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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.
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 of the thoracic spine can cause blockage of the anterior radicular artery leading to harlequin syndrome.  The sympathetic deficit on the denervated side causes the flushing of the opposite side to appear more pronounced. It is unclear whether or not the response of the undamaged side was normal or excessive, but it is believed it could be a result of the body attempting to compensate for the damaged side and maintain homeostasis. 
The ‘Harlequin Sign’ is unilateral flushing and sweating of the face and neck usually after exposure to heat or strenuous exertion. Horner’s syndrome, another problem associated with the sympathetic nervous system, is often seen in conjunction with harlequin syndrome.
Harlequin syndrome is not debilitating so treatment is not normally necessary. 
The name for the syndrome is credited to Lance and Drummond who were inspired by resemblance patient’s half-flushed faces bore to colorful Harlequin masks.
- Lance, J. W. (2005). "Harlequin syndrome". Practical Neurology 5 (3): 176–177. doi:10.1111/j.1474-7766.2005.00306.x.
- Wasner, G.; Maag, R.; Ludwig, J.; Binder, A.; Schattschneider, J.; Stingele, R.; Baron, R. (2005). "Harlequin syndrome - one face of many etiologies". Nature Clinical Practice Neurology 1 (1): 54–59. doi:10.1038/ncpneuro0040. PMID 16932492.
- Corbett, M.; Abernethy, D.A. (1999) “Harlequin syndrome.” J Neurol Neurosurg Psychiatry. Available at: http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1736279&blobtype=pdf. June 29, 2009.
- Lance, J. W.; Drummond, P. D.; Gandevia, S. C.; Morris, J. G. L. (1988) “Harlequin syndrome: the sudden onset of unilateral flushing and sweating.” Journal of Nerology, Nerosurgery, and Psychiatry (51): 635-642.
- National Institutes of Health: Office of Rare Diseases Research. (2009) “Harlequin syndrome.” Genetic and Rare Diseases Information Center (GARD). http://rarediseases.info.nih.gov/GARD/Condition/8610/QnA/22289/Harlequin_syndrome.aspx. December 9, 2011.