Selective atrophy of the type IIb muscle fibers in rheumatoid arthritis and progressive systemic sclerosis (scleroderma). A biopsy histochemical study.

We have examined biopsy material from the left m. vastus lateralis of eight patients suffering from rheumatoid arthritis and four patients with progressive systemic sclerosis (scleroderma). All were chosen according to the duration and the severity of disease, so that the broadest possible spectrum of signs and symptoms could be considered. Muscular specimens showed a selective and constant atrophy of the type IIB fibers, as revealed by the myofibrillar ATPase histochemical reaction (both at a pH of 9.4 and with pre-incubation at pH 4.35 and 4.63). Atrophy of the type I fibers was seen only occasionally. Neither structural abnormalities, such as 'motheaten' fibers, nor inflammatory reactions were observed. We think that (1) changes in skeletal muscles of patients with rheumatoid arthritis and progressive systemic sclerosis may be quite selective, and (2) the myopathy associated with rheumatoid arthritis can be differentiated from inflammatory myopathies, even in muscle biopsy specimens, on the basis of histoenzymologic investigations.