Mondo Description Horner's syndrome is a rare condition characterized by miosis (constriction of thepupil), ptosis (drooping of the upper eyelid), and anhidrosis (absence of sweating of the face). It iscaused by damage to the sympathetic nerves of the face. The underlying causes of Horner's syndrome vary greatly and may include a tumor, stroke, or other damage to a part of the brain called the brain stem ; injury to the carotid artery ;and trauma to the brachial plexus. In rare cases, Horner's syndrome is congenital (present from birth) and associated with a lack of pigmentation of the iris (colored part of the eye). Treatment of Horner's syndrome depends on the underlying cause.