Sixth nerve palsy

Cause

The nerve's emergence near the brain's base makes it vulnerable to compression from increased intracranial pressure. In adults, common causes include vasculopathic conditions (diabetes, hypertension, atherosclerosis), trauma, and idiopathic factors; less common causes include giant cell arteritis, cavernous sinus masses, multiple sclerosis, and stroke. In children, typical causes are traumatic, neoplastic (especially brainstem glioma), and idiopathic; benign, rapidly recovering isolated VIth nerve palsy may occur, sometimes after ear, nose, or throat infections.

Pathophysiology

The traditional explanation for sixth nerve palsy with increased intracranial pressure involves stretching of the nerve along its long intracranial course or compression against the petrous ligament or petrous temporal bone ridge. Collier rejected this view, arguing that the nerve's straight forward emergence from the brain stem makes it more susceptible to backward brain stem displacement from space-occupying lesions. Isolated lesions of the VI nerve nucleus cause ipsilateral gaze palsy rather than isolated VIth nerve palsy, because paramedian pontine reticular formation fibers pass through the nucleus to the opposite IIIrd nerve nucleus.

Involvement of adjacent seventh cranial nerve fibers produces ipsilateral facial palsy. Millard–Gubler syndrome involves a unilateral pontine softening from blood vessel obstruction, affecting the sixth and seventh cranial nerves and the corticospinal tract, causing VIth nerve palsy, ipsilateral facial paresis, and contralateral hemiparesis. Foville's syndrome arises from brainstem lesions affecting the Vth, VIth, and VIIth cranial nerves. In the subarachnoid space, the nerve lies near the anterior inferior and posterior inferior cerebellar and basilar arteries, making it vulnerable to compression against the clivus, typically with headache or increased ICP.

The nerve's proximity to the mastoid sinus can lead to mastoiditis and meningeal inflammation, producing Gradenigo's syndrome with VIth nerve palsy, ipsilateral hearing reduction, facial pain, paralysis, and photophobia; similar symptoms may follow petrous fractures or nasopharyngeal tumors.

Diagnosis

In adults, diagnosis is rarely difficult: onset is sudden with horizontal diplopia, and eye movement limitation is confined to abduction of the affected eye (or both eyes if bilateral). The convergent squint (esotropia) is larger on distance fixation than near fixation. Abduction limitations mimicking VIth nerve palsy may result from surgery, trauma, myasthenia gravis, or thyroid eye disease. In children, cooperation with examination is challenging; alternative diagnoses for abduction deficit include Mobius syndrome (bilateral VIth and VIIth nerve palsy with expressionless face), Duane syndrome (partial innervation of lateral rectus by IIIrd nerve branches affecting both abduction and adduction), cross fixation from infantile esotropia or nystagmus blockage syndrome, and iatrogenic injury from halo orthosis placement.

Management

Initial management aims to identify and treat the cause and relieve symptoms. In children, who rarely experience diplopia, the goal is maintaining binocular vision for proper visual development. A 6-month observation period is appropriate before further intervention, as some palsies recover without surgery. Fresnel prisms attached to glasses can compensate for inward misalignment, but they correct only a fixed deviation, so diplopia may persist when looking to the affected side; large deviations may require prism thickness that reduces vision, making temporary occlusion of one eye more appropriate.

Occlusion is never used in infants due to risk of stimulus deprivation amblyopia and because they do not experience diplopia. Botulinum toxin injected into the ipsilateral medial rectus serves several purposes: it prevents contracture of the medial rectus from unopposed action, temporarily reduces deviation to allow prismatic correction, and reveals whether the palsy is partial or complete by permitting residual lateral rectus movement. Thus, botulinum toxin works therapeutically to reduce symptoms and enhance postoperative ocular movements, and diagnostically to determine the most appropriate operation.

Did You Know?

• Patients sometimes adopt a face turned toward the affected side to control diplopia and maintain binocular vision.
• Children may suppress the esotropic eye's input due to neuroplasticity, avoiding diplopia but risking amblyopia (lazy eye).