.69 found that the reduced blink rate associated with VDT use was also accompanied by distinct patterns of blinking. For example, some patients (all of whom had symptoms of dry eye) exhibited alternating inter-blink periods of longer and shorter duration. These authors hypothesized that the change in inter-blink duration during the course of VDT operation represented cog- nitive adaptation to the computer task. Further, no significant correlation was observed between clinical measurements of the ocular tear film (tear breakup time, Schirmer I or Jones tests) and the observed blink rate during computer operation.
It has also been reported that blink rate decreases as
font size and contrast are reduced,70 or the cognitive demand of the task increases.71,72 Additionally, Sheedy et al.73 noted that voluntary eyelid squinting reduced the blink rate significantly. Therefore, the poorer image qual- ity of the electronic text (as evidenced by increased reports of blurred vision during the course of a computer task when compared with printed materials20) may adversely affect the blink rate. Interestingly, the applica- tion of topical elastoviscous solutions to the cornea does not modify the reduced blink rate associated with VDT use.74 Reduced blinking may also exaggerate the symp- toms of pre-existing dry eye, which could be exacerbated by other aspects of the work environment as noted above,

as well as factors such as contact lens wear and increasing age (particularly in females).
(3) Incomplete blinking. While blink rate has been shown
to decrease significantly with computer use,68,69,74 an additional factor to consider is the completeness of the blink, i.e. does the upper lid cover the exposed cornea completely during the blink process. Himeb- augh et al.71 analyzed the blink amplitude during a number of tasks including computer operation and observed that incomplete blinking was common, task dependent and present in all subjects. These included both individuals with symptoms of dry eye and aged- matched normals. It is unclear whether incomplete blinking is undesirable. Harrison et al.75 noted that partial blinking is associated with staining of (and presumably damage to) the inferior cornea.76,77 Yet incomplete blinking is commonly found in asymp- tomatic patients77 and provided that portion of the cornea covering the pupil is covered by the upper eyelid, one would expect to find uninterrupted clear vision. A recent study by Portello et al.78 examined both the completeness of the blink during computer operation and post-task symptoms. A significant positive correlation was observed between the per- centage of blinks deemed incomplete and the total symptom score. This is illustrated in Figure 4. These findings suggest that incomplete blinking leading to ocular dryness may be a significant cause of CVS. Additionally, Chu et al.79 noted both a higher preva- lence of incomplete blinking and higher symptom score in subjects following a reading task performed on a VDT, when compared with subjects undertaking the same task from hard copy material. These find- ings also imply that incomplete blinking may be a partial cause of CVS symptoms. Interestingly, Harri- son et al.75 observed that partial blinking may be advantageous since it does not interrupt concentra- tion on a visual task as much as complete blinks. This is consistent with the findings of Portello and Rosenfield80 who reported that increased conscious blinking during computer operation interfered with the subjects’ ability to perform the task satisfactorily.

Figure 4. Total symptom score plotted as a function of the percent- age of blinks that were deemed incomplete during the course of a 15 min computer task performed at a viewing distance of 50 cm in 21 subjects. A significant positive correlation was observed (r = 0.63; p = 0.002). Even if the two outlying subjects with more than 50% of their blinks being incomplete are removed from the analysis, the cor- relation is still statistically significant (r = 0.63; p = 0.004). Data from Portello et al.78

(4) Increased corneal exposure. Desktop computers are commonly used with the eyes in the primary posi- tion, whereas hardcopy text is more commonly read with the eyes depressed. The increased corneal expo-
sure associated with the higher gaze angle could also result in an increased rate of tear evaporation. It should also be noted that laptop computers are more typically used in downward gaze while smartphone type devices can be held in primary or downward gaze. Variations in the angle of gaze may also alter either the accommodative and/or vergence response,81–83 and therefore the level of symptoms experienced.
(5) Age and gender. The prevalence of dry eye increases with age and is higher in women than men.13,84–90 The estimated prevalence of dry eye in women and men over 50 years of age in the USA is 7.8% and
4.3%, respectively.89,90
(6) Systemic diseases and medications. While a review of this topic is beyond the scope of this paper, Moss et al.91,92 reported that the incidence of dry eye was greater in subjects with arthritis, allergy or thyroid
disease not treated with hormones. Additionally, the incidence was higher in individuals taking antihista- mines, anti-anxiety medications, antidepressants, oral steroids or vitamins, as well as those with poorer self- rated health. A lower incidence of dry eye was found with higher alcohol consumption levels.
(7) Contact lens wear. The presence of a contact lens on the anterior surface of the cornea has been shown to
alter the blink rate significantly. This may result from irritation by the lens or a more unstable tear film.69 York et al.93 examined the effect of contact lenses on blink rate during conditions of varying levels of difficulty. Subjects were required to view either an audio-visual film strip, read graded material or read material while determining how many times the letter ‘a’ appeared in the text. The authors observed that while the mean blink rate decreased with increasing task difficulty for all conditions, wearing contact lenses increased the blink rate. However, the subjects in this study were all new contact lens wearers, and the authors speculated that over time, increasing adaptation to the lenses could lessen the effect of contact lenses on blink rate. Indeed, an investigation

