There are several positional, restrictive, and innervational abnormalities of the extraocular muscles that may result in incomitant esotropia (see Table 8-1).
Sixth Nerve Palsy
Weakness of the lateral rectus muscle due to palsy of the abducens nerve results in incomitant esotropia. Sixth cranial nerve palsy occurring in the neonatal period is rare and 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, which is more common in young infants (see Chapter 12), as the unique retraction feature of this syndrome may not yet be evident. 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.
Congenital sixth nerve palsy is usually benign and transient and may be caused by the increased intracranial pressure associated with the birth process. Sixth nerve palsy in older children is associated with intracranial lesions in approximately one-third of cases, which may have additional neurologic findings. Other cases may be related to infectious or immunologic processes involving cranial nerve VI. The most common cause of isolated, transient sixth nerve palsy in a child is thought to be a virus; in an adult, a microvascular occlusive event.
Clinical features and evaluation
Older children and adults may report diplopia. Often there is a compensatory head turn toward the side of the paralyzed lateral rectus muscle, adopted to place the eyes in a position where they are best aligned. If a child presents soon after onset of the deviation, vision in the eyes is usually equal. The esotropia increases in gaze toward the side of the paralyzed lateral rectus muscle, and versions show limited or no abduction of the affected eye (Fig 8-3). Results of the saccadic velocity test show slowing of the affected lateral rectus muscle, and active force generation tests document weakness of that muscle.
A careful history should be taken, including antecedent infections, head trauma, and hydrocephalus, as well as hypertension and diabetes mellitus in adults. In light of the high prevalence of associated intracranial lesions in children with sixth nerve palsy, neurologic evaluation and magnetic resonance imaging of the head and orbit are usually indicated, even in the absence of other focal neurologic findings.
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 employed to temporarily decrease the esotropia. 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).
Figure 8-3 Sixth nerve palsy, left eye. A, Right gaze. B, Esotropia in primary position. C, Limited abduction, left eye. D, Abduction is still incomplete, but there is further abduction when the left eye is fixating, a finding that is important in the plan for surgical correction.
(Courtesy of Edward L. Raab, MD.)
See BCSC Section 5, Neuro-Ophthalmology, for further discussion of sixth nerve palsy.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.