In chronic compressive lesions of the third nerve (distal carotid, basilar, or, most commonly, posterior communicating artery aneurysm; pituitary tumor, meningioma, cholesteatoma) the pupil is almost always affected by way of dilatation or reduced light response. However, the chronicity of the lesion may permit aberrant nerve regeneration. This is manifest by pupillary constriction on adduction of the eye or by retraction of the upper lid on downward gaze or adduction.
Rarely, children or young adults have recurrent attacks of ocular palsy in conjunction with an otherwise typical migraine (ophthalmoplegic migraine). The muscles (both extrinsic and intrinsic) innervated by the oculomotor, or less commonly, by the abducens nerve, are affected. Possibly, spasm of the vessels supplying these nerves or compression by edematous arteries causes a transitory ischemic paralysis but these are speculations. Arteriograms done after the onset of the palsy usually disclose no abnormality. Although the oculomotor palsy of migraine tends to recover, after repeated attacks there may be permanent partial paresis.
A fair number of cases of fourth nerve palsies remain idiopathic even after careful investigation. The fourth nerve is particularly vulnerable to head trauma (this was the cause in 43 percent of 323 cases of trochlear nerve lesions collected by Wray from the literature). The reason for this vulnerability has been speculated to be the long, crossed course of the nerves. The fourth and sixth nerves are practically never involved by aneurysm. Herpes zoster ophthalmicus may affect any of the ocular motor nerves but particularly the trochlear, which shares a common sheath with the ophthalmic division of the trigeminal nerve. Diabetic infarction of the fourth nerve occurs, but far less frequently than infarction of the third or sixth nerves. Trochlear-nerve palsy may also be a false localizing sign in cases of increased intracranial pressure, but again, not nearly as often as abducens palsy. Trochlear-nerve palsies have been described in patients with lupus erythematosus and with Sjögren syndrome, but their basic pathology is not known. Some cases of fourth-nerve palsy are idiopathic and most of these resolve.
Superior oblique myokymia is an unusual but easily identifiable movement disorder, characterized by recurrent episodes of vertical diplopia, monocular blurring of vision, and a tremulous sensation in the affected eye; in this way it simulates a palsy. If the episodes occur during the examination, the globe is observed to make small arrhythmic torsional movements. The problem is usually benign and responds to carbamazepine. Compression of the fourth nerve by a small looped branch of the basilar artery has been suggested as the cause, analogous to several other better documented vascular compression syndromes affecting cranial nerves. This notion is supported by findings on MRI reported by Yousry and colleagues. Rare instances presage pontine glioma or demyelinating disease.
Microvascular disease is a common cause of sixth nerve palsy in diabetics, in which case there is usually pain near the lateral canthus of the eye at the onset. An idiopathic form that occurs in the absence of diabetes is also well known. Isolated unilateral or bilateral sixth nerve palsy with global headache can be the initial manifestation of raised intracranial pressure from any source—including brain tumor, meningitis, and pseudotumor cerebri; rarely, it may appear after lumbar puncture, epidural injections, or insertion of a ventricular shunt. In children, the most common tumor involving the sixth nerve is a pontine glioma; in adults, it is tumor arising from the nasopharynx.