Another type of visual defect found in children is abnormality in the external ocular muscles which control the movement of the entire eyeball in its orbit. Strabismus or crossed eyes: is caused by a lack of coordination of eye muscles; the two eyes do not simultaneously focus on the same object. In most cases one eye turns inwards towards the nose while the other focuses on the object being viewed. When in this case the deviating eye rotates inwards it is called “internal strabismus or convergent squint.” When the deviating eye turns outward it is called “exterial strabismus.”
Heterophoria: is a defect in muscular balance of the eye in which the deviation of the eye is not apparent as in strabismus. In this condition there is
A tendency for the eye to deviate from the normal position for binocular fixation,
A partially counterbalancing tendency towards simultaneous fixation through forced muscular efforts.
When the eyes tend to pull towards the nose, it is called “ESOPHORIA” and when they tend to pull away from the nose, called as “EXOPHORIA”. Heterophoria tends to cause difficulties in visual fusion that is in the ability to coordinate or fuse the two images from both eyes into a single image.
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