Idiopathic Harlequin Syndrome: A Case Report of an Uncommon Disease.

Harlequin syndrome is a rare autonomic disorder presenting with unilateral facial flushing and sweating due to sympathetic dysfunction, often idiopathic but occasionally secondary to an underlying pathology. We report a 52-year-old woman with a three-year history of left-sided hemifacial flushing and hyperhidrosis, most noticeable on exertion or heat exposure. She had comorbid diabetes, hypertension, and dyslipidemia. Neurological examination was normal. Exercise provocation reproduced the symptoms, and the iodine-starch test confirmed hyperhidrosis on the left side with anhidrosis on the right. Imaging excluded secondary causes, leading to a diagnosis of idiopathic Harlequin syndrome. The condition results from the disruption of sympathetic vasomotor pathways at the T2-T3 level, with compensatory overactivity on the unaffected side. Although benign, it can cause cosmetic and psychosocial concerns. Management includes reassurance, botulinum toxin injections, stellate ganglion blockade, or sympathectomy. Our patient opted for botulinum toxin therapy with ongoing follow-up. This case highlights the importance of recognizing idiopathic Harlequin syndrome and excluding secondary causes to avoid unnecessary interventions.