by Pointer94 on new hydrophilic contact lens wearers observed that over a 1 month lens adaptation period, task difficulty became the predominant stimulus for blink rate.
A recent report by Jansen et al.72 examined the effect
of task difficulty in adapted contact lens wearers. In com- paring the inter-blink interval when either listening to music or playing a video game, a significantly longer inter-blink interval was noted for the video game when contact lenses were not worn. However, when subjects wore their habitual contact lenses, no significant differ- ence in blink rate was observed as a function of task diffi- culty. These results suggest that soft contact lenses, even in a fully adapted wearer provide sufficient ocular surface or lid stimulation to increase the rate of blinking. Based on these results, one might speculate that if CVS is pro- duced by a decreased blink rate, symptoms should be less severe in adapted contact lens wearers. However, this pro- posal contradicts the finding that contact lens wearers are
12 times more likely than emmetropes and five times more likely than spectacle wearers to report dry eye symptoms.95 For a much fuller discussion on the topic of dry eye and contact lens wear, see the report of the defi- nition and classification subcommittee of the Interna- tional Dry eye workshop.96
As noted previously, the presence of relatively small amounts of uncorrected astigmatism (<1.0 D) may pro- duce a significant increase in symptoms of CVS.26,27 Common clinical practice is to provide patients seeking contact lenses who have astigmatism of this magnitude with spherical lenses. Accordingly, increased symptoms might occur in these individuals, not as a result of the contact lens inducing or enhancing problems associated with dry eye, but rather as a result of the uncorrected refractive error. The use of toric contact lenses or a spec- tacle overcorrection to eliminate the uncorrected astigma- tism may be appropriate here.
(8) Ocular conditions. An extensive review of dry eye dis- ease96 noted that this condition could either be caused by decreased lacrimal tear secretion or exces- sive evaporation. Either of these causes could lead to
symptoms of CVS. Decreased secretion could be due to Sjogren’s syndrome, an autoimmune condition which affects both the lacrimal and salivary glands.97 Alternatively, reduced tear output could result from either primary or secondary deficiencies or obstruc- tion of the lacrimal glands, reflex hyposecretion resulting from reduced sensory input from the tri- geminal nerve or damage to the facial nerve. Evapora- tive dry eye could be extrinsic, resulting from Meibomian gland dysfunction, an increase in exposed ocular surface area or a low blink rate or extrinsic, being due to ocular surface disorders (including

vitamin A deficiency) or diseases including allergic conjunctivitis.

Potential treatments for CVS
Potential therapeutic interventions for patients with symptoms of CVS can be divided into three main areas namely:
(1) Refractive and accommodative disorders
(2) Vergence anomalies
(3) Dry eye

Refractive and accommodative anomalies
As noted earlier, the presence of uncorrected ametropia may lead to an increase in symptoms. Given that indi- viduals may spend many hours (often continuously) viewing electronic screens, it is important that they are able to maintain a clear image of the target over time. There is little evidence to support the proposal that the accommodative demands of the VDT differ from view- ing printed materials at the same distance and gaze angle. However, the presence of any refractive or accom- modative anomaly (e.g. accommodative infacility or insufficiency53) could impact upon the patient’s level of visual comfort during the task. In examining patients with CVS, the following clinical parameters should be assessed [with all near testing being performed at the distance(s) at which the electronic screen(s) are posi- tioned]:
(1) Best corrected visual acuity
(2) Refractive error (including binocular balancing)
(3) Accommodative error (lag) at the appropriate work- ing distance
(4) Monocular and binocular amplitude of accommoda- tion
(5) Monocular and binocular accommodative facility
(6) Negative and positive relative accommodation Patients with accommodative anomalies may benefit
from measures to improve the accuracy and dynamics of their accommodative response, including vision therapy and/or the provision of lenses to provide a clear image of the target at the required viewing distance and gaze angle. If adjustments can be made to optimize the design of the workstation, these should also be discussed with the patient.
In addition, patients should be advised regarding their working times. Fixation on any near object for a sus- tained period of time, whether a computer screen or printed material may lead to asthenopia. Indeed, Henning et al.98 compared computer workers typing performance at baseline, when they were allowed three 30 s breaks plus a 3 min break each hour, and a rest break plus exercise

condition where stretching exercises were introduced dur- ing the breaks. A 5% and 15% improvement in produc- tivity was observed for the breaks and breaks plus exercise conditions, respectively. Accordingly, it seems reasonable that any patient should be advised to take breaks and to look into the distance periodically in order to reduce the accommodation and vergence responses.

