Sixth Nerve (Abducens Nerve) Palsy

The abducens nerve  (Cranial Nerve VI) 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.

Etiology: 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 sixth 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 first division of the trigeminal nerve, and the ocular sympathetics.

Microvascular changes secondary to diabetes, hypertension, and giant cell arteritis can compromise
function.

                  Sixth-Nerve Palsy. Loss of abduction of the left eye is seen in lateral gaze.

Management:
Associated signs and symptoms guide the 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 sixth-nerve palsy is likely ischemic, transient, and not indicative of underlying neurologic disease. In these cases, a glucose and erythrocyte sedimentation rate is 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.

Clinical Pearls 

1. An isolated sixth-nerve palsy is commonly due to microvascular disease, not an aneurysm.

2. Basilar skull fractures of the temporal bone are capable of producing a sixth-nerve palsy.

3. 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