Small blood vessel involvement in childhood dermatomyositis. An ultrastructural study of a case.

The electron microscopic changes in a muscle biopsy from a child with the clinical picture of acute dermatomyositis are described. Skin and muscle tissue examined by light microscopy did not revealed vascular alterations. Degeneration of isolated muscle fibers was detected with complete loss of myofilaments and disintegration of the Z-band material. Muscle arteries and veins were free of thrombi, but striking ultrastructural changes were seen in the smallest blood vessels in the muscle. These consisted mainly of vascular occlusion due to necrotic cells mixed with degenerated platelets. Dissoluted endothelial cells and denuded basement membrane were usually seen in the wall of the occluded capillaries. Cytoplasmic tubular particles were present in otherwise unaltered endothelial cells. The possibility that the capillary lesions are related to an acute immunological injury is mentioned.