Squint is defined as the misalignment of the visual axes of the two eyes. Normally the two eyes are aligned, and both the eyes see the same object. One eye develops a squint then the visual axis of the two eyes are not aligned and eyes focus on different objects.
Squint evaluation aims to assess the functional problems in vision that occurs because of squinting of the eyes. Besides cosmetic disability squint can also lead to headaches, double vision and prolonged squint can cause a permanent reduction in vision especially in children.
Squint evaluation can be done at any good eye hospital under the guidance of a squint specialist. The tests performed for assessment of squint include the following-
1. Stereopsis- This is a function of depth perception which is the first to be lost in a patient with squint. The presence or absence as well the quantification of stereopsis is done using special glasses and special charts like TNO chart, Frisby chart and Lang`s chart
2. Binocular single vision- The ability to use both eyes together gets disrupted with misalignment of the visual axes. Sometimes the patient may start using one eye more than the other, at other times the patient may perceive double vision. The patient is made to wear a pair of red green glasses and a box of four lights is shown to the patient and depending on the number of dots that the patient perceives, the patient`s binocular status is recorded. This is called the Worth Four dot test.
1. Corneal reflex test- In this test a flash of light is shone on the pupils to detect the gross position of the eyes. If the light reflex is towards the nasal side it is suggestive of divergent squint (eye is deviated outwards), if it is towards the lateral side it is suggestive of convergent squint (eye is deviated inwards).
2. Cover test- This test is done to confirm the direction of manifest squint. The apparently normal eye is covered to see the movement of the squinting eye. If the eye moves inwards towards the nose it suggests a divergent squint, if it moves away from the nose it suggests a convergent squint, if the eye moves downward it indicates an upward deviation of the eye, if the eye moves upward it suggests a downward deviation of the eye.
3. Uncover test- In this test the movement of the eye that is covered is studied too see any hidden latent squint.
4. Alternate cover test- Either eye is occluded to see the visual fixation in each eye as well to detect any presence of lazy eye depending on the preference of fixation.
5. Ocular motility- The patient is asked to follow a target in nine different directions of gaze to assess the action of the extraocular muscles. This enables the ophthalmologist to decide which muscle is overacting or underacting.
6. Prism cover test- This test is done to quantify the amount of squint. A special prism glass is placed on the eye in increasing amounts to neutralize the deviation which gives the ophthalmologist a measure of the squint. This may be done in various gazes to measure the squint in different directions.
These tests are always accompanied by other routine eye checking tests like checking vision, glass powers and a detailed evaluation of the front as well as the back part of the eye. The advice about the squint is based on this comprehensive examination.
Based on this evaluation, squint specialist determines the next steps in managing the squint which may vary from glass prescription to eye patching to an actual eye surgery for squint correction.