Vergence anomalies

It has been demonstrated that subjects reading text on a computer were most symptomatic when they converged accurately on the screen, i.e. having ortho associated phoria (AP), when compared with individuals having exo AP who were less symptomatic (see Figure 3).43 Given this finding, one might conjecture whether intentional correction of a subject’s AP to an exo posture would reduce asthenopic symptoms after a computer task. Accordingly, our labora- tory compared post-task symptoms immediately following a continuous 20 min reading task from a desktop com- puter monitor at a viewing distance of 50 cm. Each subject (n = 40) attended for two trials. In the first session, sub- jects wore the amount of prism corresponding to their AP at the computer test distance over their refractive correc- tion. In the second trial, subjects were given 3D more base- out prism than their measured AP in order to induce an AP of 3D exo. No significant difference in total ocular symptom score between the two groups was observed fol- lowing these two conditions. However, five individuals showed a significant preference for the ortho condition, whereas nine subjects showed a marked preference for the 3D exo condition. Further analysis indicated differences in binocular accommodative parameters between these two subgroups. The group whose symptoms were reduced with exo AP had significantly lower negative relative accommo- dation (NRA) and a higher accommodative response (i.e. smaller lag) when compared with the ortho AP preferred group. No significant difference in either distance or near heterophoria was observed between the two groups. Thus a subgroup of patients may exist whose symptoms of CVS can be alleviated by creating exo AP. This proposal should be examined further in a larger population.
Any vergence anomaly which would cause difficulty with maintaining clear and single vision of printed text at near (e.g. uncompensated heterophoria, convergence excess or insufficiency or vergence infacility) is likely to give rise to symptoms during sustained viewing of an electronic screen at near. Accordingly, when examining patients with CVS, the following clinical vergence param- eters should be assessed [with all near testing being per- formed at the distance(s) at which the electronic screen(s) are positioned]:
(1) Near point of convergence

(2) Near heterophoria
(3) Horizontal and vertical fixation disparity and/or asso- ciated phoria.
(4) Vergence facility
(5) Vergence ranges (negative and positive relative vergence)
(6) Stereopsis
(7) AC/A and CA/C ratios.

Dry eye

As noted previously, the presence of dry eye may play a sig- nificant role in the aetiology of CVS. Computer use has been associated with both a reduced rate of blinking and a high number of incomplete blinks when compared with viewing hard copy materials. Additionally, the environ- ments where computers are located often have low ambient humidity and forced-air heating or air conditioning which may exacerbate symptoms of dry eye. Accordingly, practi- tioners should consider both patient education and a range of therapies available to attenuate this condition.
Dry eye therapies which have been proposed to mini- mize symptoms of CVS include the use of lubricating drops, ointments and topical medications for blepharitis or allergic conditions. Additionally, blink training to increase the blink rate during computer use,99 as well as changes in ambient humidity, hydration (drinking more water) and redirection of heating and air conditioning vents have all been proposed.
The benefit of many of these therapies for minimizing CVS symptoms is unproven. For example, Acosta et al.74 observed that topical instillation of an elastoviscous solu- tion did not produce a significant change in the com- puter-induced reduction in blink rate. Mean blink rates for the computer condition with and without instillation of the elastoviscous solution were 6.4 and 6.1 blinks min)1, respectively. Accordingly, it is uncertain whether the use of lubricating or rewetting drops will indeed reduced CVS symptoms. With regard to increasing the blink rate, Portello and Rosenfield80 compared post-task symptoms when subjects were either allowed to blink voluntarily or when the blink rate was consciously increased using a metronome during computer use. Although a significantly increased blink rate was recorded in the metronome condition (23.5 vs 11.3 blinks min)1 in the control session), no significant difference in post- task CVS symptoms was found either in the entire popu- lation tested (n = 23) or in those subjects reporting the highest symptom scores in the control condition. Fur- thermore, several subjects stated that increased conscious blinking interfered with their ability to perform the task satisfactorily, which may limit the practicality of this advice.

Furthermore, Acosta et al.74 noted that blowing an air stream onto the face while subjects were playing a com- puter game did not produce a significant change in blink rate. Accordingly, to date there appears to be little experi- mental evidence to support many of the therapeutic inter- ventions that have been proposed. Further work is required to determine what aspects of dry eye treatments will indeed reduce CVS symptoms.

