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