Cause
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.
Read more about this topic: Harlequin Syndrome