CHAPTER 91

Strabismus

Teresa O. Rosales, MD

The term strabismus refers to any abnormality in ocular alignment, whether the eyes go in or out or 1 eye is higher than the other. It is among the most common eye problems observed in infants and children. The pediatrician plays an important role in the early detection and prompt referral of children with suspected ocular alignment abnormalities.

Epidemiology

Strabismus affects approximately 3% of the population, and the condition occurs most commonly in children younger than 6 years. Approximately 50% of all affected children have a positive family history of strabismus, although the exact genetic mode of inheritance is unclear. Up to 75% of otherwise healthy newborns and infants have transient intermittent strabismus during the first 3 months after birth.

Clinical Presentation

Children with ocular misalignment have an asymmetric corneal light reflex test. Eye movement is noted on cover testing. Children with paralytic strabismus may present with torticollis (ie, head tilt) in an effort to avoid double vision (ie, diplopia; Box 91.1).

Pathophysiology

Normal binocular vision is the result of the fusion of images from both eyes working synchronously across the visual field. Six extraocular muscles control all eye movements. Orthophoria is proper alignment of the eyes, and strabismus results from an imbalance in muscle movements.

Strabismus

The classification of strabismus is complex. Based on etiology, it may be considered nonparalytic (comitant) or paralytic (noncomitant). Strabismus may also be classified as congenital or acquired, intermittent or constant, and alternating or unilateral. In nonparalytic strabismus, the extraocular muscles and the nerves that control them are normal. The degree of deviation is constant or nearly constant in all directions of gaze. Nonparalytic strabismus is the most common type of strabismus occurring in children, and congenital or infantile esotropia is usually of this type. Ocular or visual defects, such as cataracts or high refractive errors, occasionally cause nonparalytic strabismus.

In paralytic strabismus, paralysis or paresis of 1 or more of the extraocular muscles produces a muscle imbalance. The deviation is asymmetric, and characteristically the degree of deviation is worse when gazing in the direction of the affected muscle. Paralytic strabismus may be congenital or acquired. Congenital paralytic strabismus may be the result of birth trauma, muscle anomalies, abnormal development of the cranial nerve nuclei, or congenital infections affecting the eyes. Congenital strabismus may occur in association with neurodevelopmental disorders, such as cerebral palsy. Acquired paralytic strabismus resulting from extraocular muscle palsies usually indicates the presence of a serious underlying condition, such as an intracranial tumor, a demyelinating or neurodegenerative disease, myasthenia gravis, progressive myopathy, or central nervous system (CNS) infection. Children may present with double vision or a compensatory head tilt to avoid double vision.

Intermittent (ie, latent) misalignment of the eyes is referred to as a phoria. Under normal conditions, the fusional mechanisms of the CNS maintain eye alignment. Eye deviation is appreciated only under certain conditions, such as illness, fatigue, or stress, or in cases in which fusion is interrupted by occluding 1 eye (eg, during cover testing). Some degree of phoria may be found in almost all individuals and typically, it is asymptomatic. Larger degrees of phoria may give rise to troublesome symptoms such as headaches, transient diplopia, or asthenopia (eg, eyestrain).

Constant misalignment of the eyes is referred to as heterotropia (ie, strabismus). This condition occurs because normal fusional mechanisms are unable to control eye deviation; children are unable to use both eyes together to fixate on an object. In alternating heterotropia, both eyes appear to deviate equally, and vision generally develops normally in each eye because children have no preference for fixation. If strabismus affects only 1 eye, the other eye is always used for fixation, and a danger exists for the development of amblyopia or vision loss in the deviating eye.

Convergent deviation, which is a turning in or crossing of the eyes, is an esodeviation (eg, esotropia, esophoria). Divergent deviation, which is a turning out of the eyes, is an exodeviation. The term hypertropia refers to conditions involving upward vertical deviations. Esodeviations are the most common type of ocular misalignment, accounting for 50% to 75% of all cases of strabismus. Vertical deviations represent less than 5% of all cases of strabismus.

Amblyopia

Amblyopia is a potential complication if strabismus is not corrected in a timely manner. Amblyopia refers to poor vision in 1 eye or, rarely, both eyes despite correction of any refractive errors. A child with no significant refractive error and with visual acuity of 1 eye that is worse than the other has amblyopia. Diagnosis of amblyopia is based on a difference in visual acuity of at least 2 lines (eg, 20/20 in 1 eye and 20/40 in the other), as measured by reading an eye chart. Amblyopia is the leading cause of preventable visual loss in children.

