Strabismus

Strabismus (Modern Latin, from Greek στραβισμός strabismos; cf. στραβίζειν strabizein "to squint", στραβός 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, which 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 in coordinating the eyes, or of one or more of the relevant muscles' power or direction of motion. Difficult strabismus problems are usually co-managed between orthoptists and ophthalmologists.

Paralytic strabismus
Forms of paralytic strabismus include


 * Third (oculomotor) nerve palsy
 * Fourth (trochlear) nerve palsy
 * Congenital fourth nerve palsy
 * Sixth (abducent) nerve palsy
 * Total (external) ophthalmoplegia
 * Progressive external ophthalmoplegia
 * Other
 * Kearns-Sayre syndrome

Other strabismus
Other forms of strabismus include:


 * Convergent concomitant/Divergent concomitant
 * Esotropia
 * Exotropia
 * Vertical strabismus
 * Hypertropia
 * Hypotropia
 * Other and unspecified heterotropia
 * Microtropia
 * Monofixation syndrome
 * Heterophoria
 * Esophoria
 * Exophoria
 * Mechanical strabismus
 * Brown's sheath syndrome
 * Other
 * Duane syndrome

Signs and symptoms
One eye moves normally, while the other points in (esotropia or "crossed eyes"), out (exotropia), up (hypertropia) or down (hypotropia).

Strabismus is often mistakenly referred to as "lazy eye" or known as amblyopia; a result from constant unilateral strabismus. It also be referred to as "crossed eyes", “wandering eyes”, or having a “cast”. Other names include "squint", "crossed eye", "spaggy eye", "google eye", "boss eye", "cock eye", "wonk eye", "codeye", "wok eye", "gam gam", "gloucester-ledbury","Ryland Optic Disorder", "disco eye", "sockeye" and "wall eye".

"Cross-eyed" means that when a person with strabismus looks at an object, one eye fixes on the object and the other fixes with a convergence angle less than zero; the optic axes overconverge. "Wall-eyed" means that when a person with strabismus looks at an object, one eye fixes on the object and the other fixes with a convergence angle greater than zero; that is, the optic axes diverge from parallel.

Pathophysiology
Strabismus can be caused when the cranial nerves III (oculomotor), IV (trochlear) or VI (abducens) have a lesion. 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 VI is particularly vulnerable to damage from brain swelling, as it runs between the clivus and brain stem.

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.

Diagnosis
During eye examinations, orthoptists, ophthalmologists 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); these are types of horizontal strabismus. "Hypertropia" is upward, and "Hypotropia" is downward; these are types of vertical strabismus, which are less common.

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, then the reflection will be in the same spot of each eye. Therefore, if the reflection is not in the same place in each eye, then the eyes aren't properly aligned.

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 cataract. Many infants are born with their eyes slightly misaligned. The best time for physicians to assess this is between ages 3 and 6 months.

Differential diagnosis
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.

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.

Whereas amblyopia (lazy eye), 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 does not change the vision; it 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. The procedure can typically be performed in about an hour, and requires about a week for recovery. Double vision can result, and occasionally vision loss can occur. 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. Most children eventually recover from amblyopia by around age 10, if they have had the benefit of patches and corrective glasses.

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.

Botulinum Toxin (Botox) may also be used in the treatment of strabismus, to improve cosmetic appearance. Most commonly used in adults, the toxin is injected in the stronger muscle, causing temporary paralysis. The treatment may need to be repeated 3–4 months later once the paralysis wears off. Common side effects are double vision, droopy eyelid, over correction and no effect. The side effects will resolve fairly quickly.

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

Prognosis
When strabismus is congenital or develops in infancy, it can cause amblyopia, in which the brain ignores input from the deviated 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."