establishing the diagnosis of ocular tilt reaction, the clinician must attend to head position and ocular cyclotorsion.

Periodic alternating skew is a rare disorder producing alternating hypertropia, typically with a 30–60 second periodicity, indicative of a midbrain lesion.

Brodsky MC, Donahue SP, Vaphiades M. Brandt T. Skew deviation revisited. Surv Ophthalmol. 2006;51(2);105–128.

Thalamic Esodeviation

Thalamic esodeviation is an acquired horizontal strabismus that may be observed in patients with lesions near the junction of the diencephalon and midbrain, most often thalamic hemorrhage. The esodeviation may develop insidiously or acutely and, in the case of expanding tumors, may be progressive. It is especially important to consider the possibility of a central nervous system lesion in children being evaluated for strabismus surgery.

Vergence Dysfunction

See Vergence Disorders in Chapter 7.

Nuclear Causes of Diplopia

The third nerve nucleus is actually a nuclear complex that contains subnuclei for 4 extraocular muscles (superior, inferior, medial recti, and inferior oblique), a single subnucleus (central caudal nucleus) for the levator palpebrae muscles, and paired subnuclei (Edinger-Westphal nuclei) for the pupillary constrictor muscles (see Chapter 1). Because the single central caudal nucleus controls both levator palpebrae superioris muscles, and the superior rectus fascicles decussate just after emerging from their subnuclei, lesions of the third nerve nuclear complex affect or spare both upper eyelids and may bilaterally affect the superior rectus muscles. Injury to the third nerve nuclear complex is uncommon but may occur secondary to reduced perfusion through a small, paramedian-penetrating blood vessel, causing unilateral damage to 1 nuclear complex; such lesions are often asymmetric and affect the oculomotor nerve fascicle on one side in addition to the nucleus.

Intraparenchymal lesions of the fourth cranial nerve (either nuclear or intra-axial) are rare, given the relatively short course of this nerve within the brainstem. A lesion of the trochlear nucleus is clinically identical to a fascicular lesion. Microvascular or inflammatory lesions may nonetheless involve the central course of the fourth nerve. On occasion, a fourth nerve palsy may be accompanied by a contralateral Horner syndrome (first-order neuron lesion) because of the proximity of the descending sympathetic pathway to the caudal portion of the nucleus. Demyelinating disease also is a rare cause of isolated fourth nerve palsy.

A selective lesion of the sixth nerve nucleus causes a horizontal gaze palsy and not an isolated abduction paresis in 1 eye; thus, patients may not experience diplopia. This occurs because the sixth nerve nucleus contains 2 populations of neurons: those that innervate (1) the ipsilateral lateral rectus muscle and (2) the internuclear motoneurons, which travel via the medial longitudinal fasciculus to innervate the contralateral medial rectus subnucleus of the oculomotor nuclear complex. Often, ipsilateral upper and lower facial weakness is also present with a nuclear sixth nerve palsy (eg,