Fascioscapulohumeral muscular dystrophy, limb girdle muscular dystrophy and a scapuloperoneal syndrome may cause shoulder weakness, however, the unilateral presentation is atypical for these disorders. Musculoskeletal causes include rotator cuff injury, biceps tendinopathy and muscle strain. He is otherwise healthy without history of diabetes mellitus, connective tissue disease or thyroid disorder. His only medication is ibuprofen, taken as needed for shoulder pain. Family history is negative for neuromuscular disease.
Review of systems is non-contributory.
If necessary, revise your differential diagnosis based on the additional clinical history. The negative family history makes muscular dystrophy and scapuloperoneal syndrome unlikely diagnoses. There is no history of precipitating factors often associated with neuralgic amyotrophy. The physical examination reveals winging of the right scapula at rest. Muscle bulk is otherwise normal and symmetric.