Article: Strabismus

For the protein Strabismus, see Strabismus (protein).

Strabismus prevents bringing the gaze of both eyes to the same point in space

Strabismus, also known as "heterotropia", "squint", "crossed eye", "wandering eye", or "wall eyed", is a disorder in which the eyes do not point in the same direction. It typically involves a lack of coordination between the extraocular muscles which prevents bringing the gaze of each eye to the same point in space, preventing proper binocular vision, which may adversely affect depth perception. The cause of strabismus can be a disorder in one or both of the eyes; for example, nearsightedness or farsightedness, making it impossible for the brain to fuse two different images.

When strabismus is congenital or develops in infancy, it can cause amblyopia, in which the brain ignores input from the deviated eye although it is capable of normal sight. Since strabismus can cause amblyopia, which is sometimes referred to as lazy eye, it is sometimes itself inaccurately referred to as lazy eye.

In addition to the visual problem, strabismus can be considered a cosmetic problem owing to the appearance of the deviated eye. One study reported that 85% of adult strabismus patients "reported that they had problems with work, school and sports because of their strabismus". The same study also reported that 70% said strabismus "had a negative effect on their self-image" [1].

Types

Strabismus may be concomitant or incomitant. Concomitant strabismus means that the strabismus is equal regardless of which direction the gaze is targeted. This indicates that the individual extraocular muscles function individually, but that they may simply not be aimed at the same target. Concomitant strabismus in a child under the age of 6 rarely indicates serious neurologic disorder. Blindness in one eye usually causes concomitant strabismus, with the eye of a child turning inward, and that of an adult turning outward.

Incomitant strabismus occurs when the degree of misalignment varies with the direction of gaze. This indicates that one or more of the extraocular muscles may not be functioning normally. Types of strabismus include:

• esotropia, or one eye turning inward;
• exotropia, or one eye turning outward;
• hypertropia, or one eye turning upward.
• hypotropia, or one eye turning downward.

Medial strabismus manifests as the inability to abduct (move laterally) one's eye. This is usually caused by damage to the abducens nerve or abducens nucleus. The result is that the eye in its normal resting state deviates medially, as the movements of the medial rectus muscle are less opposed by the denervated lateral rectus muscle.

Pseudostrabismus is the false appearance of strabismus. It generally occurs in infants and toddlers whose bridge of their nose is wide and flat. This causes the appearance of strabismus. With age the bridge of the child's nose will narrow and the folds in the corner of the eyes will go away. To detect the difference between pseudostrabismus and strabismus, a flashlight is shone in the child's eyes. When the child is looking at the light a reflection can be seen on the front surface of the pupil. If the eyes are properly aligned with one another then the reflection will be in the same spot of each eye. If strabismus is present, then the reflection from the light will not be in the same spot of each eye.

Diagnosis

If the eye being tested is the strabismic eye, then it will fixate on the object after the "good" eye is covered, as long as the vision in this eye is good enough. If it is the "good" eye, there will be no change in fixation, as it is already fixated. Depending on the direction that the strabismic eye deviates, the type of tropia or phoria may be assessed.

A simple screen for strabismus is the Hirschberg test.

Treatment and management

As with other binocular vision disorders, the primary therapeutic goal for those with strabismus is comfortable, single, clear, normal binocular vision at all distances and directions of gaze [2].

If minor and detected early, amblyopia (lazy eye) can often be corrected with enforced use of an eyepatch on the dominant eye and/or vision therapy. The use of eyepatches is unlikely to change the angle of strabismus. Advanced strabismus is usually treated with a combination of eyeglasses or prisms, vision therapy, and surgery, depending on the underlying reason for the misalignment. Surgery attempts to align the eyes by shortening, lengthening, or changing the position of one or more of the extraocular eye muscles, and is frequently the only way to achieve cosmetic improvement. Glasses affect the position by changing the person's reaction to focusing. Prisms change the way light, and therefore images, strike the eye, simulating a change in the eye position.

Early treatment of strabismus and/or amblyopia in infancy can reduce the chance of developing amblyopia and depth perception problems. Eyes that remain misaligned can still develop visual problems. Although not a cure for strabismus, prism lenses can also be used to provide some comfort for sufferers and to prevent double vision from occurring.

In adults with previously normal alignment, the onset of strabismus usually results in double vision (diplopia).

Advocates of the Bates method assert that it can reverse strabismus [3], however, this claim is rejected by the vast majority of mainstream eyecare professionals.

Wikibook

• Perfect Sight Without Glasses, chapter 21 and 22, author W.H. Bates.

See also

• Duane syndrome
• Orthoptics
• Sixth (abducent) nerve palsy