Wide-eyed Wonder: an artist's musings on three-dimensional vision

Some are color blind. I am stereo blind.

– The many faces of strabismus

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When someone mentions strabismus, people normally think of severely crossed eyes, if they have any understanding of the word at all.

Strabismus is a disorder in which the two eyes do not line up in the same direction, and therefore do not look at the same object at the same time. The condition is more commonly known as “crossed eyes.” [1]

Approximately 4% of adults have misaligned eyes. [2]

But “crossed eyes” is only the colloquial term for eye turn in, or esotropia. Eye turn out is called exotropia and often referred to as “wall eyes.” Esotropia and exotropia are the most common forms of strabismus.

ESOTROPIA or “crossed eyes”
Esotropia or strabismus with turn in can be intermittent, constant, with one eye turning in all the time, or alternating. “In an alternating esotropia the patient is able to alternate fixation between their right and left eye so that at one moment the right eye fixates and the left eye turns inward, and at the next the left eye fixates and the right turns inward. Where a patient tends to consistently fix with one eye and squint with the other, the eye that squints is likely to develop some amblyopia. Someone whose squint alternates is very unlikely to develop amblyopia because both eyes will receive equal visual stimulation.” [3]

I was diagnosed at age 22 with alternating esotropia. I was discouraged from attempting vision therapy at that time. Even now, I am attempting vision therapy with the same poor prognosis of eye teaming, because my ability to alternate is so consistent and well established.

Esotropia is further sub-categorized into 4 types: congenital (apparent at birth), infantile, accomodative, and partially accomodative.

“True congenital esotropia is an inward turn of a large amount, and is present in very few children, but the infant will not grow out of this turn … The baby with infantile esotropia usually cross fixates, which means that he or she uses either eye to look in the opposite direction. The right eye is used to look toward the left side, and the left eye is used to look toward the right side. By definition, they alternate which eye they are looking with. It is more difficult to help this type of strabismus with non-surgical methods, such as Vision Therapy and/or glasses. Sometimes, clear tape applied to the inner third of each lens (occlusion) can reduce the tendency to turn inward. Prisms may aid alignment if the turn is not too large. [4]

With accommodative esotropia “If excessive inward turning of an eye is first noted around 2 years of age, it may be due to difficulty integrating the focusing (accommodative) system with the eye alignment (binocular) system. Normally when we look across the room or beyond, our eyes are parallel, or straight. However, when we look at things up close, two things happen. We need to converge more (aim both eyes inward at the same time) and we have to input more focus, or accommodate to keep things clear.” [4]

I now wear bi-nasal occluded glasses for everything but reading, which is what is recommended for infantile esotropia. My eyes also alternate, just as the infant described. I may have started alternating as an infant and never learned to do anything else. However, I won’t rule out the notion that my turn in was partly due to inability to accomodate or focus with binocularity, as my eyes appear straight in all my childhood photos.

exotropia or divergent strabismus

EXOTROPIA or “Wall eyes”
Exotropia — a common type of strabismus — is the outward deviation of an eye (away from the nose). The deviation or eye turn may occur while fixating (looking at) distance objects, near objects or both. [5]

When the eye turns outward at all distances and at all times, it is called constant exotropia. When the eye turns outward only sometimes, it is called intermittent exotropia or alternating exotropia. When BOTH eyes turn outward or fail to converge normally (turn inward to see), this is called Convergence Insufficiency and/or Exophoria … Most exotropia is intermittent and this means that the outward eye deviation or eye turn occurs only some of the time.[5]

strabismus.org further explains:

Pros: Advantages of Intermittent Eye Turn
When the eye turn is only occasional, the visual system (including the brain) still has many opportunities to develop. That is, as long as the eyes are straight some of the time, the brain and two eyes will develop some normal functioning (binocular vision and depth perception). Consequently, good possibilities for the development of improved vision in the future will still be present.

Cons: Disadvantages of Intermittent Eye Turn
When the eye turn happens some of the time, but not all the time, the outside observer(s) might conclude that there is no serious problem and fail to seek help. Or they might think the person is simply daydreaming, lazy, or not paying attention. Even worse, without knowing that there is a physical problem, the observer might feel uneasy or mistrustful of the person with intermittent exotropia who gives poor eye contact and comes off as distracted or “shifty-eyed.” [6]

strabismus.org adds

When the outward turn of one eye occurs only at near (when the person is looking at close objects), then the intermittent exotropia can be a symptom or result of another common binocular (two-eyed) vision problem called convergence insufficiency.

This is the most common type of eye muscle or visual-motor problem in the United States, with a reported prevalence among children and adults of anywhere from 5 to 13%. Principal symptoms can include diplopia (double vision), headaches, loss of concentration while reading, carsickness, avoidance of reading, blurred vision, and/or eyestrain. Symptoms are on the rise with increased reading, desk work, and computer use. [6]

Read more about convergience insufficiency and its treatment here http://www.convergenceinsufficiency.org

Hypertropia

Upward deviation of the eye is usually due to a paresis of one of the muscles that either elevate or depress the eye. Most of the time it is due to one specific muscle. [7]

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Written by Lynda Rimke

February 19, 2011 at 5:02 pm

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