The paper presents the case of a 4-year-old boy with recurrent, unilateral episodes of facial swelling and erythema, triggered by stress and physical exertion. Initially, an allergic aetiology and angioedema were suspected; however, extensive laboratory and imaging diagnostics ruled out these diagnoses. Magnetic resonance imaging revealed an extensive, infiltrative vascular malformation involving the craniofacial structures. The clinical symptoms resulted from secondary dysfunction of the facial sympathetic fibres, leading to a diagnosis of secondary Harlequin syndrome. Pharmacological treatment with sirolimus led to gradual clinical improvement and significant regression of the lesions on follow-up imaging. The presented case report highlights that recurrent, unilateral facial swelling in a child does not always have an allergic aetiology and may be a manifestation of rare autonomic disorders or vascular pathologies.

Harlequin syndrome, recurrent facial oedema, vascular malformation, autonomic nervous system disorders, paediatrics