STRABISMUS, OR ABNORMAL ocular alignment, is one of the most common eye problems encountered in children. The misalignment may be manifest in any field of gaze, may be constant or intermittent, and may occur at distant or near fixation or both. Strabismus affects between 2% and 5% of the preschool population and is an important cause of visual and psychological disability (
1,
2). The word “strabismus” derives from the Greek word
strabismos (meaning a squinting) and probably predates the geographer Strabo, whose “peculiarly horrible and unbecoming squint was famous in Alexandria during the Roman Empire.”
Strabismus involves a number of different clinical entities. Knowledge of the terms used to describe a strabismic deviation and the more common patterns of strabismus help to predict the cause of the strabismus and determine proper treatment.
Orthophoria is the condition of exact ocular balance. It implies that the oculomotor apparatus is in perfect equilibrium so that both visual axes always intersect at the object of visual regard.
Heterophoria is a latent tendency for the eyes to deviate. This latent deviation is normally controlled by fusional mechanisms which provide binocular vision or avoid diplopia. The eye deviates only under certain conditions, such as fatigue, illness, stress, or tests that interfere with the maintenance of these normal fusional abilities (such as covering one eye). If the amount of heterophoria is large, it may give rise to bothersome symptoms, such as transient diplopia or asthenopia.
Heterotropia is a misalignment of the eyes that is manifest. The condition may be alternating or unilateral, depending on the vision. In alternating strabismus, either eye may be used for seeing while the fellow eye deviates. Because each eye is used in turn, each develops similar vision. In unilateral strabismus, only one eye is preferred for fixation while the fellow eye deviates consistently. The constantly deviating eye is prone to defective central vision during the visually immature period of life.
A convergent deviation, crossing or turning in of the eyes, is designated by the prefix “eso-” (esotropia, esophoria). Divergent deviation, or turning outward of the eyes, is designated by the prefix “exo-” (exotropia, exophoria). Vertical deviations are designated by the prefixes “hyper-” and “hypo-” (hypertropia, hypotropia). In cases of unilateral strabismus, the deviating eye is often part of the description of the misalignment (left esotropia). Most vertical deviations are described in terms of the hypertropic eye. An exception to this general rule occurs when the lower, or hypotropic, eye is restricted in its movement. The deviation is then named according to the hypotropic eye.
OCULAR ALIGNMENT IN INFANCY
Ocular deviations during the first months of life do not necessarily indicate an abnormality. Because of oculomotor instability during this time, adequate 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. Infants are often born with their eyes misaligned. During the first month of life, alignment may vary intermittently from esotropia to orthotropia to exotropia. Nixon and coworkers observed 1,219 alert infants in a newborn nursery and found that 40% seemed to have straight eyes, 33% had exotropia, and 3% had esotropia (
3). Many had variable alignment and 7% were not sufficiently alert to permit classification. Other large population studies have confirmed that strabismus is common in early infancy (
4).
NYSTAGMUS BLOCKAGE SYNDROME
The nystagmus blockage syndrome is characterized by nystagmus that begins in early infancy and is associated with esotropia. The nystagmus is reduced or absent with the fixing eye in adduction. As the fixing eye follows a target moving laterally toward the primary position and then into abduction, the nystagmus increases and the esotropia decreases. A head turn develops in the direction of the uncovered eye when the fellow eye is occluded. This abnormal head posture allows the uncovered eye to persist in an adducted position.
ACCOMMODATIVE ESOTROPIA
Accommodative esotropia is defined as a convergent deviation of the eyes associated with activation of the accommodative reflex. Esotropia that is related to accommodative effort may be divided into three major categories: refractive, nonrefractive, and partial or decompensated.
Refractive Accommodative Esotropia
Refractive accommodative esotropia usually occurs in a child between 2 and 3 years of age with a history of acquired intermittent or constant esotropia. Occasionally, children who are 1 year of age or younger present with all the clinical features of accommodative esotropia (
35,
36). The refraction of patients with refractive accommodative esotropia averages +4.75 D (
37). The angle of esodeviation is the same when measured at distance and near fixation, and is usually moderate in magnitude, ranging between 20 and 40 PD. Amblyopia is common, especially when the esodeviation has become more nearly constant.
Pathogenesis
The mechanism of refractive accommodative esotropia involves three factors: uncorrected hyperopia, accommodative convergence, and insufficient fusional divergence. When an individual exerts a given amount of accommodation, a specific amount of convergence (accommodative convergence) is associated with it. An uncorrected hyperope must exert excessive accommodation to clear a blurred retinal image. This in turn will stimulate excessive convergence. If the amplitude of fusional divergence is sufficient to correct the excess convergence, no esotropia will result. However, if the fusional divergence amplitudes are inadequate or motor fusion is altered by some sensory obstacle, an esotropia will result. Patients with lower levels of hyperopia but with significant anisometropia are also at an increased risk to develop an accommodative esotropia.
Nonrefractive Accommodative Esotropia
Children with nonrefractive accommodative esotropia usually present between 2 and 3 years of age with an esodeviation that is greater at near than at distance fixation. The refractive error in this condition may be hyperopic or myopic, although the average refraction is +2.25 D.
Pathogenesis
In nonrefractive accommodative esotropia, there is a high accommodative convergence to accommodation (AC:A) ratio: the effort to accommodate elicits an abnormally high accommodative convergence response. There are a number of ways of measuring the AC:A ratio: the heterophoria method, the fixation disparity method, the gradient method, and the clinical evaluation of distance and near deviation. Most clinicians prefer to assess the ratio using the distance-near comparison. This method allows the ratio to be evaluated more easily and quickly, since it employs conventional examination techniques and requires no calculations. The AC:A relationship is derived by simply comparing the distance and near deviation. If the near measurement in an esotropia patient is > 10 D, the AC:A ratio is considered to be abnormally high.