Harlequin syndrome

Harlequin syndrome is a rare condition affecting the autonomic nervous system, causing one side of the face to flush and sweat, usually with exercise, heat, or emotion, while the other side does not, creating a strikingly asymmetric appearance. It relates to an imbalance in the nerve pathways controlling facial blood vessels and sweat glands on one side. It is usually benign but can occasionally signal an underlying nerve problem.

It results from a disruption or imbalance in the sympathetic nerve fibres supplying one side of the face, which can be idiopathic (with no identifiable cause), or occasionally related to injury, surgery, or an underlying condition affecting these nerve pathways, such as a tumour along the nerve’s path. In many cases, especially when it develops spontaneously, no specific cause is found despite thorough investigation. It can also rarely follow certain neck or chest surgeries that affect these nerve pathways.

The characteristic symptom is asymmetric facial flushing and sweating, most noticeable with exercise, heat, or strong emotion, where one side of the face becomes visibly red and sweaty while the other remains pale and dry. It does not typically cause pain. The dramatic visual asymmetry is usually what prompts people to seek medical assessment.

In most cases, particularly when no other symptoms are present, it is a benign condition without serious underlying cause, though because it can rarely be associated with an underlying nerve problem or, very rarely, a tumour, it is generally investigated to rule these out on first presentation. Once serious causes have been excluded, ongoing management is usually just about understanding and, if desired, managing the cosmetic aspect. Any new associated symptoms, such as drooping eyelid or pupil changes, would need specific further assessment.

Diagnosis is often based on the characteristic appearance and pattern of symptoms, sometimes supported by specific autonomic function tests. Imaging, such as an MRI, may be arranged to look for any underlying structural cause along the affected nerve pathway, particularly if there are other associated neurological symptoms. A neurologist experienced in autonomic disorders typically leads this assessment.

In most cases, no specific treatment is needed beyond reassurance once serious underlying causes have been excluded, since the condition itself is not harmful. If the asymmetry is bothersome cosmetically, some treatments such as botulinum toxin injections to reduce sweating and flushing on the unaffected side have been used to even out the appearance. Treatment of any identified underlying cause, if one is found, would take priority.

When no underlying cause is found, the condition often persists as a benign quirk rather than something that is "cured," though it does not cause any harm and many people simply learn to live with the occasional visible flushing pattern. If an underlying treatable cause is identified, addressing that may improve or resolve the symptoms. Cosmetic treatments can help manage the visible asymmetry if it is bothersome, without addressing an underlying cause that may not exist.

Normal facial flushing, such as with exercise, heat, or embarrassment, typically affects both sides of the face fairly symmetrically. Harlequin syndrome is specifically characterised by a sharp, consistent difference between the two sides, with one flushing and sweating clearly more than the other. This striking asymmetry, occurring consistently with the same triggers, is what distinguishes Harlequin syndrome from ordinary flushing.

See a doctor if you notice a consistent pattern of one-sided facial flushing and sweating, so appropriate assessment and any necessary investigation can be arranged. A private neurology assessment can typically be arranged within about one to two weeks. This is particularly worth doing if there are any other new neurological symptoms alongside the facial changes, such as eyelid drooping or vision changes.