Strabismus

# Overview
Strabismus (from Greek:  strabismos, from  strabizein "to squint," from  strabos "squinting, squint-eyed") is a condition in which the eyes are not properly aligned with each other. It typically involves a lack of coordination between the extraocular muscles that prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely affect depth perception. Strabismus can be either a disorder of the brain coordinating the eyes or a disorder of one or more muscles, as in any process that causes a dysfunction of the usual direction and power of the muscle or muscles.

# Pathophysiology
Strabismus can be an indication that a cranial nerve has a lesion, particularly Cranial Nerve III (Occulomotor), Cranial Nerve IV (Trochlear) or Cranial Nerve VI (Abducens). A strabismus caused by a lesion in either of these nerves results in the lack of innervation to eye muscles and results in a change of eye position. A strabismus may be a sign of increased intracranial pressure, as CN III is particularly vulnerable to damage from brain swelling.
More commonly however, squints are termed concominant (i.e. non paralytic). This means the squint is not caused by a lesion reducing innervation. The squint in this example, is caused by a refractive error in one or both eyes. This refractive error causes poor vision in one eye and so stops the brain from being able to use both eyes together.

# Differentiating Strabismus from other Diseases

## Pseudostrabismus
Pseudostrabismus is the false appearance of strabismus. It generally occurs in infants and toddlers whose bridge of the nose is wide and flat, causing the appearance of strabismus. With age, the bridge of the child's nose narrows and the folds in the corner of the eyes go away. To detect the difference between pseudostrabismus and strabismus, a Hirschberg test may be used.

# Natural History, Complications and Prognosis

## Prognosis
When strabismus is congenital or develops in infancy, it can cause amblyopia, in which the brain ignores input from the deviated eye. Strabismus can lead to a permanent weakening of vision in the strabismic eye called amblyopia (this may not always happen), sometimes referred to as lazy eye. The appearance of strabismus may also be a cosmetic problem. 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."

# Diagnosis
During eye examinations, ophthalmologists, orthoptists, and optometrists typically use a cover test to aid in the diagnosis of strabismus. If the eye being tested is the strabismic eye, then it will fixate on the object after the "straight" eye is covered, as long as the vision in this eye is good enough. If the "straight" eye is being tested, there will be no change in fixation, as it is already fixated. Depending on the direction that the strabismic eye deviates, the direction of deviation may be assessed. Exotropic is outwards (away from the midline) and esotropic is inwards (towards the nose).
A simple screening test for strabismus is the Hirschberg test. A flashlight is shone in the patient's eye. When the patient 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, the reflection will be in the same spot of each eye. Therefore, if strabismus is present, the reflection from the light will not be in the same spot of each eye.

## Laterality
Strabismus may be classified as unilateral if the same eye consistently 'wanders', or alternating if either of the eyes can be seen to 'wander'. Alternation of the strabismus may occur spontaneously, with or without subjective awareness of the alternation. Alternation may also be seen following the cover test, with the previously 'wandering' eye remaining straight while the previously straight eye is now seen to be 'wandering' on removal of the cover.  The cover-uncover test is used to diagnose the type of strabismus (also known as tropia) present.

## Onset
Strabismus may also be classified based on time of onset, either congenital, acquired or secondary to another pathological process, such as a cataract.

# Treatment
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.
Whereas amblyopia, if minor and detected early, can often be corrected with 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).

# Other Terminology
Strabismus is often incorrectly referred to as "lazy eye" (which in fact refers to the associated condition amblyopia). It is also referred to as "squint,"  "crossed eye," "cockeye," "codeye," and "wall eye".
Cross-eyed means that when a person with strabismus looks at an object, one eye fixates the object and the other fixates with a convergence angle less than zero, that is the optic axes overconverge. Wall-eyed means that when a person with strabismus looks at an object, one eye fixates the object and the other fixates with a convergence angle greater than zero, that is the optic axes diverge from parallel.

# Related Chapters
- Duane syndrome
- Mobius syndrome
- Orthoptics
- Pediatric ophthalmology
- Sixth (abducent) nerve palsy
- Strabismus surgery