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 the sympathetic nerve fibers, which start in the part of the brain called the hypothalamus and run to the face.

Sympathetic nerve fiber injuries can result from:

• Injury to the main artery to the brain (carotid artery)
• Injury to the nerves running down the arm (brachial plexus)
• Migraine or cluster headaches
• Stroke or lesion in 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).

Eye drops and certain medications may also lead to this condition.