Sixth Cranial Nerve Palsy
CN VI is the most frequently affected nerve in an isolated ocular motor palsy. CN VI palsy presents as horizontal diplopia that worsens on ipsilateral gaze, correlating with an abduction deficit and esodeviation that increases with gaze to the affected side (Fig 7-12). A pattern of divergence insufficiency or paralysis may occur during the evolving or resolving phase of a CN VI palsy (see Chapter 8).
An ischemic mononeuropathy is the most common cause of an isolated CN VI palsy. Lesions of the cerebellopontine angle (especially acoustic neuroma or meningioma) may involve CN VI and other contiguous CNs, causing decreased facial and corneal sensation (CN V), facial paralysis (CN VII), and decreased hearing with vestibular signs (CN VIII). Chronic inflammation of the petrous bone may cause an ipsilateral abducens palsy and facial pain (Gradenigo syndrome), especially in children with recurrent middle ear infections. After exiting the prepontine space, CN VI is vulnerable to meningeal or skull-based processes, such as meningioma, nasopharyngeal carcinoma, chordoma, or chondrosarcoma. In addition, CN VI is susceptible to injury from shearing forces of head trauma or elevated intracranial pressure. In such cases, injury occurs where CN VI enters the cavernous sinus through the Dorello canal (the opening below the petroclinoid ligament). The appearance of a CN VI palsy after seemingly minor head trauma should raise concern for a preexisting pathology, such as tumor compression, that makes the nerve more susceptible to injury.
Figure 7-12 Left CN VI palsy. A, In right gaze, the eyes are aligned. B, In straight-ahead gaze, the left eye is inwardly deviated. C, In left gaze, the left eye does not abduct, causing a marked misalignment of the eyes.
(From Trobe JD. The Physician’s Guide to Eye Care. 3rd ed. San Francisco: American Academy of Ophthalmology; 2006:118. Image courtesy of W. K. Kellogg Eye Center, University of Michigan.)
Congenital CN VI palsies almost never occur in isolation. An abduction paresis that is present early in life usually manifests as Duane syndrome (see BCSC Section 6, Pediatric Ophthalmology and Strabismus, Chapter 12).
Isolated CN VI palsies in adults older than 50 years are usually ischemic. Ocular motility in such cases usually improves over time and typically resolves within 6 months. Whether or not neuroimaging is required at the time of initial diagnosis is controversial. Some experts recommend neuroimaging at the time of initial presentation. As with other isolated ocular motor cranial nerve palsies, medical evaluation is appropriate. However, a cranial MRI is mandatory if obvious improvement has not occurred after 3 months. Other diagnostic studies that may be required include lumbar puncture, chest imaging, and hematologic studies to identify an underlying systemic process such as syphilis, sarcoidosis, collagen vascular disease, or GCA. Recovery does not necessarily indicate a benign cause. Occasionally, a CN VI palsy will resolve spontaneously and then recur as a manifestation of an intracranial tumor or ocular myasthenia gravis.
Impaired abduction in patients younger than 50 years requires careful scrutiny because few such cases are caused by ischemic cranial neuropathy. Younger patients should undergo appropriate neuroimaging. A CN VI palsy may be the only presenting ocular sign of a posteriorly draining carotid-cavernous fistula (eg, white eye shunt syndrome, discussed later in this chapter). If imaging results are negative, consideration should be given to evaluation for (1) neuromuscular junction disease by obtaining acetylcholine receptor antibody titers or performing edrophonium testing; (2) for mechanical pathophysiologies, such as TED with medial rectus muscle involvement; and (3) for meningeal-based disease by obtaining a lumbar puncture. Leukemia and brainstem glioma are important considerations in children. In adolescents and young adults, demyelination may be the cause, in which case MRI with fluid-attenuated inversion recovery (FLAIR) imaging typically reveals T2 hyperintensities consistent with multiple sclerosis. (See Chapter 2 for a discussion of neuroimaging and Chapter 14 for a discussion of multiple sclerosis.) Other potential CN VI palsy mimics include Duane syndrome (type 1), spasm of near reflex, myasthenia gravis, TED, and medial orbital wall fracture with entrapment.
Tamhankar MA, Biousse V, Ying GS, et al. Isolated third, fourth, and sixth cranial nerve palsies from presumed microvascular versus other causes: a prospective study. Ophthalmology. 2013;120(11):2264–2269.
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.