harlequin syndrome (Monarch Disease Ontology id MONDO_0016040) (HSD) is an autonomic disorder occurring at any age and characterized by unilateral flushing and sweating, involving the face and sometimes arm and chest, in condition of thermal, exercise or emotional stress without sympathetic ocular manifestations. However, tonic pupils, parasympathetic oculomotor lesion and pre- or postganglionic sudomotor sympathetic deficit can rarely occur. Also known as: progressive isolated segmental anhidrosis.