Polymyositis-dermatomyositis: diagnostic and prognostic significance of muscle alkaline phosphatase.

The distribution and intensity of alkaline phosphatase deposition in 54 patients with dermatomyositis-polymyositis (PM-DM) was analyzed by the enzyme histochemical method. Increased enzyme reactivity of endomysial capillaries was found in 28% of patients, equally distributed between adult onset PM (Group I) and PM-DM with overlap in other connective tissue diseases (Group V). Patients with high endomysial capillary reactivity (R1 larger than or equal to 60) responded poorly to steroids, had an increased incidence of rheumatoid factor, and had less fiber degeneration/necrosis in their biopsies. Twenty-two percent of patients demonstrated prominent perimysial phosphatase reactivity localized in newly formed collagen and fibroblasts. Thirty patients (55%) demonstrated significant numbers of alkaline-phosphatase-positive fibers positively correlated with increased fiber degeneration/necrosis, endomysial fibrosis, increased numbers of triglyceride-containing muscle fibers, and NADH tetrazolium reductase hyperreactivity. Minimal overlap between the three enzyme distribution patterns was found. Endomysial capillary activity probably represents endothelial alkaline phosphatase induction analogous to the pattern seen normally in lower mammals (rat, rabbit, guinea pig). Alkaline phosphatase fiber reactivity probably represents a particular phase in fiber regeneration/maturation especially after denervation and is positively correlated with an increased incidence of spontaneous fibrillation potentials in PM-DM.