Snapping Shoulder Secondary to Subluxation of an Accessory Coracobrachialis

Numerous snapping syndromes have been reported in the musculoskeletal system. Identifying the cause of these symptoms can often be challenging as the underlying abnormality may not be appreciable on routine static examinations. We report a 30-year-old female who presented with an unusual snapping sensation in her right anterior shoulder. This was readily reproducible during shoulder abduction with a palpable clicking evident on clinical examination. Dynamic ultrasound revealed this to be secondary to an accessory coracobrachialis muscle, which subluxed suddenly over the anterior subscapularis tendon during abduction. An accessory coracobrachialis muscle is a rare normal variant that is often asymptomatic. Extra-articular causes of shoulder snapping have been rarely reported, and this is the first case report of an accessory coracobrachialis muscle causing a snapping shoulder phenomenon.