| CRANIAL NERVE VI PALSY SIGNS AND SYMPTOMS
The patient will present with horizontal uncrossed diplopia which worsens
at distance and in either right or left gaze. The patient will have an abduction deficit
in the involved eye and either an esophoric or esotropic position which is non-comitant.
The onset is typically sudden. If the palsy is isolated, there will be neither visual
acuity nor visual field loss. There may be some degree of head pain. The patient typically
is older and has a concurrent history of hypertension and/or diabetes. The fundus is
typically normal, except when the patient manifests a bilateral cranial nerve VI palsy,
where there typically will also be papilledema.
PATHOPHYSIOLOGY
Cranial nerve VI arises in the pons, in close association with the facial
nerve and paramedian pontine reticular formation (PPRF). Due to this arrangement, damage
to the sixth nerve within the brain stem will produce a sixth nerve palsy as well as a
facial nerve palsy or an internuclear ophthalmoplegia. Associated findings may also
include leg paralysis with sixth nerve palsy (Raymond's syndrome), or leg paralysis,
facial paralysis and sixth nerve palsy (Millard-Gubler syndrome). These additional
findings identify the location of damage as the pons, where ischemic infarct, tumor and
demyelinization are the common causes.
The sixth nerve travels through the subarachnoid space where it ascends
the clivus and enters the cavernous sinus. Within the subarachnoid space, the sixth nerve
may be stretched against the clivus as the brain stem herniates through the foramen magnum
due to increased intracranial pressure. This will give a bilateral sixth nerve palsy
(which is often intermittent) and papilledema. As the sixth nerve passes over the petrous
apex of the temporal bone, damage here can result in a sixth nerve palsy, facial pain and
hearing loss. This occurs due to inflammation of the temporal bone (Gradenigo's syndrome)
or nasopharyngeal carcinoma.
Within the cavernous sinus, the sixth nerve is joined by the
oculosympathetic nerves, and cranial nerves III, IV and V-1. Damage here will yield a
sixth nerve palsy and Horner's syndrome, as well as a concurrent CN III and IV palsy. The
etiology may be aneurysm, meningioma, pituitary adenoma, inflammation, or fistula. The
sixth nerve is also vulnerable to ischemic infarct from diabetes and hypertension; this
remains a prime cause of isolated sixth nerve palsy.
MANAGEMENT
A sixth nerve palsy combined with any of the above mentioned neurological
signs indicates a need for MRI of the appropriate area. In children, sixth nerve palsy
often occurs from a presumed viral cause and has an excellent prognosis. However, if the
palsy does not recover, or worsens over several weeks, the child should be examined for a
pontine glioma. In the adult under 50 years, obtain MRI studies of the brain. Adult over
50 with an isolated sixth nerve palsy require a workup for ischemic vascular diseases such
as diabetes and hypertension. If the patient is over the age of 65 years, order an
erythrocyte sedimentation rate (ESR) to rule out giant cell arteritis.
If no etiology is discovered on MRI or hematology studies, monitor the
patient monthly for several months until resolution (or until other signs develop which
would indicate an etiology). The vast majority of CN VI palsies due to ischemic
vasculopathy (or idiopathic etiology) will resolve without treatment in three to six
months. Fresnel prism correction or unilateral occlusion will temporarily alleviate the
diplopia.
CLINICAL PEARLS
Myasthenia gravis may mimic a sixth nerve palsy and
should always be considered in the differential diagnosis, especially if the palsy takes
on a variable course with exacerbations and remissions. Always consider the possibility of
myasthenia gravis in cases of non-restrictive, pupil sparing CN III, IV and VI nerve palsy
as well as unilateral and bilateral internuclear ophthalmoplegia.
Other reports in this section
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