Sunday, July 16, 2006

Sixth nerve palsy

Sixth nerve palsies may be complete, with a complete esotropia and abduction defect or partial. The differential includes restrictive opthalmopathy or orbitopathy affecting the medial rectus muscle. In sixth nerve palsy but not restrictive eye disease there are characteristic slowing of abducting saccades.

Although the differential is extremely broad due to the length of the nerve, there are some helpful findings. Bilateral sixth nerve palsies are common at the clivus. Other cranial neuropathies (third, fourth, first division of fifth, or an ipsilateral Horner's) suggest a cavernous sinus lesion. Headache and pappilledema suggest a "false localizing" sign if increased intracranial pressure.

Among children, the commonest etiologies are trauma and tumor, usually a pontine glioma. Among 45 young adults without trauma,aged (20-50), one third have tumor, one fourth have multiple sclerosis, 9 % have a postviral infection, 7 % have IIH, and 7 % have meningitis (Peters et al.) Among the 15 tumor patients, only 3 had an isolated sixth nerve palsy, and among 11 patients with multiple sclerosis, an isolated sixth was the presenting sign in 8 of them. Thus MRI/LP are frequently appropriate in this group.

Among older adults, especially those with vascular risk factors, a vasculopathic or ischemic cause is the commonest. Beware of a patient with a history of malignancy. Beware of GCA; consider checking a sed rate and C reactive protein. Some opthalmalogists do not recommend neuroimaging for all patients in this group, but they do suggest careful followup, and imaging if the problem progresses after 2 weeks, fails to regress by two months, or other neurologic signs and symptoms occur. BILATERAL SIXTH NERVE PALSY IS NEVER VASCULOPATHIC. Clival bony lesions may be missed on quality neuroimaging, and sometimes lumbar puncture is required to identify increased intracranial pressure or meningitis. Consider metastases!

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