Ocular palsies may have a central cause—that is, a lesion of the nucleus or the intramedullary (fascicular) portion of the cranial nerve—but more often they are peripheral. Weakness of ocular muscles because of a lesion in the brainstem is usually accompanied by involvement of other cranial nerves and by signs referable to the “crossed” brainstem syndromes of a cranial nerve palsy on one side and weakness or other deficits on the opposite side (see Table 33-3 and Chap. 44). Peripheral lesions, which may or may not be solitary, have a great variety of causes.
In the series reported by Rucker (1958, 1966), who analyzed 2,000 cases of paralysis of the oculomotor nerves, the most common sources of individual ocular motor palsies were tumors at the base of the brain or skull (primary, metastatic, meningeal carcinomatosis), head trauma, ischemic infarction of a nerve (generally associated with diabetes), and aneurysms of the circle of Willis, in that order. The sixth nerve was affected in about half of the cases; third-nerve palsies were about half as common; and the fourth nerve was involved in less than 10 percent of cases. In 1,000 unselected cases reported subsequently by Rush and Younge, trauma was a more frequent cause than neoplasm and the frequency of aneurysm-related cases was fewer than in the aforementioned series; otherwise the findings were similar. Less-common causes of paralysis of the oculomotor nerves, but nonetheless seen by most practitioners, include variants of Guillain-Barré syndrome, herpes zoster, giant cell arteritis, ophthalmoplegic migraine, carcinomatous or lymphomatous meningitis, and the granulomatous disease sarcoidosis and Tolosa-Hunt syndrome, as well as fungal, tuberculous, syphilitic, and other forms of mainly chronic meningitis. Myasthenia gravis, discussed in Chap. 46, must always be considered in cases of ocular muscle palsy, particularly if several muscles are involved and if fluctuating ptosis is a prominent feature. Thyroid ophthalmopathy, discussed further on, presents in a similar fashion but usually with proptosis and eyelid retraction and without ptosis. Actually, in the above-mentioned series, no cause could be assigned in 20 to 30 percent, although more cases are now being resolved with MRI.
The third nerve is commonly compressed by aneurysm, tumor, or temporal lobe herniation. In a series of 206 cases of third-nerve palsy collected by Wray and Taylor, neoplastic diseases accounted for 25 percent and aneurysms for 18 percent. Of the neoplasms, 25 percent were parasellar meningiomas and 4 percent were pituitary adenomas. The palsy is usually chronic, progressive, and painless. As emphasized earlier, enlargement of the pupil is a sign of extramedullary third nerve compression because of the peripheral location in the nerve of the pupilloconstrictor fibers. By contrast, infarction of the nerve in diabetics usually spares the pupil, as the damage is situated in the central portion of the nerve. The oculomotor palsy that complicates diabetes (the cause in 11 percent in the Wray and Taylor series) develops over a few hours and is accompanied by pain, which may be severe, in the forehead and around the eye. The prognosis for recovery (as in other nonprogressive lesions of the oculomotor nerves) is usually good.