|Classification and external resources|
Strabismus prevents the eyes from aiming at the same point in space
|ICD-10||H49 – H50|
Strabismus (//, from Greek strabismós), also known as heterotropia (and including the three variants cross-eye, lazy-eye and walleye), 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 thus hampers proper binocular vision, and which may adversely affect depth perception. Strabismus can present as manifest (heterotropia) or latent (heterophoria) varieties, and can be either a disorder of the brain in coordinating the eyes, or of the power or direction of motion of one or more of the relevant muscles moving the eye. Strabismus is primarily managed by ophthalmologists, optometrists and orthoptists. Strabismus is present in about 4% of children. Treatment should be started as soon as possible to ensure the best possible visual acuity and stereopsis.
If strabismus is present when the patient looks with both eyes, the condition is called manifest strabismus or Heterotropia. This condition includes horizontal tropias exotropia and esotropia which are outward and inward horizontal deviations and hypertropia and hypotropia which are when one eye is set higher or lower than the other eye. Exotropia and esotropia are also known as divergent or convergent squint respectively.
A deviation present only after binocular vision (viewing with both eyes open) has been interrupted by occlusion of one eye, is called latent strabismus or Heterophoria. This condition includes exophoria, esophoria, hyperphoria, and hypophoria.
Strabismus is divided into paretic and non-paretic types. The paretic type is due to paralysis of one or several muscles that are responsible for natural eye movements. Non-paretic strabismus is not due to paralysis of these muscles.
Paralytic strabismus has many causes including Oculomotor nerve palsy, Fourth nerve palsy, Congenital fourth nerve palsy, Sixth nerve palsy, Progressive external ophthalmoplegia, and Kearns–Sayre syndrome.
Pseudo-strabismus is a condition when a person's eye appears mis-aligned but with accurate examination no deviation is observed.
Signs and symptoms
One eye moves normally, while the other points in (esotropia), out (exotropia), up (hypertropia) or down (hypotropia).
Strabismus may be associated with "lazy eye" or "amblyopia" which is due to brain's ignoring input from one eye. Amblyopia results in impaired vision in the affected eye. During the first 7–8 years of life the brain learns how to interpret the signals that come from an eye. This process which is called visual development may be interrupted by strabismus. This is because the deviated eye is not used to focus, and the brain does not receive signals from this eye which causes failure in the normal visual development of the affected eye in childhood.
"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 visual axes over-converge. "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 visual axes diverge from parallel.
Non-paretic squints are generally concomitant, which means that the amount of deviation remains the same in all directions of gaze. Nonparetic squints are not caused by a lesion reducing innervation.
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. The primary sign of strabismus is a visible misalignment of the eyes, with one eye turning in, out, up, down or at an oblique angle.
When the misalignment of the eyes is large and obvious, the strabismus is called "large-angle," referring to the angle of deviation between the line of sight of the straight eye and that of the misaligned eye. Less obvious eye turns are called small-angle strabismus.
Typically, constant large-angle strabismus does not cause symptoms such as eye strain and headaches because there is virtually no attempt by the brain to straighten the eyes. Because of this, large-angle strabismus usually causes severe amblyopia in the turned eye if left untreated.
In most cases, the only effective treatment for a constant eye turn is strabismus surgery. Esotropia (crossed eyes) needs to be treated early in life to prevent amblyopia.
Less noticeable cases of small-angle strabismus are more likely to cause disruptive visual symptoms, especially if the strabismus is intermittent or alternating. In addition to headaches and eye strain, symptoms may include an inability to read comfortably, fatigue when reading and unstable or "jittery" vision. If small-angle strabismus is constant and unilateral, it can lead to significant amblyopia in the misaligned eye.
Both large-angle and small-angle strabismus can be psychologically damaging and affect the self-esteem of children and adults with the condition, as it interferes with normal eye contact with others, often causing embarrassment, anger, and awkwardness.
School children, teenagers and adults may experience psychosocial difficulties if they have noticeable strabismus.A study showed that adults and children perceive a squinting right eye as more distubing than a squinting left eye, and that children perceive an inward squint (esotropia) as worse than an outward squint (exotropia).
During eye examination, a test which is called cover test, is typically used in the diagnosis and measurement 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 are not properly aligned.
Strabismus may be classified as unilateral if the one eye consistently deviates, or alternating if either of the eyes can be seen to deviate. 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 examined eye remaining straight while the previously straight eye is now seen to be deviated on removal of the cover. The cover-uncover test is used to diagnose the type of strabismus (also known as tropia) present.
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.
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.
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 eye patch on the dominant eye and/or vision therapy, the use of eye patches is unlikely to change the angle of strabismus. Strabismus is usually treated with a combination of eyeglasses, 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 and restoring[clarification needed] binocular vision. The procedure can typically be performed in about an hour, and requires about one or two weeks for recovery. Adjustable sutures may be used to permit refinement of the eye alignment in the early postoperative period.
Double vision can rarely result, especially immediately after the surgery, and vision loss is very rare. 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 in infancy may reduce the chance of developing amblyopia and depth perception problems. Most children eventually recover from amblyopia if they have had the benefit of patches and corrective glasses. It has long been considered that amblyopia remains permanent if not treated within a critical period, namely before the age of about 7 years; however, recent discoveries give reason to challenge this view and to adapt the earlier notion of a critical period to account for stereopsis recovery in adults.
Eyes that remain misaligned can still develop visual problems. Although not a cure for strabismus, prism lenses can also be used to provide some temporary comfort for sufferers and to prevent double vision from occurring.
Botulinum Toxin type A (BT-A) was approved to treat strabismus by the Food and Drug Administration in 1989. 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 or diplopia.
When strabismus is congenital or develops in infancy, it can cause amblyopia, in which the brain ignores input from the deviated eye. Even with therapy for amblyopia, stereoblindness may occur. 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". It is possible that after surgery the squint returns again, therefore, a second operation is sometimes required to straighten the eyes.
- Strabismus surgery
- Pediatric ophthalmology
- Palsy of cranial nerve III (oculomotor), IV (trochlear), VI (abducens)
- Bates method
- Duane syndrome
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