Harlequin syndrome (HSD) is an autonomic disorder occurring at any age and characterized by unilateral flushing and sweating, involving the face and sometimes arm and chest, in condition of thermal, exercise or emotional stress without sympathetic ocular manifestations. However, tonic pupils, parasympathetic oculomotor lesion and pre- or postganglionic sudomotor sympathetic deficit can rarely occur.

The upper eyelid margin is positioned 3 mm or more lower than usual and covers the superior portion of the iris (objective); or, the upper lid margin obscures at least part of the pupil (subjective).