Abducens nerve palsy from an occult high flow carotid cavernous fistula.

A 37-year-old man with a one-year history of hypertension noted pulsatile tinnitus and binocular horizontal diplopia. On examination he had left abduction weakness without proptosis or orbital congestion. Neuroimaging revealed a spontaneous high flow posterior draining carotid cavernous fistula and an internal carotid artery dissection. The mechanism of fistula formation most likely resulted from the carotid dissection with pseudoaneurysm formation and rupture into the cavernous sinus. This case illustrates a unique cause of an abducens palsy due to an occult carotid cavernous fistula. Cranial neuroimaging techniques are discussed, including computed tomography angiography, a relatively new modality used to image intracranial blood vessels.