What is Horner syndrome?

Horner syndrome is a nerve disorder in which the nerve pathway on one side of the face is disrupted, causing a drooping eyelid, decreased pupil size and less sweated on the side affected, states Mayo Clinic. The condition occurs due to spinal cord injury, tumor or stroke, although in some people, it develops without any known underlying problem.

In patients affected by Horner syndrome, there is sometimes a slight elevation of the lower eyelid, a condition known as upside-down ptosis, notes Mayo Clinic. The two pupils will be noticeably different in size, and there will be delayed opening or little dilation of the affected pupil.