Conclusions

As noted above, the use of electronic devices to view small type for many hours, frequently at close working distances, has become commonplace in modern society in patients of all ages. Many individuals use multiple devices such as a desktop and laptop computer as well as one or more hand- held devices. These present a variety of visual demands that are significantly different from those of printed materials in terms of working distances, gaze angles and text sizes. It is no longer reasonable to assume that a patient will read text at a viewing distance of approximately 40 cm with their eyes depressed. Accordingly, a significant change in both optometric testing methods and the design of ophthalmic lenses (particularly for the correction of presbyopia) will probably be required.
Given that the prevalence of symptoms (including eye- strain, headaches, ocular discomfort, dry eye, diplopia and blurred vision) may be as high as 90%, it is likely that an increasing number of patients will present for eye examina- tions due to symptoms associated with CVS. Practitioners need to consider what are appropriate examination proce- dures and treatment regimens for these individuals. Near testing at a single distance and gaze angle such as is com- monly employed when a nearpoint card is positioned in the primary position at a viewing distance of 40 cm is not adequate. The assessment of oculomotor functions at mul- tiple viewing distances and gaze angles may be required. One should note that this cannot be achieved when the patient is viewing through a phoropter, and nearpoint test- ing in free space, with the patient wearing a correction mounted in a trial frame is required.
In addition, prescribing routines may need to be recon- sidered. For example, small refractive errors (such as astig- matism between 0.50 and 1.00 D), which might have been left uncorrected in the past (particularly in contact lens wearers), should be corrected in a patient who is viewing an electronic screen for an extensive period of time. Similarly, instances of low to moderate oculomotor anomalies or cases of dry eye, which might previously have been left uncor- rected may be of sufficient magnitude to cause significant symptoms when combined with prolonged computer use.
It is worth noting that the symptoms of CVS associated with accommodation and vergence disorders do seem, in

most cases, to be a result of viewing a visually demanding near target for an extended period of time and not spe- cific to the electronic monitor. In contrast, symptoms of dry eye do appear to be directly related to computer use due to the position of the monitor (producing increased corneal exposure), reduced blink rate, increased partial blinking and other environmental factors. Further research is required to determine the efficacy of dry eye treatments in reducing symptoms of CVS.
Given the remarkably high number of hours per day that many (or perhaps most) individuals now spend viewing small text on electronic screens at close working distances and varying gaze angles, it is incumbent upon all eye care practitioners to have a good understanding of the symptoms associated with, and the physiology underlying CVS. As modern society continues to move towards greater use of electronic devices for both work and leisure activities, it seems likely that the visual demands that these place upon our patients will only continue to increase. An inability to satisfy these visual requirements could present significant lifestyle difficulties for patients.

Acknowledgement

I would like to acknowledge the assistance of the follow- ing colleagues in this research: Yuliya Bababekova, Jaclyn Benzoni, Christina Chu, Juanita Collier, Rae Huang, Jen- nifer Hue, Marc Lay, Joan Portello, Daniel Recko, Eliza- beth Wickware and Abraham Zuckerbrod.

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18/Oct/2019

Computer Vision Syndrome

The computer has become a part of the everyday life at present. Even in Sri Lanka a considerable percentage of the work force uses computer for their job. In the world it has been estimated that nearly 60 million people experience vision problems as a result of computer use. This computer related ocular condition is called Computer Vision Syndrome (CVS). Millions of new cases occur each year.

Some decades back, before the advent of computers, the office work involved a range of activities, including typing, filing, reading and writing etc. All these activities are different from each other and needed different types of posture and vision, causing a natural break from each activity. With the computer all these activities were combined and needed no change of posture or vision of the user from his desktop. It certainly improved the quality of the work and efficiency but caused ocular problems, such as dry eye, redness, irritation, eye strain, tired eyes, temporary blurred vision, light sensitivity and muscular problems that stem from using a computer. All these symptoms collectively referred to as computer vision syndrome, which comprised of ocular surface abnormalities or accommodative spasms and/or extra-ocular (ergonomic) aetiologies due to improper posture such as neck and upper back pain and headache.

The major contributors to CVS is thought to be the dry eye, the visual effects of video display terminals (VDT) such as lighting, glare, display quality, refresh rates and radiation and positioning of computer monitors.

Visual mechanisms on VDT screen

The focusing systems of human eyes are not meant for electronically generated characters on the VDT. It responds perfectly to the images that have well defined edges with good background contrast. (eg: solid black letters on white background). VDT letters are made up of small dots or pixels. Each pixel is bright at its center and with decreasing brightness towards its outer edge.

The human eyes find it very difficult to focus on the pixel characters. The eyes focus on the plane of the computer screen but cannot sustain that focus. Then it will relax on to a focus behind the screen. This point is called the resting point of accommodation (RPA) or dark focus. RPA is different from person to person but it is somewhat away than the normal working distance to the computer. Therefore the eyes are constantly relaxing to RPA and straining to refocus on to the screen constantly.

The constant changing of focusing by the ciliary body creates fatigue to the eye and causes accommodative symptoms pertaining to CVS.

