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Sixth Cranial Nerve Palsy
(Abducens Nerve Palsy)
Sixth cranial nerve palsy affects the lateral rectus muscle, impairing eye abduction. The eye may be slightly adducted when the patient looks straight ahead. The palsy may be secondary to nerve infarction, Wernicke encephalopathy, trauma, infection, or increased intracranial pressure, or it may be idiopathic. Determining the cause requires MRI and often lumbar puncture and evaluation for vasculitis.
Sixth cranial (abducens) nerve palsy typically results from small-vessel disease, particularly in diabetics as part of a disorder called mononeuritis multiplex (multiple mononeuropathy). it may result from ischemia, hypertension (sometimes), or compression of the nerve by lesions in the cavernous sinus (eg, nasopharyngeal tumors), orbit, or base of the skull. The palsy may also result from increased intracranial pressure, head trauma, or both. Other causes include meningitis, meningeal carcinomatosis, Wernicke encephalopathy, aneurysm, vasculitis, multiple sclerosis, pontine stroke, and, rarely, low CSF pressure headache (eg, after lumbar puncture). Children with respiratory infection may have recurrent palsy. However, the cause of an isolated 6th cranial nerve palsy is often not identified.
Symptoms include binocular horizontal diplopia when looking to the side of the paretic eye. Because the tonic action of the medial rectus muscle is unopposed, the eye is slightly adducted when the patient looks straight ahead. The eye abducts sluggishly, and even when abduction is maximal, the lateral sclera is exposed. With complete paralysis, the eye cannot abduct past midline.
Palsy resulting from nerve compression by a hemorrhage (eg, due to head trauma or intracranial bleeding), a tumor, or an aneurysm in the cavernous sinus causes severe head pain, chemosis (conjunctival edema), anesthesia in the distribution of the 1st division of the 5th cranial nerve, optic nerve compression with vision loss, and paralysis of the 3rd, 4th, and 6th cranial nerves. Both sides are typically affected, although unevenly.
A 6th nerve palsy is usually obvious, but the cause is not. If retinal venous pulsations are seen during ophthalmoscopy, increased intracranial pressure is unlikely. CT is often done because it is often immediately available. However, MRI is the test of choice; MRI provides greater resolution of the orbits, cavernous sinus, posterior fossa, and cranial nerves. If imaging results are normal but meningitis or increased intracranial pressure is suspected, lumbar puncture is done.
If vasculitis is suspected clinically, evaluation begins with measurement of ESR, antinuclear antibodies, and rheumatoid factor. In children, if increased intracranial pressure is excluded, respiratory infection is considered.
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