The wide spectrum of myofibrillar myopathy suggests a multifactorial etiology and pathogenesis

Background: Myofibrillar myopathy (MFM) is characterized by nonhyaline lesions (foci of myofibrillar destruction) and hyaline lesions (cytoplasmic inclusions composed of compacted myofibrillar residues) on light and electron microscopy. Immunocytochemistry demonstrates the abnormal expression of desmin and numerous other proteins. The clinical, laboratory, and histologic features of MFM are heterogeneous, making a diagnosis difficult. Results: We diagnosed eight patients with MFM over the preceding 3 years. MFM was inherited in an autosomal dominant pattern in one patient, developed sporadically in five patients, and was induced by an experimental chemotherapy, Elinafide (Knoll, Parsippany, NJ), in two patients. Age at onset ranged from 14 to 64 years. The pattern of weakness was variable but involved proximal and distal muscles. Five patients had evidence of a cardiomyopathy. Electromyography demonstrated muscle membrane instability and small, polyphasic motor unit potentials. Serum creatine kinase levels were normal to moderately elevated (<10x normal). Light and electron microscopy demonstrated the characteristic pattern of nonhyaline and hyaline lesions and the associated abnormalities on immunocytochemistry. Conclusions: Patients demonstrate a wide spectrum of clinical, laboratory, and histologic abnormalities. Chemotherapy-induced MFM has abnormalities on immunocytochemistry similar to the those of hereditary and sporadic cases. The pathogenesis of MFM is likely heterogeneous. However, MFM is distinctive in that it can preferentially affect distal muscles and has a frequent association with cardiomyopathy. The cardiomyopathy may be amenable to treatment with pacemaker insertion or cardiac transplantation.