Nuclear Causes of Diplopia
The CN III 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 CN III nuclear complex either affect or spare both upper eyelids and may affect the contralateral superior rectus muscle. Injury to the CN III nuclear complex, while uncommon, 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 may affect the CN III fascicle on one side in addition to the nucleus.
Intraparenchymal (either nuclear or intra-axial) lesions of CN IV are rare, given the relatively short course of the nerve within the brainstem. A lesion of the CN IV nucleus is clinically identical to a fascicular lesion. Microvascular, inflammatory, neoplastic, or demyelinating lesions may involve the central course of CN IV. Occasionally, a CN IV 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. A relative afferent pupillary defect may also be associated with a CN IV palsy due to the pupillary fibers running in the nearby brachium of the superior colliculus.
Table 7-1 Supranuclear Ocular Motor Lesions That Produce Strabismus and Diplopia
A selective lesion of the CN VI nucleus causes a horizontal gaze palsy and not an isolated abduction paresis in 1 eye; therefore, patients with this lesion may not experience diplopia. This occurs because the CN VI nucleus contains 2 populations of motoneurons: (1) those that innervate the ipsilateral lateral rectus muscle and (2) those that travel via the medial longitudinal fasciculus to innervate the contralateral medial rectus subnucleus of the CN III nuclear complex. Often, ipsilateral upper and lower facial weakness is also present with a nuclear CN VI palsy due to the adjacent facial nerve fascicle (eg, facial colliculus syndrome).
Eliott D, Cunningham ET Jr, Miller NR. Fourth nerve paresis and ipsilateral relative afferent pupillary defect without visual sensory disturbance. A sign of contralateral dorsal midbrain disease. J Clin Neuroophthalmol. 1991;11(3):169–172; discussion 173–174.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.