Harlequin syndrome is a condition characterized by asymmetric sweating and flushing on the upper thoracic region of the chest, neck and face. Harlequin syndrome is considered an injury to the autonomic nervous system (ANS). The ANS controls some of the body's natural processes such as sweating, skin flushing and pupil response to stimuli. Such individuals with this syndrome have an absence of sweat skin flushing unilaterally; usually on the one side of the face, arms and chest. It is an autonomic disorder that may occur at any age. Harlequin syndrome affects fewer than 200,000 people in the United States. Symptoms associated with Harlequin syndrome are more likely to appear when a person has been in the following conditions: exercising, warm environment and intense emotional situation. Since one side of the body sweats and flushes appropriately to the condition, the other side of the body will have an absence of such symptoms. This syndrome has also been called the "Harlequin sign" and thought to be one of the spectrum of diseases that may cause Harlequin syndrome. It can also be the outcome of a one sided endoscopic thoracic sympathectomy (ETS) or endoscopic sympathetic blockade (ESB) surgery. Harlequin syndrome can also be seen as a complication of VA (veno-arterial) extracorporeal membrane oxygenation (ECMO). This involves differential hypoxemia (low oxygen levels in the blood) of the upper body in comparison to the lower body. Signs and symptoms The "Harlequin sign" is unilateral flushing and sweating of the face, neck, and upper chest usually after exposure to heat or strenuous exertion. Horner syndrome, another problem associated with the sympathetic nervous system, is often seen in conjunction with harlequin syndrome. Since Harlequin syndrome is associated with a dysfunction in the autonomic nervous system, main symptoms of this dysfunction are in the following: Absence of sweat(anhidrosis) and flushing on one side of the face, neck, or upper thoracic area. In addition, other symptoms include cluster headaches, tearing of the eyes, nasal discharge, abnormal contraction of the pupils, weakness in neck muscles, and drooping of one side of the upper eyelid. Causes One possible cause of Harlequin syndrome is a lesion to the preganglionic or postganglionic cervical sympathetic fibers and parasympathetic neurons of the ciliary ganglion. It is also believed that torsion (twisting) of the thoracic spine can cause blockage of the anterior radicular artery leading to Harlequin syndrome. The sympathetic deficit on the denervated side causes