Dermatomyositis associated with impairment in both muscle aerobic and anaerobic function

We report a 50-year-old Japanese woman with dermatomyositis in whom an anti-mitochondrial antibody was detected. Muscle biopsy demonstrated periodic acid Schiff- (PAS) positive vacuoles in addition to infiltrates of mononuclear cells. Histochemical analysis showed reduced phosphorylase activity. An aerobic exercise test demonstrated that the concentrations of serum lactate and pyruvate were elevated before corticosteroid therapy but decreased after the therapy. On the other hand, in forearm ischemic exercise tests, the responses of serum lactate and pyruvate were attenuated before corticosteroid therapy but recovered after the therapy. These findings indicate that an inflammatory mechanism interfered with myophosphorylase activity and muscle aerobic function.