Following surgery of an intracranial mass lesion, a 60-year-old woman complained of a droopy left eyelid and diplopia.

Our patient had a recent craniotomy for an intracranial mass. On examination, she had complete left eyelid ptosis. Upon lifting her drooped eyelid, she had exotropia (lateral deviation) and hypotropia (downward deviation) of the left eye. She also had anisocoria; the left pupil measured 5 mm in diameter, and the right pupil measured 2 mm in diameter. There was no constriction directly or consensually of the left pupil. She had impaired adduction (medial rectus muscle), supraduction (superior rectus and inferior oblique muscles), and infraduction (inferior rectus muscle) of the left eye. On attempted adduction of the left eye, there was normal depression and intorsion (superior oblique muscle). She had full abduction of the left eye (lateral rectus muscle). Range of eye movements of the right eye was completely normal.

Normal contraction of the medial rectus muscle produces adduction (inward turning), whereas abduction (outward turning) is driven by the lateral rectus muscle. The superior and inferior recti muscles are best evaluated on abduction. Elevation in abduction is caused by the superior rectus muscle. Depression of the globe in abduction is caused by the inferior rectus muscle. The oblique muscles are best evaluated on adduction. Elevation of the eye on adduction is caused by the inferior oblique muscle. Depression of the globe in adduction is caused by the superior oblique.

Why could she not open her left eye? With a third cranial nerve lesion, eye opening is impaired. The levator palpebrae superioris, innervated by the third nerve, plays the major role in eyelid opening. On the other hand, with a cranial nerve VII lesion, eyelid closure (orbicularis oculi) is impaired, and the palpebral fissure is wider. Why did she have anisocoria? Two iris muscles regulate pupil size. The sphincter (pupilloconstrictor) is innervated by the parasympathetic system, and the dilator (pupillodilator) is innervated by the sympathetic system. As a result of parasympathetic dysfunction, there was unrestrained activity of the sympathetic pupillodilator and the left pupil became larger and unreactive to light.