There is a break in a series of nerve fibers that begin in the part of the brain called the hypothalamus and go to the face and eyes. These nerve fibers are involved with sweating, the pupils, and the muscles of the upper and lower eyelids. Damage to nerve fibers can be the result of a) Injury to the carotid artery b) Injury to the brachial plexus. l) Migraine or cluster headaches d) Stroke, tumor, or other brain stem damage. e) Tumor in the upper part of the lung, between the lungs and the neck. f) Injections or surgeries performed to interrupt nerve fibers and relieve pain (sympathectomy). g) Injury to the spine. The condition can occur with a lack of color (pigmentation) of the iris.

Treatment: There is no specific treatment for Horner syndrome, it can often disappear when the undiagnosed disease is treated effectively.