1.2.2Exodeviations

Exotropia is a disorder of ocular alignment characterized by an outward deviation of the eye or eyes (Fig. 1.2). Exotropia is less common than esotropia among Western populations [1]; however, it is the predominant form of strabismus in the East [2]. Regardless of the relative prevalence, the age at presentation for children with exotropia tends to be older than for those with esotropia [4]. Amblyopia is less commonly associated with exotropia than esotropia [5].

1.2.2.1 Intermittent Exotropia

Intermittent exotropia is an acquired, intermittent deviation of 10 or more PD unassociated with other ocular, paralytic, or neurologic disorders. It is the second most commonly diagnosed form of strabismus (at approximately 17%) in the United States [1] and the most commonly diagnosed subtype of exodeviation with an incidence of 1 in 155 children [9]. In a recent populationbased study, it was reported to occur nearly twice as often in girls compared with boys [10].

1.2.2.2Congenital Exotropia

Congenital exotropia includes children with a constant exodeviation that develops by 6 months of age.Although this condition is rare, many children will have associated neurologic or other disorders and should undergo CNS imaging [11]. This form of exotropia results in amblyopia much more often than other subtypes of divergent strabismus.

1.2.2.3Convergence Insu ciency

Convergence insu ciency describes children who are generally orthotropic at distance fixation but whose eyes do not converge su ciently at near fixation, leaving an

Fig. 1.2 A child with exotropia

exodeviation at near. It is the second most commonly diagnosed type of exodeviation and comprises approximately one in five children with exotropia [9] with an incidence of 1 in 411 children [9]. However, this disorder is likely to be under-diagnosed given the obscure symptoms and relatively imperceptible nature of the deviation to outside observers.

1.2.2.4Abnormal Central Nervous System Exotropia

Exotropic children with a congenital or acquired developmental or neurological disorder may be grouped under CNS defects regardless of the age at onset.Approximately, 15% of children with exotropia may have neurologic abnormalities, most commonly cerebral palsy and developmental delay [9].

1.2.2.5 Sensory Exotropia

Sensory exotropia includes children with a unilateral or bilateral ocular condition that prevents normal fusion, most commonly anisometropic amblyopia or cataract [9]. Children with sensory disturbances are more likely to develop exotropia (24 of 235 children, or 10.2%) than esotropia (15 of 221 children, or 6.8%) [12]. This di erence may be in part due to the age at onset of visual impairment. Havertape and coauthors have shown that children with a unilateral or bilateral visual loss by 6 months of age are more likely to develop sensory esotropia, whereas those with an acquired visual loss are much more likely to develop sensory exotropia [13].

Summary for the Clinician

■Exotropia is the predominant form of strabismus among Asian populations; however, it is less common than esotropia in the West.

■Intermittent exotropia is the most commonly diagnosed form of exodeviation.

■Amblyopia is less commonly associated with exotropia than esotropia.

1.2.3Hyperdeviations

Hypertropia, or a vertical displacement of one eye relative to the other, is the least diagnosed form of strabismus [1]. Nearly one-third of all cases are associated with fourth cranial nerve palsy (Fig. 1.3), corresponding to an incidence of 1 in 1,264 children [14]. Other causes of