Assessing anisocoria and Horner's syndrome

The first step is to determine which pupil is abnormal. Ptosis and opthalmoparesis on the side of the larger pupil suggests a third nerve paresis on that side. Ptosis on the side with the smaller pupil suggests a Horner's syndrome. If the pupils appear equal in normal lighting, try to darken the room. Anisocoria that is more pronounced in a darker room suggests a Horner's syndrome on the side with the smaller pupil. If its more pronounced in light, it suggests a parasympathetic defect on the side with the larger pupil.

Horner's syndrome may have upper lid ptosis and sometimes reverse lower lid ptosis, giving the (false) impression of enopthalmos. Acutely there is also hyperemia or the conjunctiva. Horner's in isolation usually is a postganglionic Horner's and it usually is due to one of two conditions, cluster headache and carotid dissection, which are not confused clinically.