• Inflammatory myopathies comprise an uncommon, heterogeneous group of disorders characterized by immunologically mediated injury and inflammation of mainly the skeletal system. They may occur alone or with other immune-mediated diseases, particularly systemic sclerosis. -Robbins
• Classification of Polymyositis/Dermatomyositis
• Primary Idiopathic Polymyositis (or) Dermatomyositis
• Polymyositis (or) Dermatomyositis associated with neoplasia – seen in adults > 40 y/o, should always be checked!
• Polymyositis associated with collagen vascular disease

Dermatomyositis – a typical rash plus muscle weakness is diagnostic

• Clinical presentation for both Dermatomyositis and Polymyositis: Onset is acute to subacute (days to weeks), Weakness is typical: bilaterally symmetric, proximal, with initial symptoms in pelvic or shoulder girdle muscles (ex: getting up from a chair and climbing steps become increasingly difficult). Dysphagia seen in 1/3 of patients (not found early in any other hereditary myopathies). Muscle pain may be present, but not necessary for dx. Interstitial lung disease, vasculitis, and myocarditis, may be present in some cases.
• Clinical presentation in Dermatomyositis ONLY:
• Heliotrope discoloration – rash taking form of a heliotrope discoloration of the upper eyelids with periorbital edema.
• Grotton lesions - scaling erythematous eruption over knuckles, elbows, and knees.
• Labs: Elevated Creatine Kinase (x5–x10). EMG shows a myopathic pattern that can be associated with signs of muscle irritability such as fibrillations. Sedimentation rate is normal.
• Differential Diagnosis: Inclusion Body Myositis, Muscular dystrophy (e.g. limb girdle), Metabolic myopathy (e.g. Acid Maltose deficiency), Polymyalgic Rheumatica (no weakness, just pain. CK normal, Sed rate increased).
• Histology: NOT AN INFLAMMATORY ATTACK OF MUSCLE! Inflammatory infiltrates seen around small blood vessels and in perimysial connective tissue. Perifascicular atrophy is seen and attributed to the state of hypoperfusion caused by the endothelial attack and fibrosis. Muscle fiber necrosis and regeneration may be seen.
• Pathogenesis: Cause unknown. Capillaries seem to be main targets, attacked by Abs and complement, giving rise to foci of ischemic myocyte necrosis.
• Treatment: Corticosteroids are drug of choice Þ Prednisone 1.0-1.5 mg/kg/day.
• Prognosis: Most patients do well but many require chronic immunosuppression. Factors which predict good prognosis: Age (kids Þ complete remission), early treatment (acute onset better than chronic), patients who lack corticosteroid side effects.

• Clinical presentation: see above.
• Labs: see above.
• Differential Diagnosis: see above.
• Histology: Lymphocytes seen in the endomysium. Surround and invade healthy muscle fibers. Both necrotic and regenerating fibers are present. No evidence of vascular injury.
• Pathogenesis: Cause unknown. Seems to be caused by cell-mediated injury. CD8⁺ cytotoxic T cells and Mf s are seen near damaged muscle fibers, and the expression of HLA-I molecules is increased on the sarcolemma of normal fibers.
• Treatment: see above.
• Prognosis: see above.

• Clinical presentation: Unlike the above two entities, inclusion-body myositis begins with the involvement of distal muscles, especially extensors of the knee (quads) and flexors of the wrists and fingers. Muscle weakness may be asymmetric. Insidious disorder, typically affecting patients > 50 y/o.
• Pathogenesis: Cause unknown. CD8⁺ cytotoxic T cells found in the muscle, but immunosuppressive therapy is not beneficial.
• Histology: Diagnostic finding is rimmed vacuoles… aka holes in myocytes!!! Vacuoles contain tubular filamentous proteins. Pattern of inflammatory cell infiltrate is similar to that seen in polymyositis.