Some of the common pathologic changes in the muscle fiber as viewed with the electron microscope are considered. These include myofilament and Z band abnormalities, alterations of the sarcotubular system, mitochondrial aberrations and glycogen accumulations. Correlations with light microscopic findings are mentioned when pertinent. The diagnostic significance of the various ultrastructural changes is discussed.
Focal myositis is a benign pseudotumor of skeletal muscle, the cause of which is obscure. It usually appears as a steadily enlarging mass within the muscle of an extremity. The pathologic features are those of an inflammatory myopathy in which lymphocytic infiltrates, muscle cell necrosis, and regeneration are seen. Four cases of focal myositis were studied using enzyme histochemistry and electron microscopy. The changes of denervation atrophy, reinnervation, and fiber hypertrophy were found. Intramuscular nerve damage manifested by axonal swelling, demyelination, and endoneurial fibrosis was evident. The findings suggest that a denervating process plays an important role in the pathogenesis of focal myositis.
Thirty-two patients with adult-onset polymyositis uncomplicated by cancer or systemic connective tissue disease were studied. Muscle biopsy specimens were examined with direct immunofluorescence microscopy and results were compared with those in 94 control subjects. Sarcolemmal and sarcoplasmic staining were observed in both groups and considered to be nonspecific. Immune deposits in the muscle microvasculature were present in some cases of systemic lupus erythematosus and dermatomyositis but were not present in polymyositis. Our data suggest that the finding of vascular immunofluorescence excludes the diagnosis of adult polymyositis and implies that the pathogenesis of this disease and other idiopathic inflammatory myopathies may differ.
