Background: Myopathy associated with plasma cell dyscrasia is rare. Reported cases include paraneoplastic and paraproteinaemic disease (poly/dermatomyositis-like; light chain deposition disease; amyloidosis), or focal mass lesions. Histologic data are scarce.

Case: A 74-year-old male presented with five months of progressive, symmetric shoulder and quadriceps weakness and wasting. Electrophysiologic studies suggested myositis, ESR was greater than 120 mm/hr, and the CK was around 1000 IU/L. Deltoid biopsy was initially reported as inflammatory myopathy with dystrophic features. Upon review, sheets of CD138-positive plasma cells filled with abundant Russell bodies were identified, mimicking round atrophic muscle fibres. Muscle fascicles showed dense focal lymphoplasmacytic inflammation, muscle fibre necrosis, and fibroadipose replacement. Plasma cells were kappa light chain restricted. A diagnosis of plasma cell dyscrasia was made.

Conclusion: We report a case of symmetric proximal muscle weakness and wasting with a histologic diagnosis of light chain restricted plasma cell infiltration and lymphoplasmacytic myositis. Symmetry of muscle involvement suggests a combination of paraproteinaemic, paraneoplastic, and diffusely infiltrative processes. This case may add a new histologic variant to the spectrum of myopathies associated with plasma cell dyscrasia.

Conflictsof Interest:

None.