resected, and the true muscle advanced and secured back to the sclera at the proper position. (See Chapter 14, Fig 14-1, and the accompanying text discussion.)

Nystagmus and Esotropia

Several types of nystagmus are associated with esotropia. Fusion maldevelopment nystagmus syndrome (FMNS; also known as latent and manifest latent nystagmus) is a feature of infantile esotropia. Ciancia syndrome (discussed earlier in this chapter) is a severe form of infantile esotropia associated with an abducting nystagmus. Nystagmus blockage syndrome occurs in children with congenital nystagmus who use convergence to “damp,” or decrease the amplitude or frequency of, their nystagmus. See also Chapter 13.

Incomitant Esotropia

The term incomitant esotropia is used when the esodeviation varies in size in different positions of gaze.

Sixth Nerve Palsy

Palsy of the sixth cranial nerve (abducens) occurring within the first 2 months of life is not common and is usually transient. Most cases of suspected congenital sixth nerve palsy are actually infantile esotropia with cross-fixation. Congenital sixth nerve palsy may be difficult to differentiate from esotropic Duane retraction syndrome in young infants (see Chapter 12), as the unique retraction feature of this syndrome may not be evident in an infant who has shallow orbits and who may not track well to extreme side gaze. Esotropic Duane retraction syndrome is a much more frequent cause of persistent abduction deficit in infants than is sixth nerve palsy. A distinguishing characteristic is that for an equal amount of abduction deficit, the deviation in primary position is usually much larger in sixth nerve palsy than it is in esotropic Duane retraction syndrome.

See BCSC Section 5, Neuro-Ophthalmology, for further discussion of sixth nerve palsy.

Pathogenesis

Congenital sixth nerve palsy is usually benign and transient. Some surmise that it is caused by the increased intracranial pressure associated with the birth process. In any case, it usually resolves spontaneously. Sixth nerve palsy occurs much more frequently in childhood than in infancy. Approximately one-third of cases in older children are associated with intracranial lesions and may have other associated neurologic findings. Other cases may be related to infectious or immunologic processes that involve cranial nerve VI. Isolated transient sixth nerve palsy is thought to be caused most commonly by a virus in a child or a microvascular occlusive event in an adult.

Evaluation

Older patients may report diplopia and often have a compensatory head turn toward the side of the paralyzed lateral rectus muscle, which they have adopted to relieve the diplopia. If the child presents soon after onset, vision in the eyes is usually equal. The esotropia increases in gaze toward the paralyzed lateral rectus muscle. Saccadic velocities show slowing of the affected lateral rectus muscle, and active force generation tests document weakness of that muscle. Versions show limited or no abduction of the affected eye (Fig 8-2).

Figure 8-2 Sixth nerve palsy, left eye. A, Right gaze. B, Esotropia in primary position. C, Limited abduction, left eye. D, Still incomplete but further abduction of left eye when fixating, a finding important in the plan for surgical correction. (Courtesy of

Edward L. Raab, MD.)

A careful history should be taken, including antecedent infections, head trauma, hydrocephalus, hypertension and diabetes mellitus in adults, or other possible inciting factors for sixth cranial nerve weakness. In light of the high prevalence of associated intracranial lesions in children with sixth nerve palsy, neurologic evaluation and gadolinium-enhanced magnetic resonance imaging of the head and orbit are usually indicated, even in the absence of other focal neurologic findings.

Management

Patching may be necessary to prevent or treat amblyopia if the child is not using a compensatory head posture or if the child is very young. Press-on prisms are sometimes used to correct diplopia in primary position. Correction of a significant hyperopic refractive error may help prevent the development of an associated accommodative esotropia. Botulinum toxin injection of the ipsilateral medial rectus muscle is sometimes used to decrease the esotropia and prevent secondary contracture. If the deviation does not resolve after 6 months of treatment, surgery may be indicated. Options include horizontal rectus muscle surgery if abduction is at least partially preserved or vertical rectus muscle transposition surgery if abduction is absent (see Chapter 14).

Other Forms of Incomitant Esotropia

Medial rectus muscle restriction may result from thyroid eye disease, medial orbital wall fracture, excessive medial rectus muscle resection, or congenital fibrosis of the extraocular muscles. Duane retraction syndrome and Möbius syndrome begin as paralytic disorders, and secondary restriction of the medial rectus may develop later. Patients with high myopia may develop esotropia due to displacement of the posterior globe between the lateral and superior rectus muscles (see Chapter 12).

For further discussion of these special forms of strabismus, see Chapter 12 of this volume and BCSC Section 5, Neuro-Ophthalmology.