Histologic Changes of Muscle in Dermatomyositis

Normal muscle is composed of fascicles of individual muscle fibers all about the same size. One characteristic of the fascicle in DM is atrophy of muscle fibers at the edge of the fascicle--perifasicular atrophy. Note this change in the two photos at low and high magnification below (A & B). Another feature is inflammation around perimysial blood vessels (A). In B a few inflammatory cells are present around muscle fibers. A macrophage at the arrow is digesting a fiber.




This low-power view of another fascicle shows active inflammation and muscle fiber degeneration and regeneration. Some muscle fibers are normal in size and dark pink. Others, which are degenerating, are smaller and lighter pink. They are sometimes vacuolated and may have central nuclei rather than the normal peripheral nuclei (find some). Small fibers with a bluish tinge and enlarged nuclei are regenerating fibers. Inflammatory cells including lymphocytes and macrophages cannot be distinguished from muscle fiber nuclei at this power. Fibers that are larger than normal, as occur in some of the muscular dystrophies, do not occur in DM.


At higher magnification, some of the hypercellularity is caused by proliferation of the muscle nuclei and some is caused by the presence of lymphocytes and macrophages between muscle fibers.

Note especially the capillaries, which have enlarged endothelial cells. The one at the arrow is narrowed, whereas that at the upper left has a more normal lumen.

The pathogenesis of the changes involves deposition of complement components on the endothelial cell with destruction of the capillary and resultant ischemic changes. Therapy can reverse these changes: capillaries and muscle fibers revert to normal.