|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, 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 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 development of the best possible visual acuity and stereopsis.
Diagnosis and Classification
During an eye examination, a test such as cover testing or the Hirschberg test, are used in the diagnosis and measurement of strabismus and its affect on vision. There are several classifications that are made when diagnosing strabismus.
Strabismus can be manifest or latent. A manifest deviation, or heterotropia, is present while the patient views a target binocularly, with no occlusion of either eye. The patient is unable to align the gaze of each eye to achieve fusion. A latent deviation, or heterophoria, is only present after binocular vision has been interrupted, typically by covering one eye. This type of patient can typically maintain fusion despite the misalignment that occurs when the positioning system is relaxed. Intermittent strabismus is a combination of both of these types, where the patient can achieve fusion, but occasionally or frequently falters to the point of a manifest deviation.
Strabismus may also be classified based on time of onset, either congenital, acquired, or secondary to another pathological process. Many infants are born with their eyes slightly misaligned, and typically is outgrown by six to twelve months of age. Aquired and secondary strabismus is developed after normal binocular vision has developed. In adults with previously normal alignment, the onset of strabismus usually results in double vision.
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 triggered by various tests during an eye exam.
Horizontal deviations are classified into two varieties. Eso- describes inward or convergent deviations towards the midline. Exo- describes outward or divergent misalignment. Hyper- is the term for an eye whose gaze is directed higher than the fellow eye while hypo- refers to an eye whose gaze is directed lower. Torsional strabismus occurs when the eyes rotate around the anterior-posterior axis to become misaligned, which is quite rare.
The directional prefixes are combined with -tropia and -phoria to describe various types of strabismus. For example, a constant left hypertropia exists when a patient's left eye is always aimed higher than the right. A patient with an intermittent right esotropia has a right eye that occasionally drifts toward their nose, but at other times is able to align with the gaze of the left eye. A patient with a mild exophoria can maintain fusion during normal circumstances, but when the system is disrupted, the relaxed posture of their eyes is slightly divergent.
Strabismus can be further classified as follows:
- Paretic strabismus is due to paralysis of one or several extraocular muscles.
- Non-paretic strabismus is not due to paralysis of extraocular muscles.
- Comitant strabismus is a deviation that is the same magnitude regardless of gaze position.
- Non-comitant strabismus has a magnitude that varies as the patient shifts their gaze up, down, or to the sides.
Non-paretic strabismus is generally concomitant. Paretic strabismus can be either comitant or non-comitant.
When the misalignment of the eyes is large and obvious, the strabismus is called large-angle, referring to the angle of deviation between the lines of sight of the eyes. Less severe eye turns are called small-angle strabismus. The degree of strabismus can vary based on whether the patient is viewing a distant or near target.
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 esotropia due to less sclera being visible nasally. With age, the bridge of the child's nose narrows and the folds in the corner of the eyes become less prominent.
Signs and symptoms
When observing a patient with strabismus, the misalignment of their eyes may be quite apparent. A patient with a constant eye turn of significant magnitude is very easy to notice. However, a small magnitude or intermittent strabismus can easily be missed upon casual observation. In any case, an eye care professional can conduct various tests, such as cover testing, to determine the full extent of the strabismus.
Symptoms of strabismus include double vision and/or eye strain. To avoid double vision, the brain may adapt by ignoring one eye. In this case, there is often no noticeable symptoms other than a minor loss of depth perception. This deficit may not be noticeable in someone that has had strabismus since birth or early childhood as they have likely learned do judge depth and distances using monocular cues. However, a constant unilateral strabismus causing constant suppression is a risk for amblyopia in children. Small-angle and/or intermittent strabismus are more likely to cause disruptive visual symptoms. In addition to headaches and eye strain, symptoms may include an inability to read comfortably, fatigue when reading and unstable or "jittery" vision.
The extraocular muscles control the position of the eyes. Thus a problem with the muscles or the nerves controlling them can cause paralytic strabismus. The muscles are controlled by cranial nerves III, IV, or VI. An impairment of cranial nerve III causes the associated eye to deviate down and out and may or may not affect the size of the pupil. Impairment of cranial nerve IV, which can be congenital, causes the eye to drift up and perhaps slightly inward. Sixth nerve palsy causes the eyes to deviate apart and has many causes due to the relatively long path of the nerve. Increased cranial pressure can compress the nerve as it runs between the clivus and brain stem. Also, if the doctor is not careful, twisting of the baby's neck during forceps delivery can damage cranial nerve VI.
Evidence indicates that a cause for strabismus may lie with the input that is provided to the visual cortex. This allows for strabismus to occur without the direct impairment of any cranial nerves or extraocular muscles.
Strabismus may cause amblyopia due to the brain ignoring one eye. Amblyopia is the failure of one or both eyes to achieve normal visual acuity despite normal structural health. During the first 7–8 years of life the brain learns how to interpret the signals that come from an eye through a process called visual development. Development may be interrupted by strabismus if the child always fixates with one eye and rarely or never fixates with the other. To avoid double vision, the signal from the deviated eye is suppressed, and the constant suppression of one eye causes a failure of the visual development in that eye.
Also, amblyopia may cause strabismus. If there is a great difference in clarity between the images from the right and left eyes, there may not be sufficient input to correctly reposition the eyes. Other causes of a visual difference between right and left eyes, such as asymmetrical cataracts, refractive error, or other eye disease, can also cause or worsen strabismus.
Accommodative esotropia is a form of strabismus caused by refractive error in one or both eyes. Due to the near triad, when a patient engages their accommodation to focus on a near object, there is an increase in the signal sent by cranial nerve III to the medial rectus muscles, drawing the eyes inward. If the accommodation needed is more than the usual amount, such as with people with significant hyperopia, the extra convergence can cause the eyes to cross.
People of all ages may experience psychosocial difficulties if they have noticeable strabismus. Both large-angle and small-angle strabismus can negatively impact self-esteem, as it interferes with normal eye contact, often causing embarrassment, anger, and feelings of awkwardness. A study showed that adult and children observers perceive a right heterotropia as more disturbing than a left heterotropia, and that children observers perceive an esotropia as worse than an exotropia. Successful surgical correction of strabismus is known to have positive effects on psychological well-being, even when implemented with adult patients.
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 therapy is used for treating strabismus in certain circumstances. In 1989, the Food and Drug Administration approved Botulinum Toxin type A (BT-A) as a treatment for strabismus in patients over 12 years old. Most commonly used in adults, the technique is also used for treating children, in particular children affected by infantile esotropia. The toxin is injected in the stronger muscle, causing temporary and partial 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 typically resolve also within 3–4 months.
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
- Convergence insufficiency
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