brain, spinal cord, and nerve disorders
Horner Syndrome
Introduction
Horner syndrome affects one side of the face, causing the eyelid to droop, the pupil to become small (constricted), and sweating to decrease. The cause is disruption of the nerve fibers that connect the brain to the eye. Horner syndrome may occur on its own or result from a disorder that disrupts nerve fibers connecting the brain with the eye. The upper eyelid droops, the pupil remains small, and the affected side of the face may sweat less. Doctors test the pupil to see whether it can widen and may do imaging tests to look for a cause. The cause, if identified, is treated. (See also Overview of the Autonomic Nervous System .) Horner syndrome can develop in people of any age. 