Ultrastructural pathology of skeletal muscle in various rheumatic diseases.

Seventy-three muscle biopsies from patients with various rheumatic diseases were analyzed using immunofluorescence, light, and ultrastructural microscopy. Pathologic data were correlated with clinical variables of local muscle and systemic disease. Light and immunofluorescence findings were generally normal. Ultrastructure differed from normals, showing a spectrum of nonspecific changes. There were no disease specific pathologic features. Myofibrillar damage was the most common pathologic change, with atrophy or degeneration occurring in a majority of biopsies. Semiquantitative analysis showed a general correlation between the extent of pathologic change and muscle weakness.