Definition of Horner syndrome:

Horner syndrome is a rare condition that affects the nerves to the eye and face.

Causes, incidence, and risk factors:

Horner syndrome can be caused by any interruption in a set of nerve fibers that start in the part of the brain called the hypothalamus and run to the face.

Sympathetic nerve fiber injuries can result from:

• Injury to one of the main arteries to the brain (carotid artery)
• Injury to nerves in the neck called the brachial plexus
• Migraine or cluster headaches
• Stroke, tumor, or other damage to a part of the brain called the brainstem
• Tumor in the top of the lung

Rarely, Horner syndrome may be present at birth (congenital). The condition may occur with a lack of color (pigmentation) of the iris (colored part of the eye).