Friday, October 28, 2005

Sixth Cranial Nerve

Cranial nerve VI, the abducens nerve, innervates the lateral rectus muscle which abducts the eye (move away from the midline). The abducens nerve emerges from the ipsilateral abducens nucleus between the caudal pons beneath the floor of the fourth ventricle and the medulla (the pontomedullary junction). The abducens nerve exits the skull through the superior orbital fissure (one of the holes in the skull behind the eye).
Looking for a 6th nerve palsy is a good screening sign in children with suspected meningitis. As the abducens emerges near the bottom of the brain, it is often the first nerve compressed when there is any rise in intracranial pressure. The lateral rectus muscle of the eye that the abducens nerve innervates is opposed by the action of the medial rectus muscle. Damage to the abducens nerve causes medial strabismus as the individual is no longer able to control lateral eye movement via the lateral rectus motor neurons.

A sixth nerve palsy can often be localized by the accompanying symptoms. The patient with an inability to move either eye to the affected side may have a lesion in the abducens nucleus. This results from loss of motor neurons or interneurons that project to the contralateral medial longitudinal fasciculus.
In the ventral pons a lesion in the fasciculus of the 6th nerve may also involve an adjacent pyramidal tract to produce a contralateral hemiplegia.
Increased intracranial pressure may stretch the 6th nerve in the subarachnoid portion that exits in the brainstem and travels upward to the dural attachment at the clivus. Cerebral edema for example may cause a downward displacement of the brainstem and may damage the nerve or simple swelling may compress the nerve. Nausea and vomiting associated with a sixth nerve palsy should prompt one to think of this mechanism.
Within the cavernous sinus the sixth nerve travels with sympathetic fibers and may produce a post ganglionic Horner's syndrome to produce anisocoria. The opposite pupil will seem dilated. Alternatively the 3rd nerve lesion may accompany a 6th nerve lesion in the cavernous sinus in such things as Tolosa Hunt syndrome (inflammation in the cavernous sinus associated with ophthalmoplegia).


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