is caused by an attack on the sympathetic nervous system
. It manifests as four precise clinical signs: drooping of the upper eyelid, or ptosis; constriction of the pupil, or miosis; the eye being sunk into the socket, or enophthalmos; and dilation of the blood vessels along with decreased sweating around the neck and face, called anhidrosis. Horner's syndrome is secondary to a damaged sympathetic nerve
in the neck. It is commonly caused by lesions on the central nervous system, such as in certain forms of stroke
, or by the compression of this nerve by different processes.