Strabismus

DEFINITION/OVERVIEW

Strabismus denotes a misalignment of the eyes. It is one of the most common eye problems encountered in children, affecting approximately 4% of children younger than 6 years of age. Strabismus can result in vision loss (amblyopia) and can have important and lifelong psychological effects. Early detection and treatment of strabismus are vital to prevent permanent visual impairment. In order to provide the best opportunity for children with strabismus to develop normal binocular vision, alignment of the visual axes must occur at an early stage of visual development. Several terms are used in characterizing the various forms of strabismus and are important in understanding its etiology, treatment, and prognosis.

Orthophoria is the ideal condition of exact ocular balance. It defines that state when the oculomotor apparatus is in perfect equilibrium so that the eyes remain coordinated and perfectly aligned. Even when binocular vision is interrupted, as by occlusion of one eye, truly orthophoric individuals maintain straight eyes. True orthophoria is uncommon, the majority of individuals have a small latent deviation (heterophoria).

Heterophoria is a latent tendency for the eyes to deviate. The latent deviation is normally controlled by fusional mechanisms that provide binocular vision or avoid diplopia (double vision). The eyes deviate only under certain conditions, such as fatigue, illness, stress, or during tests that interfere with maintenance of these normal fusional controls (such as covering one eye). If the amount of heterophoria is large, it may give rise to bothersome symptoms, such as intermittent diplopia, headaches, or asthenopia (eyestrain). Some degree of heterophoria is found in the majority of normal individuals; it is usually asymptomatic.

Heterotropia is a misalignment of the eyes that is constant. It occurs because of an inability of the fusional mechanisms to control the deviation. Tropias can be alternating, involving both eyes, or unilateral. In an alternating tropia, there is no preference for fixation of either eye, and both eyes drift with equal frequency. Because each eye is used periodically, vision in each eye usually develops normally although binocular vision will not. A unilateral tropia is a serious

situation because only one eye is constantly misaligned. The undeviated or ‘fixating’ eye becomes the preferred eye, resulting in loss of vision or amblyopia of the deviated eye.

It is also necessary to describe the type of deviation that is present. The prefixes eso-, exo-, hyper-, and hypoare added to the terms phoria and tropia to further delineate the type of strabismus. Esophorias and esotropias are inward or convergent deviations of the eyes, commonly known as crossed eyes. Exophorias and exotropias are divergent or outward-turning deviations, walleyed being the lay term. Hyperdeviations and hypodeviations designate upward or downward deviations of an eye. In cases where only one eye is seen to be misaligned, the deviating eye is often part of the description of the misalignment (left esotropia).

Ocular deviations may be constant in all fields of gaze or they may change depending on where the patient is looking. This characteristic of a strabismus describes its ‘comitancy’. A comitant strabismus does not change with movement of the eyes into different gazes whereas an incomitant strabismus does. Detecting the presence of an incomitant strabismus can be extremely important. Many forms of incomitant strabismus may be neurologic in nature and demand urgent evaluation, especially if they are of acute onset. It is customary to divide strabismus disorders into comitant and incomitant forms as will be done in this chapter.

DIAGNOSIS

Many techniques are used to assess ocular alignment and movement of the eyes to aid in evaluating and diagnosing strabismic disorders. In a child with strabismus, as with any ocular disorder, assessment of visual acuity is essential. Decreased vision in one eye requires evaluation for a strabismus or other ocular abnormalities, which may be difficult to discern on a brief screening evaluation. Strabismic deviations of only a few degrees in magnitude, too small to be evident by gross inspection, may lead to amblyopia and devastating vision loss.

Corneal light reflex testing is perhaps the most rapid and easiest performed diagnostic test for strabismus. It is especially useful in children who are uncooperative and in those who have poor ocular fixation. The Hirschberg corneal reflex test is performed by projecting a light source onto the cornea of both eyes simultaneously as a child

looks directly at the light. Comparison should then be made of the placement of the corneal light reflex in each eye. In straight eyes, the light reflection appears symmetric and, because of the relationship between the cornea and the macula, slightly nasal to the center of each pupil. When a strabismus is present, the reflected light is asymmetric and appears displaced in one eye. The Krimsky method of corneal reflex testing requires a prism that is placed over one or both eyes to align the light reflections (41, 42). The amount of prism needed to align the reflections is used to measure the degree of deviation. Although corneal light reflex testing is a useful screening tool, it may fail to detect a small deviation or an intermittent strabismus.

Cover tests for strabismus allow for more accurate and detailed strabismus measurements but require a child’s attention and cooperation, good eye movement, and reasonably good vision in both eyes. If any of these elements are lacking, the results of a cover test may be invalid. Cover tests include the cover–uncover test and the alternate cover test. In the cover–uncover test, a child looks at an object in the distance, 6 m (20 feet) is standard. As the child looks at the distant object, the examiner covers one eye and watches for movement of the uncovered eye. If no movement occurs, there is no apparent misalignment of that eye. After one eye is tested, the same procedure is repeated on the other eye. When performing the alternate cover test, the examiner rapidly covers and uncovers each eye, shifting back and forth from one eye to the other. If the child has an ocular deviation, the eye rapidly moves as the cover is shifted to the other eye. Both the cover–uncover test and the alternate cover test should be performed at both distance and near fixation. The cover–uncover test differentiates manifest deviations (tropias), from latent deviations (phorias).

Strabismus during infancy

Many children will display unstable ocular alignment during the first few months of life and ocular deviations during this period do not necessarily indicate an abnormality. Infants are rarely born with their eyes aligned. During the first months of life, alignment may vary intermittently from esotropia to orthotropia to exotropia. A number of large population studies have confirmed that strabismus is common in early infancy. Because of this, adequate

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41, 42 Krimsky method of light reflex testing. A prism is placed in front of one (or both) eyes to center the corneal light reflexes.

assessment of alignment usually is not made until the patient is approximately 3 months of age and any angle of strabismus that is present is stable.

Pseudostrabismus

Pseudostrabismus (pseudoesotropia) is one of the most common reasons a pediatric ophthalmologist is asked to evaluate an infant. Pseudoesotropia is characterized by the false appearance of strabismus when the visual axes are actually aligned. This appearance is usually caused by a flat, broad nasal bridge, prominent epicanthal folds, or a narrow interpupillary distance. An observer may perceive less white sclera nasally than would be expected, and the impression is that the eye is turned in toward the nose, especially when the child looks to either side. Parents may comment that the eye almost disappears when their child looks to the side.

Pseudoesotropia can be differentiated from a true strabismus when the corneal light reflex is seen to be centered in both eyes and/or when the cover–uncover test shows no refixation movement. Tightening the epicanthal folds by pinching the bridge of the nose can also be effective in demonstrating that the ‘crossing’ is not real. Once the diagnosis of pseudoesotropia has been confirmed, parents can be reassured that the child will outgrow the appearance of esotropia. As the child grows, the bridge of the nose becomes more prominent and displaces the epicanthal folds, and the medial sclera becomes proportional to the amount visible on the lateral aspect of the eye.