Harlequin syndrome is a condition characterized by asymmetric sweating and flushing on the upper thoracic region of the chest, the neck, and the face. It is caused by sustaining an injury to the sympathetic nervous system. Listed as a rare disease, Harlequin syndrome affects fewer than 200,000 people in the United States. It can also be the outcome of a unilateral endoscopic thoracic sympathectomy (ETS) or endoscopic sympathetic blockade (ESB) surgery, see endoscopic thoracic sympathectomy.