Osteochondroma of the clavicle causing Horner's syndrome

A 17-year-old girl was admitted with a 1-year history of right-sided headaches and unilateral ptosis for 6 months. She was diagnosed with Horner's syndrome. Ultrasound and CT scan demonstrated an osteochondroma of the posterosuperior aspect of the medial clavicle compressing adjacent structures, including the right internal jugular vein and right common carotid artery. Cartilage sequence MRI showed an osteochondroma with a 5 mm cartilage cap. There were no suspicious features of malignancy. The patient was referred to a specialist centre for surgical resection. Osteochondroma of the clavicle presenting with Horner's syndrome is extremely rare, with only one previously reported case. In this case report, we discuss the anatomy of the oculosympathetic chain, which consists of a three neuron arc. The disruption of the neuronal arc, at any point, may result in Horner's syndrome. We also discuss the various causes of Horner's syndrome and its radiological investigation.