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Strabismus

Also called squint, heterotropia, crosseye, or walleye, strabismus is a condition of eye malalignment that results from the absence of normal, parallel, or coordinated eye movement. In children, it may be concomitant, in which the degree of deviation doesn't vary with the direction of gaze; inconcomitant, in which the degree of deviation varies with the direction of gaze; congenital (present at birth or during the first 6 months); or acquired (present during the first 2½ years).

Strabismus can also be latent (phoria), apparent when the child is tired or sick, or manifest (tropia). Tropias are categorized into four types: esotropia (eyes deviate inward), exotropia (eyes deviate outward), hypertropia (eyes deviate upward), and hypotropia (eyes deviate downward).

Strabismus affects about 2% of the population. Incidence of strabismus is higher in patients with central nervous system (CNS) disorders, such as cerebral palsy, mental retardation, and Down syndrome.

The prognosis for correction varies with the timing of treatment and the onset of the disease. Muscle imbalances may be corrected by glasses, patching, or surgery, depending on the cause. However, residual defects in vision and extraocular muscle alignment may persist even after treatment.

Causes

Strabismus is frequently inherited, but its cause is unknown. Controversy exists over whether or not amblyopia ("lazy eye") causes or results from strabismus. In adults, strabismus may result from trauma.

Strabismic amblyopia is characterized by a loss of central vision in one eye that typically results in esotropia (from fixation in the dominant eye and suppression of images in the deviating eye). Strabismic amblyopia may result from hyperopia (farsightedness) or anisometropia (unequal refractive power).

Esotropia may result from muscle imbalance and may be congenital or acquired. In accommodative esotropia, the child's attempt to compensate for the farsightedness affects the convergent reflex, and the eyes cross.

Malalignment of the eyes leads to suppression of vision in one of the eyes. It causes amblyopia if it develops early in life, before bifoveal fixation is established.

Signs and symptoms

• Eyes that appear crossed
• Eyes that do not align in the same direction
• Uncoordinated eye movements
• Double vision
• Vision in only one eye with loss of depth perception

Diagnosis

Parents of children with strabismus will typically seek medical advice. Older persons with strabismus commonly seek treatment to correct double vision, to improve appearance because of changes caused by thyroid ophthalmopathy (thyroid eye disease), or following eye injury. A careful, detailed patient history is essential not only for the diagnosis, but also for the prognosis and treatment of strabismus.

The following ophthalmologic tests help diagnose strabismus:

• Visual acuity test
• Hirschberg's method detects malalignment.
• Retinoscopy
• Maddox rods test
• Convergence test
• Duction test
• Cover-uncover test
• Alternate-cover test

A neurologic examination determines whether the condition is muscular or neurologic in origin. It should be performed if the onset of strabismus is sudden or if the CNS is involved.

Treatment

Initial treatment depends on the type of strabismus. For strabismic amblyopia, therapy includes patching the normal eye and prescribing corrective glasses to keep the eye straight and to counteract farsightedness (especially in accommodative esotropia). 

Surgery is often necessary for cosmetic and psychological reasons to correct strabismus that results from basic esotropia or residual accommodative esotropia after correction with glasses.

Timing of surgery varies with individual circumstances. For example, a 6-month-old infant with equal visual acuity and a large esotropia will have the deviation corrected surgically. But a child with unequal visual acuity and an acquired deviation will have the affected eye patched until visual acuity is equal, then undergo surgery.

Surgical correction includes recession (moving the muscle posteriorly from its original insertion) or resection (shortening the muscle). A recent surgical procedure uses an adjustable suture.

Possible complications include over­correction or undercorrection, slipped muscle, and perforation of the globe. Postoperative therapy may include patching the affected eye and applying combination antibiotic-steroid eyedrops. Eye exercises and corrective glasses may still be necessary; surgery may have to be repeated.