is not known why some persons become esotropic and others exotropic after unilateral vision loss. Although both sensory esotropia and sensory exotropia occur in infants and young children, exotropia predominates in older children and adults. If the vision in the exotropic eye can be improved, peripheral fusion may sometimes be reestablished after surgical realignment, provided the sensory exotropia has not been present for an extended period. Loss of fusional abilities, known as central fusion disruption, can lead to constant and permanent diplopia when adult-onset sensory exotropia has been present for several years before vision rehabilitation and realignment. In these patients, intractable diplopia may persist, even with well-aligned eyes.

Consecutive Exotropia

Exotropia that occurs after a period of esotropia is called consecutive exotropia. In rare cases, exotropia may spontaneously develop in a patient who was previously esotropic but who has not had surgery. Much more commonly, consecutive exotropia develops after previous surgery for esotropia (postsurgical exotropia), usually developing within a few months or years after the initial surgery. However, in some patients with infantile esotropia, the exotropia may not develop until adulthood. Treatment of postsurgical exotropia depends on many factors, including the type and amount of previous surgery, the presence of duction limitations, and lateral incomitance.

Other Forms of Exotropia

Exotropic Duane Retraction Syndrome

Duane retraction syndrome can present with exotropia, usually accompanied by deficient adduction and a head turn away from the affected eye. See Chapter 12 for further discussion.

Neuromuscular Abnormalities

A constant exotropia may result from third nerve palsy, internuclear ophthalmoplegia, or myasthenia gravis. These conditions are discussed in Chapter 12 and in BCSC Section 5, Neuro-Ophthalmology.

Dissociated Horizontal Deviation

Dissociated strabismus complex may include vertical, horizontal, and torsional components (see Chapter 11). When a dissociated abduction movement is predominant, the condition is called dissociated horizontal deviation (DHD). Though not a true exotropia, DHD can be confused with a constant or intermittent exotropia. Dissociated vertical deviation and latent nystagmus often coexist with DHD (Fig 9-2). In rare cases, patients may manifest both DHD and intermittent esotropia. DHD must be differentiated from anisohyperopia associated with intermittent exotropia. In these patients, the exotropic deviation is present when the patient fixates with the normal eye but is absent during fixation with the hyperopic eye because of accommodation. Treatment of DHD usually consists of unilateral or bilateral lateral rectus recession in addition to any necessary oblique or vertical muscle surgery.

Figure 9-2 Dissociated strabismus complex. A, When the patient fixates with the left eye, a prominent dissociated vertical deviation is shown in the right eye. B, However, when the patient fixates with the right eye, a prominent dissociated horizontal

deviation is shown in the left eye. (Reproduced from Wilson ME. Exotropia. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 1995, module 11.)

Convergence Paralysis

Convergence paralysis is a condition distinct from convergence insufficiency and usually secondary to an intracranial lesion. It is characterized by normal adduction and accommodation, with exotropia and diplopia present at attempted near fixation only. Convergence paralysis is most commonly associated with dorsal midbrain syndrome (see BCSC Section 5, Neuro-Ophthalmology). Patients appearing to have convergence paralysis due to malingering or lack of effort may be distinguished by the absence of a pupillary reaction when they are instructed to focus at near.

Treatment of convergence paralysis is difficult and often limited to use of base-in prisms at near to alleviate the diplopia. Plus lenses may be required if accommodation is limited. Monocular occlusion is indicated if diplopia cannot be otherwise treated.