NQ-107: (D) Dermatomyositis

Diagnosis: Perifascicular atrophy in dermatomyositis, modified Gomori’s trichrome stain

Pathology of the case: In this preparation with modified Gomori’s tricrhrome, fibers at the periphery of the fascicles are significantly smaller than those at the center of the core. This finding, known as perifascucular atrophy, is often seen in dermatomyositis. There is no inflammatory cell infiltration in this image. However, one must know that the extent and degree of inflammatory cell infiltration in dermatomyositis is very variable.

• Dermatomyositis is an inflammatory myopathy of unknown etiology, though many patients with dermatomyositis have circulating autoantibodies. A muscle biopsy in a patient with dermatomyositis will show a characteristic perifascicular atrophy of muscle fibers, with relative sparing of the more central fibers, in a background of chronic inflammation. The findings in dermatomyositis are the result of a capillaritis (vasculitis) that is likely induced by anti-endothelial antibodies. The perifascicular atrophy is thought to be the result of damage to the most distal aspects of the capillary network, which is seen along the periphery of the fascicle. The inflammatory cells are a response to the capillaritis. On electron microscopy, tubuloreticular cytoplasmic inclusions (TRIs) may be seen in endothelial cells of patients with dermatomyositis. While these are not specific to dermatomyositis, as they may also be seen in other autoimmune diseases, they may be helpful in distinguishing dermatomyositis from other inflammatory myopathies (e.g. polymyositis, inclusion body myositis) as TRIs do not occur in those conditions.

Polymyositis is an inflammatory myopathy mediated by cytotoxic T cells, with an unknown etiology. A muscle biopsy in a patient with polymyositis will show chronic inflammation and muscle fiber necrosis. It does not show the perifascicular pattern seen in dermatomyositis.

Neurogenic atrophy is muscle fiber atrophy that occurs following denervation. Acutely, the muscle fibers become atrophic and angular in shape. Over time, as adjacent nerve fibers begin to reinnervate the atrophic muscle fibers, the fibers will regenerate and recover their original size and shape. ATP-ase staining to differentiate muscle fiber types will demonstrate fiber-type grouping (large groups of fibers of one fiber type rather than the normal “checker board” fiber pattern), which is characteristic of reinnervation after denervation.

Spinal muscular atrophy is a degenerative disease characterized by loss of lower motor neurons resulting in progressive weakness. A muscle biopsy in a child with SMA will show signs of acute denervation (atrophy), without reinnervation (no fiber-type grouping).

Type 2 fiber atrophy is often seen in patients who have received steroids for a prolonged period of time. It may also be seen in settings of disuse of a muscle group.