Pathologic changes in muscle in primary fibromyalgia syndrome.

Primary fibromyalgia syndrome, also inappropriately called "fibrositis," is a clinically recognizable nonarticular rheumatic condition with diffuse and chronic musculoskeletal aching and stiffness, accompanied by exaggerated tenderness at characteristic sites on physical examination. Results of muscle biopsy from 12 well-defined cases of primary fibromyalgia syndrome without any history of trauma have been published recently. Light microscopic examination revealed no evidence of inflammation. Histochemical analysis demonstrated type II fiber atrophy in seven patients and the "moth-eaten" appearance of type I fibers in five patients. Electron microscopic findings were most impressive, and included myofibrillar lysis with deposition of glycogen and abnormal mitochondria, as well as subsarcolemmal accumulation of glycogen and mitochondria in all 12 patients and papillary projections of sarcolemmal membrane in 11 patients. Mechanisms of these significant muscle changes in primary fibromyalgia syndrome are uncertain but may include subclinical injury of muscle spasm.