Extraocular Motor Nerve Palsies

In extraocular motor nerve palsies, dysfunction affects the third, fourth, and sixth cranial nerves. These nerves are responsible for innervating eye movement. The superior branch of the oculomotor (cranial III) nerve innervates the levator superioris muscle of the upper eyelid and the superior rectus muscle of the eye; the inferior branch innervates the interior rectus, the medial rectus, and the inferior oblique muscles. It also supplies the intrinsic pupillary and ciliary body muscles, which control lens shape and accommodation. The trochlear (cranial IV) nerve innervates the superior oblique muscles, which control downward rotation, intorsion, and abduction of the eye. The abducens (cranial VI) nerve innervates the lateral rectus muscles, which control inward movement of the eye.

Causes

The most common causes of extraocular motor nerve palsies include diabetic neuropathy, trauma, and pressure from an aneurysm or a brain tumor. Other causes vary depending on the cranial nerve involved.

Oculomotor (third nerve) palsy. or acute ophthalmoplegia, also results from brain stem ischemia or other cerebrovascular disorders, poisoning (lead, carbon monoxide, botulism), alcohol abuse, infections (measles, encephalitis), trauma to the extraocular muscles, myasthenia gravis, or tumors in the cavernous sinus area.

Trochlear (fourth nerve) palsy also may result from closed-head trauma (for example, a blowout fracture) or sinus surgery.

Abducens (sixth nerve) palsy also results from increased intracranial pressure, brain abscess, cerebrovascular accident, meningitis, arterial brain occlusion, infections of the petrous bone (rare), lateral sinus thrombosis, myasthenia gravis, and thyrotropic exophthalmos.

Diagnostic tests

A patient with extraocular motor nerve palsy needs to supply a full health history and undergo a complete neuroophthalmologic examination before diagnosis is confirmed. Differential diagnosis of third, fourth, or sixth nerve palsy depends on the specific motor defect exhibited by the patient.

Depending on the patient's symproms, blood studies may be ordered to detect diabetes; computed tomography scanning, magnetic resonance imaging, or skull X-rays make be taken to rule out intracranial tumors; and cerebral angiography may be used to evaluate possible vascular abnormalities such as aneurysm.

If sixth nerve palsy results from an infection, culture and sensitivity tests may be used to identify the causative organism and determine therapy.

Treatment

Appropriate treatment varies depending on the cause. For instance, neurosurgery may be necessary for a brain tumor or an aneurysm. For infection, massive doses of I.V. antibiotics may be appropriate. After treatment for the primary condition, the patient may need to perform exercises that stretch the neck muscles to correct acquired torticollis. Other care and treatments depend on residual symptoms.

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