Binocular Vision - Disorders

Disorders

To maintain stereopsis and singleness of vision, the eyes need to be pointed accurately. The position of each eye in its orbit is controlled by six extraocular muscles. Slight differences in the length or insertion position or strength of the same muscles in the two eyes can lead to a tendency for one eye to drift to a different position in its orbit from the other, especially when one is tired. This is known as phoria. One way to reveal it is with the cover-uncover test. To do this test, look at a cooperative person's eyes. Cover one eye of that person with a card. Have the person look at your finger tip. Move the finger around; this is to break the reflex that normally holds a covered eye in the correct vergence position. Hold your finger steady and then uncover the person's eye. Look at the uncovered eye. You may see it flick quickly from being wall-eyed or cross-eyed to its correct position. If the uncovered eye moved from out to in, the person has exophoria. If it moved from in to out, the person has esophoria. If the eye did not move at all, the person has orthophoria. Most people have some amount of exophoria or esophoria; it is quite normal. If the uncovered eye also moved vertically, the person has hyperphoria (if the eye moved from up to down) or hypophoria (if the eye moved from down to up). Such vertical phorias are quite rare. It is also possible for the covered eye to rotate in its orbit. Such cyclophorias cannot be seen with the cover-uncover test; they are rarer than vertical phorias.

During the cover-uncover test, a person with some phoria will notice a brief episode of double vision or diplopia after uncovering the eye. This is a normal consequence of the eye's being briefly misaligned. If the diplopia is enduring, that is considered a disorder.

The cover-uncover test can also be used for more problematic disorders of binocular vision, the tropias. In the cover part of the test, the examiner looks at the first eye as he or she covers the second. If the eye moves from out to in, the person has exotropia. If it moved from in to out, the person has esotropia. People with exotropia or esotropia are wall-eyed or cross-eyed respectively. These are forms of strabismus with amblyopia. When the covered eye is the non-amblyopic eye, the amblyopic eye suddenly becomes the person's only means of seeing. The strabismus is revealed by the movement of that eye to fixate on the examiner's finger. There are also vertical tropias (hypertropia and hypotropia) and cyclotropias.