Horner's syndrome secondary to asymptomatic pneumothorax in an adolescent.

Horner's syndrome is uncommon in the paediatric population, but is seen more in infancy, and most cases are either congenital or related to birth trauma, head and neck tumours or thoracic surgery. We report an unusual cause of Horner's syndrome in a healthy adolescent boy secondary to a large, spontaneous, but asymptomatic, primary pneumothorax.