Amblyopia may be classified into 3 major categories: deprivation amblyopia, refractive amblyopia, or strabismic amblyopia. Generally, deprivation amblyopia is the result of obstruction of vision caused by a unilateral lesion or developmental defect in 1 of the structures of the eye or its visual pathways. Causes of deprivation amblyopia include congenital cataract, ptosis, corneal opacity, retinal detachment, retinoblastoma, coloboma, optic nerve defect (eg, optic nerve hypoplasia), and orbital tumor. These conditions cause a lack of formation of a retinal image or a blurred retinal image, usually in 1 eye. Refractive amblyopia refers to a blurring of the retinal image resulting from large or asymmetric refractive errors. Amblyopia is usually considered a unilateral abnormality, but it may be bilateral in cases of large refractive errors (usually, astigmatism or hyperopia). In strabismic amblyopia, the immature or developing brain suppresses images from the deviating eye to prevent diplopia. Strabismic amblyopia is most commonly associated with strabismus that manifests in children younger than 4 years. If the condition that causes amblyopia is not corrected while the brain’s visual pathways are still malleable (eg, before approximately 6–7 years of age), children may experience some degree of permanent visual loss.

Differential Diagnosis

The differential diagnosis of strabismus may be divided into 3 categories: transient neonatal strabismus, congenital or infantile strabismus, and acquired strabismus. It is important to differentiate true strabismus from the illusion of deviation created by facial asymmetry or anatomic variations (ie, pseudostrabismus).

Transient Neonatal Strabismus

Eye alignment in normal newborns and infants during the first 2 to 3 months after birth may vary from normal to intermittent esotropia or exotropia. These deviations are believed to result from CNS immaturity and resolve spontaneously in most infants by 4 months of age. If such deviations are constant or persist beyond this age, the child should be referred to an ophthalmologist for further evaluation.

Congenital or Infantile Strabismus

Congenital or infantile strabismus is deviation that occurs within the first 6 months after birth. Because the deviation may not always be present at birth, the term “infantile” may be more accurate. The differential diagnosis of infantile strabismus is presented in Box 91.2. Pseudoesotropia is an illusion or apparent deviation; it is not a true deviation. In many infants, the broad, flat nasal bridge and prominent epicanthal folds may obscure a portion of the sclera near the nose and create the appearance of esotropia (Figure 91.1). This illusion resolves as children mature. Symmetric corneal light reflexes or normal cover tests differentiate pseudoesotropia from true esotropia.

Esotropia is among the more common types of childhood strabismus. The constant deviation of infantile esotropia typically is readily apparent because of the large angle of deviation. Affected children usually have good bilateral vision because of the alternation of fixation from 1 eye to the other. Cross-fixation, in which children look to the left with the adducted right eye and to the right with the adducted left eye, may be evident because of the large angle of deviation. Rarely, esotropia may be caused by abducens palsy (ie, palsy of the sixth cranial nerve) in isolation or association with other cranial nerve palsies (eg, Möbius syndrome).

Infantile exotropia is less common than esotropia. Like esotropia, exotropia manifests within the first 6 months after birth and is characterized by a large angle of deviation. Causes of infantile exotropia include trochlear palsy (ie, congenital third nerve palsy, a type of paralytic strabismus) and abnormalities of the bones of the orbit (eg, Crouzon syndrome).

Acquired Strabismus

Acquired strabismus may result from a variety of causes (Box 91.2). Accommodative esotropia and intermittent exotropia are the 2 most common types of acquired strabismus. Accommodative esotropia typically manifests in children between 2 and 4 years of age but may occur as early as 6 months or as late as 8 years. Children with hyperopia use accommodation (ie, attempts to focus) to see clearly. The accommodative reflex is closely linked to convergence; when accommodation occurs, so does convergence. If children have severe hyperopia (ie, farsightedness), the amount of convergence that occurs with accommodation may be severe and may result in the development of esotropia. Such esotropia is usually intermittent initially and only gradually becomes constant. Often, the deviating eye becomes amblyopic.

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