What is Strabismus?
Strabismus is a visual defect in which the eyes are misaligned and point in different directions. Strabismus is a fairly common condition among children – approximately 3% of the population has an eye misalignment of some sort. Strabismus can also occur later in life, for a variety of reasons.
There are three general types of eye misalignment – the eyes can turn towards the nose (eso-deviation), towards the ears (exo-deviation), or one eye can be higher than the other (hyper-deviation). Notice that the plural “eyes” is used here–even though only one eye may appear to be deviating, in fact, it is almost always the case that the misalignment involves both eyes. Even when one eye appears to look straight ahead and the other eye appears to turn in another direction, it is more accurate – and useful – to consider the eyes as misaligned relative to each other.
For example, if both eyes actually turn in a little towards the nose (eso-deviation) but the right eye is in the straight-ahead position (it is the “preferred” eye), this will cause the left eye to shift towards the right (towards the nose) even more. Understandably most people would say, “the left eye is turned and the right is straight” but, in fact, the eyes are turned towards each other.
Strabismus can be present all of the time (a tropia), or only when the person is fatigued, sick, or under testing conditions (a phoria), or somewhere in between (an intermittent tropia). Therefore, someone can have an eso-phoria (eyes turn towards the nose only when tired, etc.), an intermittent eso-tropia (eyes sometimes turn towards the nose on their own), or an eso-tropia (eyes always turn towards nose).
Why Do We Need Properly Aligned Eyes
With proper alignment of the eyes, both eyes look at the same object at the same time and send two separate (and slightly different) pictures to the brain. There are specialized brain cells (binocular cells) that take these two slightly separate pictures and put them together into a single three-dimensional (3-D) perception; only when both eyes point in the same direction at the same time is 3-D vision (depth perception) possible.
With strabismus, the eyes are looking in very different directions, so the brain is given two very different images to interpret. The binocular cells cannot combine these very different pictures into a single perception, so depth perception (3-D vision) is not possible; the brain perceives two separate pictures. When a young child develops strabismus, their brain avoids double vision by ignoring one of the pictures – normal binocular cells cannot develop, or are lost, in children who ignore one image. Such children lose their depth perception, sometimes irreversibly. Adults who develop strabismus cannot just ignore the second image, so they develop double vision (diplopia). Adults who develop strabismus typically have normal binocular cells but these cells are not being used when the eyes are misaligned.