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Definitions and Epidemiology

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Strabismus occurs when the visual axes of the eyes are misaligned. It is one of the most common disorders encountered in pediatric ophthalmology, estimated to affect approximately 3% to 5% of children. The type of strabismus is defined by the direction of misalignment and whether the deviation is latent, intermittent, or constant. A phoria describes a latent strabismus that is present when one eye is covered. The eyes return to normal alignment when the eye is uncovered and the patient views under normal binocular viewing conditions. A tropia describes strabismus that is present when both eyes are viewing. Intermittent strabismus is present when the eyes vary between being misaligned and straight.

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When the eyes are horizontally misaligned, they are esotropic if they are turned toward each other (“cross-eyed”) and exotropic if they are directed away from each other (“wall-eyed”). If the eyes are vertically misaligned, the type of strabismus is described by the deviating eye. If the deviating eye is lower than the straight eye, it is hypotropic. If it is higher than the straight eye, it is hypertropic. Vertical strabismus is much less common than horizontal strabismus in children.

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If the eyes are aligned normally, they are orthophoric. A deviation is comitant if the amount of misalignment does not change in different positions of gaze. This is the most common situation in primary strabismus. A deviation is incomitant if the amount of misalignment changes as the eyes move in various directions. This occurs in patients with cranial nerve palsies or other forms of strabismus with limited extraocular movements. Horizontal eye movements are called adduction when the moves in an inward direction and abduction when it moves outward. Pseudostrabismus is present when the eyes appear misaligned, but they are optically straight.

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Anatomy and Embryology

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Eye movements result from contraction of the extraocular muscles. Horizontal movements are produced by the medial and lateral rectus muscles. Vertical movements are produced by the inferior and superior rectus muscles. The superior oblique muscle causes downward and intorsional movements. The inferior oblique muscle causes upward and extorsional movements (Figure 34–1).

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FIGURE 34–1
Graphic Jump Location

The extraocular muscles. Top: View of right eye from above. Bottom: Frontal view of left eye.

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Movements of the extraocular muscles are controlled by inputs from the third, fourth, and sixth cranial nerves. The third cranial nerve controls 4 of the 6 extraocular muscles (medial rectus, inferior rectus, superior rectus, and inferior oblique muscles), in addition to the sphincter muscle of the iris and the levator muscle of the eyelid. It begins in the rostral midbrain at the level of the superior colliculus, where it is composed of separate subnuclei that subserve the different muscles controlled by the nerve. The fourth cranial nerve controls the superior oblique ...

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