Giant cell myositis confined to the lower extremities without associated thymoma or myasthenia gravis: a case report.

Giant Cell Myositis (GCMm) is an exceptionally rare inflammatory myopathy, historically reported in association with Myasthenia Gravis (MG), Thymoma, or Giant Cell Myocarditis (GCMc). The prognosis is often dictated by the frequently co-occurring, highly lethal GCMc. We describe the case of a 31-year-old male presenting with a three-month history of progressive, painful, and diffuse pseudohypertrophy of the lower extremities. Laboratory findings, including normal creatine kinase, were largely unremarkable, but a muscle biopsy demonstrated the pathognomonic infiltration by multinucleated giant cells. Extensive work-up, including cardiac magnetic resonance imaging and a full autoantibody panel, showed no evidence of cardiac, ocular, or associated thymic disease or MG. This is, to our knowledge, the first reported case of GCMm presenting as an isolated skeletal muscle disease without any known associated systemic conditions, underscoring the importance of considering GCMm in the differential diagnosis for unexplained limb pseudohypertrophy, even in the absence of typical comorbidities or profound muscle weakness.