Asthenopia is derived from greek word asthen-opia meaning weak eye condition
It manifests with non specific symptoms such as fatigue, headache, pain in and around the eyes, burning of eyes, blurred or double vision. It is brought on by concentrated use of eyes for visual tasks like reading , computer work or close visual activities. All these activities cause tightening and spasm of ciliary muscles.
Asthenopia is proving to be a major problem in school going children especially with the new norms of on line classes with constant and continuous near work.
Apart from refractive errors, binocular vision anomalies are the most common visual disorders in children. As increasingly more children will be visiting eye specialists with asthenopia symptoms , it is imperative that a complete binocular vision assessment is made and a targeted therapy towards the cause will ensure relief from symptoms and satisfied patients.
Various studies have proved convergence insufficiency (ci) and accommodation infacility (ai) to be a leading cause of cvs*, accounting for 30% -50% of the cases.
Binocular vision assessment
1) detailed history with emphasis on the type of symptoms and its relation to the duration of screen time or digital exposure
2) cyclopegic refraction
3) dry eyes assessment with tbut, schirmers test etc
4) orthoptic evaluation
A)detection of heterophoria
B)measurement of accommodation facility
c)measurement of vergence amplitudes
1) traditional method involves using prisms and synaptophore
2) latest computerised software based programmes like bynocs
Bynocs is a patented computer based programme designed to assist general ophthalmologist in accurate diagnosis of binocular vision anamolies and offers a range of software based exercises as therapy. It offers a revolutionary change in the management of asthenopia patients by easier and simpler methods replacing traditional orthoptics.
The four programmes for the management of asthenopia are:
1) measurement of heterophoria
2) diagnosis and management of accommodation infacility
3) diagnosis and management of convergence insufficiency
4) diagnosis and management of divergence excess
Wearing anaglyph glasses, patient sits in front of the computer screen. Parameters are set depending on the distance of the patient from the screen and also the colors are matched with the glasses.
Patient is presented with two semi circles of red and blue colours. With the help of the arrow keys, patient moves one of the semi circles and aligns it with the other to complete the circle.
The software then calculates the amount of phoria and gives the results in mm and dioptres for exo, eso and cyclo deviations.
Accommodation is a unique mechanism by which our eyes focus the diverging rays from a near object on to the retina to get a clear image. Two indices of importance are:
1) amplitude of accomodation: it is the maximum potential increase in optical power that an eye can achieve in adjusting its focus.
Measured in diopters, it the reciprocal to the closest distance of the object from the eye, when seen clearly.
Based on dander and duane’s data and the hoffstetter’s work, expected age related accommodative amplitudes can be predicted by the formula, 16- (0.3 x age) .
For an emmetrope the expected values are:
At 3 years: 16 – ( 0.3 x 3) = 15 diopters of aa
At 10 years: 16 – (0.3 x 10) = 13 diopters of aa
It is the ability to rapidly change the power of the crystalline lens to various focus distances like changing the focus from near to distance and back again.
It measures the ability to make rapid accommodative changes under monocular and binocular conditions.
It is measured by using accommodative flippers and presenting the target at 40 cms.
The patient is presented by a series of broken “c” symbols. And is asked to hold the flipper close to the eye ( plus lens first) & then flip the lens on the minus side on clearing the target.
The number of flips made per minute are recorded as cycles/ min, where a full cycle consists of both plus and minus lenses,
An average child has an accommodation facility of 5-8 cycles/ min.
When the accommodative power is significantly less than the normal physiological limits for the patient’s age, he or she is diagnosed as having accommodative insufficiency. It is caused by ciliary muscle weakness.
It causes blurred near vision, discomfort & strain associated with near tasks, fatigue, difficulty with attention and concentration while reading.
Accommodation infacility: repeated change of focus from distance to near and vice versa cause asthenopia
It is disjugate movement in which both eyes rotate inwards for bifoveal single vision. It does not deteriorate with age like accommodation but can be affected in visually demanding situations.
Fusional convergence is a reflex produced to ensure that similar retinal images are projected onto corresponding retinal areas.
Near point of convergence:
It is the closest point at which an object can be seen as single during bifoveal vision and the normal range is between 5-10 cms.
Far point of convergence:
It refers to the relative position of the eyes at rest and is usually at infinity.
Amplitude of convergence;
It is the difference in the power needed to turn the eyes from their far point to near point of convergence.
Convergence insufficiency (ci):
It is the inability to maintain the two eyes together while working at a near distance. It is a sensory and neuromuscular anomaly of the binocular visual system.
Ci is characterised by exophoria at near, increased npc ( > 10 cms)and low fusional convergence amplitude ( < 15-20 prism dioptres). Some patients may have concurrent accommodation infacility.
Ci may be primary or idiopathic or may be associated with refractive errors and muscular imbalances.
It is commonly seen in children with increased school work, computer users and people who read books for long hours. It is one of the main causes of asthenopia.
It can be measured either manually with the help of prism bars or with the software program of bynocs.
A series of images, which are based on the principle of random dot stereogram are presented. On seeing through anaglyph glasses, the image produces a sense of depth giving a 3-d picture to the patient. With the help of arrow keys, the patient is asked to click on the position of smaller square within the bigger square.
The software records the break point ( point at which the convergence breaks causing exotropia and diplopia) and the recovery point ( point at which patient is again able to maintain convergence and single vision)
The normal values expressed in prism dioptres are:
Break point: 30 prism diopters
Recovery point: 20 prism dioptres.
Values below this conveys the presence of ci and the patient requires treatment.
Ci is treated with exercises after proper refractive correction.
1)pencil push ups as home therapy
2) computerised program as combined office based and home based therapy.
Convergence insufficiency treatment trial (citt) clearly demonstrated that computer based exercises combined with office based orthoptics were far more effective than pencil push ups.
bynocs presents clinically proven orthoptic exercises which improve the convergence power in 8-12 sessions depending on the degree of ci. It has shown improvement in the symptoms of 80-90% of patients.
Extreme cases of convergence spasm or convergence paralysis are usually neurologic or psychological in origin and the treatment is directed accordingly.
It is the simultaneous outward and opposite rotation of the two eyes to form a single bifoveal vision.
Fusional divergence is a reflex produced to ensure that similar images are projected onto the corresponding retinal areas.
Amplitude of divergence:
Same as convergence
The normal range of divergence amplitude is 4-6 prism dioptres for distance and 8-12 prism dioptres for near.
It is characterised by intermittent or constant eso deviation , which is more for distance. It is treated with prism glasses.
It is characterised by large exo deviation, which is more at distance than near. It is prevelant in 0.05%of population and is more common in women.
It causes blurring of vision for distance.
Treatment is orthoptics exercises with bynocs