Excerpt from Abducens Nerve Palsy

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Background

Cranial nerve VI, also known as the abducens nerve, innervates the ipsilateral lateral rectus (LR), which functions to abduct the ipsilateral eye. It has the longest subarachnoid course of all the cranial nerves; therefore, its syndromes are similar to those of the fourth nerve because of their long intracranial courses. The sixth nerve nucleus is located in the pons, just ventral to the floor of the fourth ventricle and just lateral to the medial longitudinal fasciculus (MLF). About 40% of its neurons protect into the ipsilateral MLF only to cross over to the contralateral side and ascend to innervate that contralateral medial rectus subnucleus to participate in contralateral eye adduction.

Patients usually present with binocular horizontal diplopia (double vision producing a side-by-side image with both eyes open) and esotropia in primary gaze. The deviation, as would be expected, is noted to be greater when the patient fixates with the paretic eye at distance. Patients also may present with a head-turn to maintain binocularity and binocular fusion and to minimize diplopia.

It is rare to find true congenital sixth nerve palsy. A typical workup of a sixth nerve palsy involves excluding paresis of other cranial nerves (including VII and VIII), a check of ocular muscle motility and evaluating pupillary responsiveness. Checking deep tendon reflexes (DTRs) and motor function to exclude corticospinal tract involvement also is important. MRI is indicated for any brainstem findings to exclude pontine glioma in children (most have papilledema and nystagmus without other cranial nerve involvement) and in adults that show no improvement.

In young adults, a lumbar puncture (LP) for cerebrospinal fluid (CSF) analysis is completed to exclude meningitis in patients who have no history of diabetes or hypertension and who have a negative head CT scan. Senior patients should have blood testing for an erythrocyte sedimentation rate (ESR) and/or a C-reactive protein to screen for giant cell (temporal, cranial) arteritis. Poor or no resolution should prompt a full neurologic evaluation and a consideration of other possible diagnoses (eg, congenital esotropia, Möbius syndrome, Duane syndrome).

Pathophysiology

Only the ipsilateral lateral rectus that is solely innervated by the involved peripheral sixth cranial nerve is affected; therefore, only deviations in the horizontal plane are produced. In isolated cases of peripheral nerve lesions, no vertical or torsional deviations are present. Central nervous system lesions of the abducens nerve tract are localized easily secondary to the typical findings associated with each kind of lesion. Damage to the sixth nerve nucleus results in an ipsilateral gaze palsy. The lack of a contralateral adduction defects makes it easy to differentiate nuclear from a fascicular or nonnuclear lesion.

Abducens palsy frequently is seen as a postviral syndrome in younger patients and as an ischemic mononeuropathy in the adult population.

Frequency

United States

Sixth nerve palsies fall into the following categories: 8-30% idiopathic, 10-30% miscellaneous, 3-30% trauma, 0-6% aneurysm, and 0-36% ischemic.

The sixth cranial nerve is the most commonly affected of the ocular motor nerves. In children, it is second most common after the fourth nerve, with an incidence of 2.5 cases per 100,000 in the population.

Mortality/Morbidity

A young patient should have an aggressive workup because of the greater likelihood of a neoplasm causing the palsy. Patients older than 40 years require a less aggressive workup because of the greater likelihood of the etiology being more benign in nature (eg, ischemic mononeuropathy, vasculopathy).

Age

Cranial nerve VI palsy can occur in all age groups; however, the etiology varies depending on the age group.

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