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Extraocular Motor Nerve Palsies

Dysfunctions of the third, fourth, and sixth cranial nerves are called extraocular motor nerve palsies. Each of these nerves innervates specific muscles.

• The oculomotor (third cranial) nerve innervates the inferior, medial, and superior rectus muscles; the inferior oblique extraocular muscles; the pupilloconstrictor muscles; and the levator palpebrae muscles.
• The trochlear (fourth cranial) nerve innervates the superior oblique muscles.
• The abducens (sixth cranial) nerve innervates the lateral rectus muscles.

The superior oblique muscles control downward rotation, intorsion, and abduction of the eye. Complete dysfunction of the third cranial nerve is called total oculomotor ophthalmoplegia and may be associated with other central nervous system abnormalities.

Causes

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

• Third-nerve (oculomotor) palsy (acute ophthalmoplegia) may be congenital or acquired and causes diplopia. It may result from an aneurysm (particularly in the posterior communicating artery); microvascular disease, such as diabetes or hypertension; tumor; or trauma. Rare causes include uncal herniation, cavernous sinus mass lesion, orbital disease, herpes zoster, and leukemia. In children, a cause may be ophthalmic migraine.
• Fourth-nerve (trochlear) palsy also results from closed head trauma (blowout fracture) or sinus surgery.
• Sixth-nerve (abducens) palsy more commonly results from increased intracranial pressure; vasculopathic entities, such as diabetes, hypertension, and atherosclerosis; trauma; and idiopathic thyroid disease. Less common causes include giant cell arteritis, cavernous sinus mass (meningiomas, aneurysms, or metastasis), multiple sclerosis, and cerebrovascular accident.

Signs and symptoms

The most characteristic clinical effect of extraocular motor nerve palsies is diplopia of recent onset, which varies in different visual fields, depending on the muscles affected.

• Typically, the patient with third-nerve palsy exhibits ptosis, exotropia (in which the eye looks outward), pupil dilation, and unresponsiveness to light; the eye is unable to move and cannot accommodate.
• The patient with fourth-nerve palsy displays diplopia and an inability to rotate the eye downward or upward. The patient develops a head tilt to compensate for vertical diplopia.
• Sixth-nerve palsy causes one eye to turn; the eye cannot abduct beyond the midline. To compensate for diplopia, the patient turns his head to the unaffected side.

Diagnosis

A complete neuro-ophthalmologic examination and thorough patient history are needed to diagnose these palsies. Differential diagnosis of third-, fourth-, or sixth-nerve palsy depends on the specific motor deficit exhibited by the patient.

Treatment

Identification of the underlying cause is essential because treatment of extraocular motor nerve palsies varies accordingly. Neurosurgery is necessary if the cause is an intracranial tumor or an aneurysm. For giant cell arteritis, high­dose corticosteroids are given I.V.; this is called pulse therapy.