A case of sporadic late-onset nemaline myopathy with monoclonal gammopathy of undetermined significance: long-term observation of neurological symptoms after autologous stem-cell transplantation.

A 47-year-old woman presented with progressive limb weakness. A neurological examination revealed proximal dominant symmetrical muscle weakness in her limbs, and electromyography revealed complex repetitive discharges and short motor unit potentials with positive sharp waves in the biceps. We observed early recruitment in the quadriceps, and laboratory tests revealed normal creatine kinase. Serum protein electrophoresis showed monoclonal IgG-lambda, but the bone marrow aspiration specimen was normal. A muscle biopsy revealed nemaline rod accumulations in the muscle fibers; based on the results, we diagnosed the patient with sporadic late-onset nemaline myopathy with monoclonal gammopathy of undetermined significance (SLONM-MGUS). We administered repeated intravenous immunoglobulin, but her limb weakness continued, and she developed a restrictive ventilatory defect. The patient received melphalan, followed by autologous stem-cell transplantation (ASCT). Her upper extremity strength and respiratory capability improved within one year after ASCT; however, it was not until six years after ASCT that her atrophied lower extremities strengthened. A discrepancy in the timeline of treatment response between the upper or respiratory muscles and the atrophied lower limb was characteristic in the patient, suggesting that the efficacy of ASCT on SLONM-MGUS should be evaluated in the long term, especially in severely atrophied muscles. In addition, this case showed that ASCT for SLOMN-MGUS is an effective treatment option in Asian populations.