Harlequin sysndrome, first described in 1988, is characterized by sweating and flushing (or lack of) on one side of the face only, typically after exercise or exposure to heat.
It's estimated to affect around 1 in 1000 individuals. The cause is presently unknown, though it's presumed to be associated with imbalances in the sympathetic nervous system.
Although the exact mechanism for Harlequin syndrome is still unclear, understanding what is affected with this syndrome is important. The majority of cases are thought to occur when nerve bundles in the head and neck are injured.
Source : Wikipedia