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Miyoshi myopathy (dysferlinopathy), described in Japan, belongs to the heterogeneous group of distal myopathies. The disease occurs mainly in young adults (on average 20 years of age) and the first symptoms are weakness and atrophy of the calves, which is sometimes asymmetrical, leading to inability to jump, run or walk on tiptoes. Muscular weakness subsequently progresses to the upper and lower limbs. Facial, bulbar and cardiac muscles are spared. Serum creatine kinase levels are markedly elevated, usually 10 to 150-fold higher than normal. This increase occurs in the preclinical stages of the disease. The electromyograhy (EMG) shows a myopatic pattern. In contrast with the other distal myopathies, muscle biopsy reveals dystrophic changes without the presence of rimmed vacuoles. Inflammatory infiltrates are a frequent finding. In Japan, the incidence of Miyoshi myopathy is approximately 1 in 440 000 inhabitants, but it has been also described in the USA, Italy, Spain, Germany, The Netherlands, Africa and Brazil. Miyoshi myopathy is transmitted as an autosomal recessive trait. The causative gene has been mapped to 2p13 and encodes the dysferlin protein, one of the components of muscular fibre membrane. The same gene is involved in autosomal recessive LGMD2B (limb-girdle muscular dystrophy type 2B). Dysferlin expression is decreased or absent in Miyoshi myopathy, providing an excellent diagnostic tool for this distal myopathy. |