Strabismus is a vision condition in which the eyes are not aligned properly and point in different directions. In strabismus one eye looks directly at the object, while the other eye is misaligned inward (esotropia), outward (exotropia), upward (hypertropia) or downward (hypotropia).
Strabismus can be constant or intermittent. The misalignment also might always affect the same eye (unilateral strabismus), or the two eyes may take turns being misaligned (alternating strabismus).
Strabismus occurs in about 3% of children. Without timely treatment about 50% of children with strabismus have some visual loss due to amblyopia (functional reduction in visual acuity resulting in one of the two eyes is almost not used in the vision process).
Symptoms and sings
Symptoms of strabismus usually include:
- Double vision
- Eye strain or discomfort
The only obvious sign of strabismus is a visible misalignment of the eyes, with one eye turning in, out, up, down or at an oblique angle. In some cases, however, a small magnitude or intermittent strabismus can easily be missed upon casual observation.
Some patients with strabismus have normal and equal visual acuity. However, constant unilateral strabismus causing amblyopia in children and is due to cortical suppression of the image in the deviating eye to avoid confusion and diplopia.
Each eye has six extraocular muscles that control movement and position of the eye. For normal binocular vision, functioning of these muscles for both eyes must be coordinated perfectly. Strabismus occurs when neurological or anatomical problems interfere with the control and function of the extraocular muscles.
The problem may be associated with dysfunction of the extraocular muscles themselves, or with visual centers in the brain that control binocular vision. Genetic factors also may play a role. If there is a family history of the condition, children should be checked by an ophthalmologist earlier and more frequently than children from families where the condition does not occur.
- Eye examination
- Neurologic examination
- Hirschberg test
- Cover-uncover test
Strabismus can be detected during comprehensive examination that includes a thorough history, family history, assessment of visual acuity, pupil reactivity, and the extent of extraocular movements. Neurologic examination is also required to be done.
The Hirschberg test, or Hirschberg corneal reflex test is a good screening test, but it is not very sensitive for detecting small deviations. The child looks at a light and the light reflection (reflex) from the pupil is observed. Normally, the reflex appears symmetric (ie, in the same location on each pupil). The light reflex for an exotropic eye is nasal to the pupillary center, whereas the reflex for an esotropic eye is temporal to the pupillary center.
The cover-uncover test involves the patient fixating on an object, one eye is then covered while the other is observed for movement. No movement should be detected if the eyes are properly aligned, but manifest strabismus is present if the uncovered eye shifts to establish fixation once the other eye, which had fixed on the object, is covered. The test is then repeated on the other eye.
In a variation of the cover test, called the alternate uncover test, the child is asked to fixate on an object while the examiner alternately covers one eye and then the other, back and forth. An eye with a latent strabismus shifts position when it is uncovered. In exotropia, the eye that was covered turns in to fixate, and in esotropia, it turns out to fixate. Deviations can be quantified by using prisms positioned such that the deviating eye need not move to fixate. The power of the prism is used to quantify the deviation and provide a measurement of the magnitude of misalignment of the visual axes. The unit of measurement used by ophthalmologists is the prism diopter. One prism diopter is a deviation of the visual axes of 1 cm at 1 m.
- Patching or atropine drops for attendant amblyopia
- Contact lenses or eyeglasses
- Eye exercises
- Surgical alignment of the eyes
Treatment of strabismus aims to equalize vision and then align the eyes. Treatment of children with amblyopia requires measures to encourage use of the amblyopic eye, such as patching the better eye or administering atropine drops into the better eye to provide a visual advantage to the amblyopic eye; improved vision offers a better prognosis for development of binocular vision and for stability if surgery is done. Patching is not, however, a treatment for strabismus. Eyeglasses or contact lenses are sometimes used if the amount of refractive error is significant enough to interfere with fusion, especially in children with accommodative esotropia. Orthoptic eye exercises can help correct intermittent exotropia with convergence insufficiency.
Surgical treatment is generally done when nonsurgical methods are unsuccessful in aligning the eyes satisfactorily. Surgical repair consists of loosening (recession) and tightening (resection) procedures, most often involving the horizontal rectus muscles. Surgical repair is typically done in an outpatient setting. Rates for successful realignment can exceed 80%.
It is important to remember that the earlier strabismus is treated surgically, the more likely it is that the affected eye will develop normal visual acuity and the two eyes will function together properly as a team.