Harlequin syndrome is a condition characterized by asymmetric

Asymmetry

Asymmetry is the absence of, or a violation of, symmetry.-In organisms:Due to how cells divide in organisms, asymmetry in organisms is fairly usual in at least one dimension, with biological symmetry also being common in at least one dimension....

 sweating and flushing on the upper thoracic

Thorax

The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...

 region of the chest, the neck, and the face. It is caused by sustaining an injury to the sympathetic nervous system

Sympathetic nervous system

The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

,the part of the nervous system that reacts to stress and flight-or-fight circumstances. Listed as a rare disease

Rare disease

A rare disease, also referred to as an orphan disease, is any disease that affects a small percentage of the population.Most rare diseases are genetic, and thus are present throughout the person's entire life, even if symptoms do not immediately appear...

, Harlequin syndrome affects less 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

Endoscopic thoracic sympathectomy

Endoscopic thoracic sympathectomy is a surgical procedure where certain portions of the sympathetic nerve trunk are destroyed. ETS is used to treat hyperhidrosis, facial blushing, Raynaud's disease and reflex sympathetic dystrophy. By far the most common complaint treated with ETS is palmar...

.