[1] Individuals with this syndrome have an absence of sweat skin flushing unilaterally, usually on one side of the face, arms and chest.

[2] Symptoms associated with Harlequin syndrome are more likely to appear under the following conditions: vigorous exercise, warm environments and intense emotional situations.

[5] The "Harlequin sign" is unilateral flushing and sweating of the face, neck, and upper chest usually after exposure to heat or strenuous exertion.

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.

[3] One possible cause of Harlequin syndrome is a lesion to the preganglionic or postganglionic cervical sympathetic fibers and parasympathetic neurons of the ciliary ganglion.

[7] It is also believed that torsion (twisting) of the thoracic spine can cause blockage of the anterior radicular artery leading to Harlequin syndrome.

[3] In an MRI, a radiologist may observe areas near brain or spinal cord for lesions, or any damage to the nerve endings.

[6] In cases where the individual may feel socially embarrassed, contralateral sympathectomy may be considered, although compensatory flushing and sweating of other parts of the body may occur.

Two female patients with metastatic cancer, ages 37-years-old and 58-years-old, were scheduled for placement of an intrathecal pump drug delivery system.

Another case study was based on a 6-year-old male visiting an outpatient setting for one sided flushes during or after physical activity or exposed to heat.

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.