Isolated third nerve palsies.

In the diagnostic assessment of patients with N III palsies, the examiner should first endeavor to determine if the palsy is isolated, that is, unassociated with other contributory neurologic findings. If any other abnormalities are present, they should be used to localize the lesion and to help direct ensuing neuroradiologic studies. If the palsy is isolated, the presence of pupil sparing, together with advanced age or a vasculopathic background probably indicates that an extra-axial infarction of the nerve has taken place. The patient may safely be observed periodically without radiographic studies. In younger patients or those without significant vasculopathy, the status of the pupil should not be a major determinant of management. Moreover, the clinician must be aware that pupil sparing is expected when the vulnerable superior division of N III is selectively involved in cavernous sinus compressive lesions. "Pseudo pupil-sparing" in aberrant N III regeneration and in coexisting parasympathetic and sympathetic pupillomotor paresis is a pitfall to be avoided. When one of the muscles subserved by N III appears to be misfunctioning, the diagnosis is rarely that of a partial N III palsy. Instead, the causative lesion is more likely to be in the muscles themselves, the neuromuscular junction, or in the gaze pathways converging on the N III subnuclei.