Dermatomyositis is connective-tissue disease that is characterized by inflammation of the muscles and the skin. Its cause is unknown, but it may result from either a viral infection or an autoimmune reaction. Up to 50% of the cases may be a paraneoplastic phenomenon, indicating the presence of cancer.
X-ray findings include dystrophic calcifications in the muscles.
The main symptoms include skin rash and muscle pain. Characteristic signs include a heliotrope rash (a purple/red rash affecting the eyelids, cheeks, forehead and nasolabial folds) and Gottron's papules (purple/red raised plaques over the knuckles and extensor regions).
The diagnosis of dermatomyositis can be confirmed by muscle biopsy. There are two classic microscopic findings of dermatomyositis. They are:
A mixed B- and T-cell perivascular inflammatory infiltrate
Perifascicular muscle fiber atrophy
Dermatomyositis is associated with autoantibodies, especially anti-Jo-1 antibody.
Cross sections of muscle reveal muscle fascicles with small, shrunken polygonal muscle fibers on the periphery of a fascicle surrounding central muscle fibers of normal, uniform size.
Aggregates of mature lymphocytes with small, dark nuclei and scant cytoplasm are seen surrounding vessels. Other inflammatory cells are distinctly uncommon. Immunohistochemistry can be used to demonstrate that both B- and T-cells are present in approximately equal numbers.
The mechanism is conjectured to be complement-mediated damage of microscopic vessels with muscle atrophy and lymphocytic inflammation secondary to tissue ischemia.
Dermatomyositis must be differentiated from other common, lymphocyte predominant inflammatory myopathies. If present, the characteristic perifascicular atrophy makes this distinction trivial.
There is some overlap in the microscopic appearances of different inflammatory myopathies, but some helpful differences are often present. The rimmed vacuoles of inclusion body myositis (IBM) are absent in dermatomyositis. Polymyositis is characterised by diffuse or patchy inflammation of the muscle fascicles, a random pattern of muscle atrophy, and T-cell predominance with T-cells seen invading otherwise viable appearing muscle fibers.