Subarachnoid Damage to the Oculomotor Nerve

From the ventral midbrain, through the interpeduncular fossa and to its entry into the cavernous sinus, the third nerve lies in the subarachnoid space, where it is exposed to hemorrhages from aneurysms arising from the supracli-noid carotid artery, mostly at the exit of the posterior communicating artery. Rupture of such an aneurysm produces paralysis of the third nerve, but also the dramatic symptoms of acute subarachnoid bleeds, including abrupt headache of the worst sort, reduced levels of consciousness up to complete coma, and pronounced meningismus. The ophthalmologist will not be confronted by this syndrome in his/her own office.

jl Note

In about one third of cases the course is more gradual, beginning with an incomplete internal and external oculomotor nerve paralysis, which precedes onset of extreme levels of head pain referred to the orbital apex. The ocular presentation may lead the patient to consult the ophthalmologist, who should then immediately arrange for emergent neuroradiologic study. Only a few days following such a presentation can one expect an abrupt subarachnoid hemorrhage, a frequently fatal event that can be avoided with timely surgical management of the aneurysm.

Other causes of damage to the subarachnoid portion of the oculomotor nerve include severe head trauma, in which the nerve can suffer a contusion or traction injury. A post-traumatic supratentorial hemorrhage can also lead to a secondary oculomotor nerve paralysis, in which ventral displacement of the brainstem causes a compression of the nerve on fixed structures at the skull base. Another mechanism for the same type of injury is a space-occupying su-pratentorial process (hemorrhage, tumor) that causes the uncus to herniate, compressing the third nerve against the margin of the tentorium. This type of paralysis begins with an ipsilateral pupillary mydriasis (Hutchinson's pupil). If the process is not controlled, it leads to loss of consciousness and an external ophthalmoplegia.

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