Definition:

Myosin storage myopathy (MSM), also called hyaline body myopathy, is a rare congenital myopathy with variable inheritance characterized by the presence of sub-sarcolemmal hyaline bodies in type I muscle fibers and predominantly proximal muscle weakness. Clinically, patients exhibit variable age of onset ranging from birth through childhood, and occasionally middle age. Symptoms also vary, but typically include slowly progressive muscle hypertonia, scapularperoneal weakness, and respiratory insufficiency. MSM has been associated with 4 missense mutations in the MYH7 gene, which encodes slow/beta-cardiac myosin heavy chain (MyHC). The disease causing mutations in MYH7 are located in the alpha-helical coiled-coil tail.