Harlequin syndrome is a rare autonomic disorder characterized by hemifacial anhidrosis and impaired flushing due to disruption of the sympathetic pathway. While most cases are idiopathic, secondary causes such as structural lesions, vascular events, or iatrogenic injury are recognized. Post-traumatic etiologies are uncommon but clinically important, particularly when standard imaging is unrevealing.

A 44-year-old woman presented with left-sided facial dryness and inability to sweat during physical exertion, first noted after a motor vehicle collision with whiplash injury four years earlier. Over several years, she consulted multiple specialists including internal medicine, dermatology, and rheumatology, without a unifying diagnosis. MRI of the brain and cervical spine was normal. Clinical examination and patient-provided photographs demonstrated unilateral facial flushing with contralateral anhidrosis during exertion, consistent with Harlequin syndrome. Given the temporal relationship to trauma and absence of alternative etiologies, post-traumatic sympathetic pathway injury was considered the most likely cause.

Clinical examination and review of patient photographs revealed marked right hemifacial flushing and left hemifacial anhidrosis with exertion. MRI of the brain and cervical spine showed no structural lesions.

This case illustrates whiplash injury as a rare cause of Harlequin syndrome and underscores the importance of clinical recognition in the absence of radiographic findings. Awareness of post-traumatic sympathetic dysfunction can prevent prolonged diagnostic delays and unnecessary investigations. Careful history, examination, and localization along the sympathetic chain remain critical in establishing the diagnosis.