Third nerve palsies that result from head injuries or intracranial aneurysms may sometimes show symptoms of aberrant regeneration and only partially heal. The usual characteristics of oculomotor nerve palsy, caused by compression of the third cranial nerve, are severe ptosis, deficiencies in elevation, depression, and adduction, and a dilated, poorly responding pupil on the afflicted side. The parasympathetic fibres are usually spared from a vasculopathic lesion and impacted by compressive lesions because they are situated in the peripheral segment of the oculomotor nerve as it leaves the brain stem. When pupillary mydriasis coexists with acute third cranial nerve palsy, an aneurysm at the junction of the internal carotid and posterior communicating arteries has to be thoroughly and quickly explored using the necessary neuroimaging techniques.
