The abducens nerve innervates the lateral rectus muscle and is the most common single muscle palsy, causing loss of abduction and resultant horizontal diplopia, worse in ipsilateral gaze. Associated findings are dependent on the location of the lesion. Within the pons, involvement of the corticospinal tract results in contralateral hemiparesis. The abducens has the longest intracranial course of any nerve, and therefore is vulnerable to stretching or compression secondary to elevated intracranial pressure, trauma, neurosurgical manipulation, and cervical traction. Also, any meningeal process (infectious, inflammatory, or neoplastic) can affect this portion of the 6th nerve. Aneurysmal compression is uncommon.
Prior to entering the cavernous sinus, the nerve crosses the petrous portion of the temporal bone. Trauma with temporal bone fracture can result in a combination of sixth- and seventh-nerve palsies. Cavernous sinus pathology is suggested by the involvement of the internal carotid artery, venous drainage of the eye and orbit, trochlear and oculomotor nerves, the 1st division of the trigeminal nerve, and the ocular sympathetics. Microvascular changes secondary to diabetes, hypertension, and giant cell arteritis can compromise function.
Management and Disposition
Associated signs and symptoms guide the ED workup. CT or MRI is indicated if brain stem or cavernous sinus involvement is suspected. Pathology localizing to the subarachnoid space should prompt consideration for CT scanning and subsequent spinal tap. In the elderly, an isolated 6th-nerve palsy is likely ischemic, transient, and not indicative of underlying neurologic disease. In these cases, a glucose and erythrocyte sedimentation rate are appropriate; these patients can be followed as outpatients provided close follow-up is arranged.
There is no treatment for the palsy itself except for patching the affected eye if diplopia is bothersome.
An isolated sixth-nerve palsy is commonly due to microvascular disease, not an aneurysm.
Basilar skull fractures of the temporal bone are capable of producing a sixth-nerve palsy.
A sixth-nerve palsy associated with a Horner is usually localized to the cavernous sinus, since sympathetic fibers, as they traverse from the internal carotid artery to the oculomotor nerve, may briefly accompany the abducens nerve.
Sixth-Nerve Palsy. Loss of abduction of the left eye is seen in lateral gaze. (Photo contributor: Frank Birinyi, MD.)
Sixth-Nerve Palsy. Isolated sixth-nerve palsy of the right eye after acute head trauma. (Photo contributor: Kevin J. Knoop, MD, MS.)