Studies on CVS

By the year 1992, a total 1307 surveys were completed by American optometrists and reported that the majority of VDT patients have symptoms that are different from other near point workers in relation to glare, lighting viewing conditions and spectacle requirements. Travers and Stanton identified a trend in symptomatology. They reported that the symptoms appeared to increase as the duration of VDT exposure increases. It has been estimated in the USA that the diagnosis and treatment of CVS costs about $ 2 billion each year

Computer vision syndrome and children

Children have more access to computers either at home or at schools now. Parents also encourage children at very early years to use the computers. It is believed that heavy computer use among children put them at the risk of getting early myopia. Several studies have shown it.

  • 25% – 30% of computer using children need corrective glasses – A study at the University of California at Bekerly School of
  • The percentage of patients who work with myopia has increased from 12.1% to 20.4% since 1995 according to a study by the department of health in

According to American Optometric Association the impact of computer use on children is vision involves three factors.

  1. Children have a limited degree of self- They perform a task on computer for hours and hours without breaks. This prolonged activity causes CVS.
  2. Children are very They assume that what they see and how they see is normal, even if the vision is problematic. Therefore parents need to monitor their work.
  3. As children are smaller than adults the computer work station set up arranged for adult usage does not suit the requirement of children. Computer users should view the screen slightly downwards at a 15°

Symptomatology

As described earlier symptoms can be categorized as,

  1. Accommodative or asthenopic symptoms
  2. Ocular surface related symptoms
  3. Extra ocular symptoms

The table shows relevant symptoms and diagnosis

Symptom category SysmptomsDiagnosis
AsthenopicEye strain Tired eyes Sore eyes Dry eyesBinocular vision Accommodation
Ocular surface relatedWatery eyes Irritation
Contact lens problems
Visual problemsBlurred vision
Poor focusing change Double vision Presbyopia
Refractive error Accommodation Binocular vision
Extra ocularNeck pain
Back pain Shoulder pain
Presbyopic correction Computer location

Some patients have marginal vision disorders such as difficulties in accommodation or binocular visual problem that do not cause problems when performing less demanding visual tasks. Prolonged VDT usage causes diminished power of accommodation and removal of the near point of convergence causing deviation of the eye for near vision. These symptoms are most likely transient with workers returning to their baseline accommodative values by the end of the day or week. Too much of accommodative efforts may be a causative factor in the development of myopia. Luberto et al observed a transient myopia in 20% of VDT workers at the end of their work shift. It is still questionable whether VDTs are associated with a risk of myopia progression in adults compared to paperwork.

Dry eye is thought to be the primary cause of ocular fatigue. When working with a VDT the blink rate is decreased and the exposed ocular surface area is increased causing desiccation of

the eyes. It is thought that the blink rate is further decreased in dark settings where it is difficult to read. The factors that involved in drying of the ocular surface are;

  1. Environmental factors – dry air ventilation fans, static build up, dusty environment, photocopy toner etc.
  2. Reduced blink Normally people blink 10– 15 times per minute. Studies have shown that the rate is significantly diminished when working at a computer.
  1. Increased exposure. In normal reading the eyes look downwards causing the lids to cover the part of the cornea minimizing the evaporation of On the contrary the computer operators view it in a horizontal gaze causing wider opening of the palpebral fissure that lead to increased evaporation through exposed area.
  2. Sex – It is found that the prevalence of dry eye is slightly higher among
  3. Age – Tear production normally decreases with age. Post-menopausal women are the group of individuals who are mostly
  4. Systemic diseases and medications – Dry eye is associated with some systemic diseases. (eg: Sjogren syndrome, rheumatoid arthritis, and several auto immune ) Treatment diuretics, antihistamines psychotropic and some antihypertensives are associated with dry eyes, and patient taking such treatment are more prone to get CVS.
  5. Contact lens use – Office workers wearing contact lenses are more likely to suffer due to Contact lens comfort is highly dependent on lubrication of the eye.
  6. Other ocular pathology – Dysfunctions of the lid glands as in blepharitis affecting Meibomian glands causes lack of adequate lipid layer in tears that causes more

Visual effects due to computer screen display

Visual effects are due to number of display characteristics such as character size structure, style and the image contrast and stability.

Display quality

As discussed earlier images are produces mainly by pixels and raster (horizontal lines). The images formed by them lack sharp edges. Slightly blurred characters create an under stimulation of accommodation and causes lag of accommodation behind the screen. Resolutions of the monitors are associated with visual fatigue. Resolution of monitors has improved drastically over the past decade.

Lighting and glare

Bad lighting conditions of the surrounding area of the computer can adversely affect the eyes of the user. Bright illumination from surrounding (head fluorescents, large windows, desk lamps) can wash out screen character images. It creates a glare and reflection. It causes annoyance and visual fatigue. It has been shown that surrounding luminance significantly reduces accommodation amplitude. Glare causes delay in reading time and when it is not possible to change the surrounding lighting system anti-glare filters are used to reduce it. But some studies (a study with 25, 064 participants) investigated for incidence of asthenopia did not show that the filters reduce the occurrence of asthenopia.

