Concurrent Sporadic Late‐Onset Nemaline Myopathy and an Excessive Glycogen Accumulation Associated With Monoclonal Gammopathy

BACKGROUND: Monoclonal gammopathy-associated myopathies (MGAMs) include light chain (AL) amyloid myopathy, sporadic late-onset nemaline myopathy (SLONM), and vacuolar myopathy with monoclonal gammopathy and stiffness (VAMMGAS). These subtypes usually occur separately, although rare overlap has been described. We report a patient with monoclonal gammopathy and concurrent SLONM and excessive glycogen accumulation, resembling VAMMGAS but with distinct features. METHODS: Case report with clinical, electrophysiological, pathological, and therapeutic characterization of a patient with IgG-kappa monoclonal gammopathy and coexisting SLONM and glycogen accumulation within muscle fibers. RESULTS: A 69-year-old man developed subacute progressive axial and limb weakness, head drop, dysphagia, and weight loss. Examination showed proximal/distal weakness, neck extensor weakness, and lumbar hyperlordosis. EMG revealed myopathic motor unit potentials without electrical myotonia or complex repetitive discharges. Serum studies identified IgG-kappa monoclonal protein with elevated kappa light chain and ratio. Muscle biopsy demonstrated nemaline rods (1% of fibers) and scattered fibers with non-rimmed vacuoles (0.3% of fibers) containing PAS-positive, diastase-labile material consistent with concurrent SLONM and glycogen storage myopathy-like pathology. Genetic testing for congenital nemaline and glycogen storage myopathies was negative. Plasma-cell directed therapy with daratumumab, lenalidomide, and dexamethasone led to rapid functional improvement and M-protein reduction. Further gains followed autologous stem cell transplantation. CONCLUSION: This case expands the MGAM spectrum, highlighting co-occurrence of SLONM and an excessive glycogen accumulation responsive to immunotherapy and transplantation. These findings suggest a spectrum of related disease processes. Recognition of this combined pathology is clinically important, as affected patients may benefit from targeted immunotherapy or stem cell transplantation.