Monoclonal Gammopathy of Clinical Significance-Associated Glycogen Storage Myopathy: A Novel Acquired Muscle Disease.

Monoclonal gammopathy of clinical significance (MGCS)-associated myopathy is a category of diseases in which the clonal plasma cells are responsible for muscle damage. MGCS-associated myopathy includes amyloid light chain (AL) amyloidosis-associated myopathy, sporadic late-onset nemaline myopathy with monoclonal gammopathy (SLONM-MG), and non-amyloid light chain deposition disease (LCDD)-associated myopathy. On the other hand, glycogen storage myopathy (GSM) is a well-known genetic condition affecting the glycogen biosynthesis or degradation pathways. Nevertheless, in exceedingly rare situations, GSM can be acquired and associated with MGCS.  Recently, the acquired MGCS-associated GSM was recognized within the category of MGCS-associated myopathy. Herein, we report the case of a 62-year-old male patient who developed an MGCS-associated GSM. The weakness was subacute and affected axial, proximal, and distal muscles. An obvious asymmetry also characterized the clinical presentation of this myopathy, and serum creatine kinase was normal. The patient responded significantly to chemotherapy based on a protocol of eight cycles of melphalan-dexamethasone. Due to the treatability of MGCS-associated GSM, it is of utmost importance to differentiate this exceptional and acquired myopathy from the usual genetic GSM.