Refresh rates

Refresh rate is the number of times per minute the screens repainted to produce an image (measured in Hz). If the rate is too slow characters will start to flicker. It causes annoyance fatigue and headache. The critical fusion frequency (CFF) is the refresh rate that human eyes do not distinguish the pulsating beams flicker as separate entities. It is 30-50 Hz. Therefore it is recommended to have 75Hz that removes flicker at all brightness levels. Studies have shown that much higher refresh rates decreases ocular symptoms and improves the reading rate too. Liquid crystal displays (LCD) have very high refresh rates compared to cathode ray tube (CRT). LCD is an advance in screen technology that minimizes ocular discomfort.

Radiation

It was earlier thought that the radiation emission from VDT causes health effects to the user. Ionizing radiation is known to cause human cellular changes and break down the chemical bonding and change the neutral molecules. But VDTs do not produce or emit alpha, beta or gamma rays or hard x-radiation. Small amounts of soft x-rays are produced. But it is also contained by the monitors’ glass screen. Studies have shown that there is no evidence to support that VDT operators face hazards like skin cancers, spontaneous abortions and ocular abnormalities due to ionizing radiation. Research should be done to define the risk of electromagnetic radiation produced by VDT.

 

Prevention and treatment of CVS

It is multidirectional because different people have different complaints. Treatment needs ocular therapy as well as workplace adjustments.

Lighting

Light should not be too bright and set in a way that it does not throw bright light in to the eyes or on to the computer screen producing glare Excessive fluorescent lighting should be reduced and window lighting should be filtered with curtains or blinds or tinting. Anti-glare filters may not reduce the symptoms of asthenopia but provide visual comfort.

 

VDT positioning

Postural difficulties at the VDT lead to pain in the back neck and shoulders. It is important to keep the monitor at a proper distance and height. Studies have shown that improvement of physical ergonomics reduces discomfort at computer stations and improves performance.

It is recommended that the eye should be 35-40 inches away from the monitor. (Earlier it was thought to be 16-30 inches but it has been proved now that at shorter distances people get more eyestrain).

It is also recommended that the screen should be placed 10-20 degrees below the eye level. When the screen is higher than this, the user turns the head back and causes muscle strain on the trapezius and neck muscles. When the monitor is lower, the gaze is downwards and exposes less ocular surface reducing tear filters evaporation. Studies have shown that raised monitor has no beneficial effect on postural stress of the cervical spine.

Work breaks

When regular breaks are given work efficiency improves. This has been proved by studies. Short frequent breaks are recommended. A quick walk at the break (around the office) will give a stretching to the fatigued muscles. It provides the change of focus of the eye and relaxation. It is believed that looking away at a distance at least twice an hour is sufficient for prevention of visual fatigue.

Lubricating eye drops

Symptoms caused by dry eyes can be relieved by lubricating drops. Over-the-counter tear substitutes are available at pharmacies. It can be used periodically to rewet the ocular surface. It helps in maintaining the balance of salts and acidity too, while working with the computer. It is important to find the proper lubricating drop for the computer user. Higher viscosity of lubricants reduces the visual acuity.

Computer spectacles

An occasional computer user may be able to use his own spectacle and use the computer without much problem. But those who spend more than 1 – 2 hours per day can benefit from computer

glasses. Progressive lenses are thought to be better but still the users will find a “perfect-spot” with the lens and use only that spot for viewing the monitor. This also resulting annoyance and head/neck strain. Occupational progressive lenses are now designed to have a large area on the top half of the lens for mid distance viewing (arms length viewing) and a bottom half for near viewing (for the key board). Presbyopes need special attention when deciding on the right glass for the computer. A micro environment glass (MEGS) is presently under study which will address the ocular surface pathology also. Removal of accumulation of airborne particles and irritants are expected to include in this design.

– Saman Wimalasundera

Senior Lecturer in Community Medicine and Ophthalmologist, Community Ophthalmology Centre, Faculty of Medicine, University of Ruhuna, Galle

References

  1. Abelson How to fight computer vision syndrome.Reviews of Ophthalmology1999; 114-6.
  2. Sheedy JE. Presbyopia and computer users. Refract Eye Care Ophthalmology 1999; 3: 5-9.
  3. Berqvist UO, Knave Eye discomfort and work with visual display terminals Scandinavian Journal of Work Environment Health 1994; 20: 27-33.
  4. Carter JB. Banister Musculo skeletal problems in VDT work: a review. Ergonomics 1994; 37: 1623-48.
  5. Cheu Good vision at work. Occular Health Safety 1998; 67: 20-4.
  1. Campbell FW, Durden K. The visual display terminal issues, a consideration of its physiological, psychological and clinical background, Ophthalmic Physiology 1994; 3: 1623-
  2. Berm M. et al An occupational study of employees with VDT associated symptoms – The importance of stress Medicine 1996; 12: 51-4.
  3. Bachman Computer specific spectacle lens design preference of presbyopic operators. Journal of Occupational Medicine 1992; 34: 1023-7.
  4. Berman SM Greenhouse DS Bailey IL, et al. Human electroretinogram responses to video displays, Fluorescents lighting and other high frequency Optometry Visual Science 1991; 68: 645-62.
  5. Acosta MC, Galler J, Belmoute C. The influence of eye solutions on blinking and ocular comfort at rest and during work at video display Experimental Eye Research 1999; 68: 663-9.
  6. Bauer W, Witting Influence of screen and copy holder positions on head posture, muscle activity and user judgment. Applied Ergonomics 1998; 29: 185-92.

18/Oct/2019

Therapy for Amblyopia: A newer perspective

Amblyopia is a reduction of best‑corrected visual acuity that cannot be contributed to the structural abnormality of the eye. The prevalence of amblyopia is between 2% and 5%.[1] Amblyopia is associated most commonly with early childhood strabismus and anisometropia and less commonly with metropia and vision deprivation such as congenital cataract.

The conventional treatments for amblyopia include refractive correction, occlusion, and atropine penalization. Optimal refractive correction alone can resolve in  at least one‑third of cases with untreated anisometropic amblyopia and even some untreated strabismic amblyopia.[2] If amblyopia is not resolved, occlusion or pharmacological penalization with atropine on the better eye is often prescribed simultaneously or soon after refractive correction is provided.

Even with spectacle correction plus occlusion or atropine penalization, there are still one‑third of amblyopia have poor response to treatment. Eyes with poor initial visual acuity, the presence of significant astigmatism, and age of over 6 years are risk factors of treatment failure.[3] Compliance with amblyopia treatments has a major effect on response to therapy.

Compliance of conventional amblyopic treatment is generally low. Discomfort from eye patches, difficulties with vision from occluding the better eye, psychological distress, uncomfortable effect of bright sunlight to the atropine‑treated eye, and ocular sensitivity to atropine are the causes of poor compliance.[4]

Reduced Connectivity between Brain Areas

Recent studies have shown that the physiological basis of amblyopia is mainly located at visual cortex and lateral geniculate nucleus. Functional agnetic resonance imaging study in human amblyopia suggests that V1 may be the earliest anatomic site in the visual pathway.[5,6] Optimized voxel‑based morphometry indicates that human amblyopes have reduced gray matter volume in visual cortical region.[7] Amblyopic deficit not only involves circumscribed visual areas as visual cortex and lateral geniculate nucleus but also reduced the effective connectivity in different visual areas.[8] The effective connectivity loss was found correlated with the degree of amblyopia. Feedforward and feedback connectivities are similarly affected.

Plasticity of Visual System

The effect of treatment for amblyopia usually decreases after critical period which is thought to be 6 years of age and is thought attributing to decreased brain plasticity.

The plasticity of visual system is greatest in early infancy. It is triggered by maturation of inhibitory, gamma‑aminobutyric acid (GABA)‑producing interneurons.[9] In amblyopes, there is suppression from the better‑seeing eye over the amblyopic eye.[10] GABA is thought to play a key role in suppression of inputs from the amblyopic eye within the visual cortex.

The Role of Suppression in Amblyopia

Early concept think that suppression simply follows amblyopia. Treatments of amblyopia focus on occlusion or penalization of the better eye because the input from the amblyopic eye is weaker. The treatment does not concern suppression. The current concept thinks that suppression plays the causal role in amblyopia. Disruption of binocular function causes suppression leading to amblyopia.

Using dichoptic motion coherence threshold technique,[11] quantitative measurement of interocular suppression is assessed in strabismic and anisometropic amblyopia. It is found that deeper suppression is associated with poorer vision in the amblyopic eye.

Due to limited success of conventional treatment for amblyopia and the new concept of brain plasticity, a variety of treatment strategies were investigated. These include dichoptic treatments and pharmacological therapy.

Dichoptic Treatment

Suppressive interactions within the visual cortex are a viable target for amblyopia treatment. Theory of dichoptic training bases on the concept that the binocular circuitry from the weak amblyopic eye is actively suppressed by the strong fellow eye.[12] Dichoptic training tasks reduce fellow eye contrast to rebalance the contrast between the eyes. Dichoptic treatment of amblyopia promotes binocular vision and reduces inhibitory interactions within the visual cortex. Reduce suppression within the visual cortex was found enhancing improvements in binocular visual function in adult amblyopes. Repeated exposures to dichoptic motion coherence threshold stimuli effectively reduce suppression in adults with amblyopia, which in turn improve visual acuity and stereopsis. These visual improvements are sustained and have so far been demonstrated in adults well beyond the critical period of visual development.

Behavioral treatments including perceptual learning, dichoptic training, and video game are found improving visual function in adult amblyopia. A meta‑analysis found that these new methods yielded a mean improvement of visual acuity of 0.17 logMAR with 32% of patients achieving gains ≥0.2 logMAR.[13]

Although dichoptic amblyopic therapy shows significant visual improvement in children and adult amblyopic patient, there is no home‑based dichoptic training design and most of the training needs a supervisor.[14] No long‑term comparison of conventional occlusion treatment and dichoptic treatment has been studied. Design of home‑based dichoptic amblyopic treatment with long‑term randomized control trial needs further investigation.

Pharmacological Treatment

In this issue, Singh et al. reviewed studies on pharmacological therapy for amblyopia. The drugs in this review include levodopa‑carbidopa combination and antidepressants such as fluoxetine, GABA antagonists, and cytidine 5’‑diphosphocholine (choline or citicoline).

The effect of levodopa was studied on many aspects. It can increase endogenous expression of nerve growth factor, increase expression of N‑methyl‑D‑aspartate receptor‑1‑subunit in visual cortical neurons which is reduced in amblyopia, improve visual evoked potential response, increase visual acuity, and decrease fixation point scotomas.

Chronic administration of fluoxetine promotes the recovery of visual functions in adult amblyopic animals by reducing the intracortical inhibition and increasing the expression of brain‑derived neurotrophic factor in the visual cortex.

GABA antagonist was found able to restore binocularity. However, it also has serious adverse effect. Significant visual improvement was found in citicoline administration.

However, most of these drugs are still in experimental stage. Further evaluations of efficacy and side effect of these drugs are needed.

May-Yung Yen

Department of Ophthalmology, Shu-Tien Urology Ophthalmology Clinic, The Yin Shu-Tien Medical Foundation, 2Department of Ophthalmology, Taipei Veterans General Hospital, 3Department of Ophthalmology, School of Medicine, National Yang-Ming University,

Taipei, Taiwan, Republic of China

Address for correspondence:

Dr. May-Yung Yen, Department of Ophthalmology, Taipei Veterans General Hospital, No. 201, Section 2, Shih-Pai Road, Taipei 11217, Taiwan,

Republic of China. E-mail: myyen11@gmail.com

 

References

  1. Preslan MW, Novak Baltimore vision screening project. Phase 2. Ophthalmology 1998;105:150‑3.
  2. Cotter SA, Edwards AR, Arnold RW, Astle WF, Barnhardt CN, Beck RW, et Treatment of strabismic amblyopia with refractive correction. Am J Ophthalmol 2007;143:1060‑3.
  3. Hussein MA, Coats DK, Muthialu A, Cohen E, Paysse EA. Risk factors for treatment failure of anisometropic J AAPOS 2004;8:429‑34.
  4. Wang Compliance and patching and atropine amblyopia treatments. Vision Res 2015;114:31‑40.
  5. Hess RF, Thompson B, Gole G, Mullen KT. Deficient responses from the lateral geniculate nucleus in humans with amblyopia. Eur J Neurosci 2009;29:1064‑70.
  6. Barnes GR, Hess RF, Dumoulin SO, Achtman RL, Pike GB. The cortical deficit in humans with strabismic amblyopia. J Physiol 2001;533(Pt 1):281‑97
  7. Mendola JD, Conner IP, Roy A, Chan ST, Schwartz TL, Odom JV, et Voxel‑based analysis of MRI detects abnormal visual cortex in children and adults with amblyopia. Hum Brain Mapp 2005;25:222‑36.
  8. Li X, Mullen KT, Thompson B, Hess RF. Effective connectivity anomalies in human amblyopia. Neuroimage 2011;54:505‑16.
  9. Tailor VK, Schwarzkopf DS, Dahlmann‑Noor Neuroplasticity and amblyopia: Vision at the balance point. Curr Opin Neurol 2017;30:74‑83.
  10. Sengpiel F, Jirmann KU, Vorobyov V, Eysel Strabismic suppression is mediated by inhibitory interactions in the primary visual cortex. Cereb Cortex 2006;16:1750‑8.
  11. Li J, Hess RF, Chan LY, Deng D, Yang X, Chen X, et Quantitative measurement of interocular suppression in anisometropic amblyopia: A case‑control study. Ophthalmology 2013;120:1672‑80.
  12. Li J, Thompson B, Lam CS, Deng D, Chan LY, Maehara G, et al. The role of suppression in Invest Ophthalmol Vis Sci 2011;52:4169‑76.
  13. Tsirlin I, Colpa L, Goltz HC, Wong Behavioral training as new treatment for adult amblyopia: A meta‑analysis and systematic review. Invest Ophthalmol Vis Sci 2015;56:4061‑75.
  14. Foss Use of video games for the treatment of amblyopia. Curr Opin Ophthalmol 2017;28:276‑81.



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