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Both diseases belong to a group of connective tissue disorders known as idiopathic inflammatory myopathies. They are a multisystem disease characterized by necrotizing inflammatory myopathy of striated muscles and a skin rash, both of unknown etiology.

Approximately 5-10:100,000 in the general population. Female preponderance (male-to-female ratio = 1:2) but no racial predisposition.

Both are autoimmune-mediated diseases, but the mechanisms are different and the exact etiology is unknown.

  • Primary idiopathic polymyositis (adult)
  • Primary idiopathic dermatomyositis (adult)
  • Childhood dermatomyositis or polymyositis with necrotizing vasculitis
  • Polymyositis associated with connective tissue disorder (i.e., overlap syndrome, mixed connective tissue syndrome)
  • Polymyositis or dermatomyositis associated with malignant tumors

Whether the pathophysiologies for dermatomyositis and polymyositis are different or the same is controversial. For some authors, these two diseases differ from each other mainly by the presence of dermatologic features (dermatomyositis) or the lack thereof (polymyositis). In the opinion of the proponents of a different pathophysiology, the two diseases differ as follows:

  • Dermatomyositis: Activation of the humoral part of the immune system with antibodies directed against small arterioles and capillaries of the muscles seems to be the main culprit. Deposition of the complement factors C5b-9 (so-called membrane attack factor) results in antibody-mediated cell death. As the inflammatory process continues, capillary destruction results in atrophy, necrosis of muscle fibers, and, finally, muscle weakness and muscle tenderness.
  • Polymyositis: Abnormal cellular immune response is considered responsible for the symptoms of this disease. Some evidence exists for a cytotoxic, T-cell-mediated attack directed toward muscular components. The factors triggering this cascade are unknown, but viruses could play a role (picornavirus-like structures have been found in affected muscle cells). Electron microscopy revealed tubular inclusions in endothelial cells of affected skin and muscle vessels and in surrounding monocytes, similar to those seen in other viral infections. Other viruses that have been blamed include retroviruses (HIV, HTLV-1) and coxsackievirus B.

Pathognomonic features are heliotrope rash (violaceous erythema and edema) of the periorbital region and the eyelids, a reddish rash on the face and upper torso, and Gottron papules (violaceous-colored papules found on metacarpophalangeal and distal interphalangeal joints [knuckles] and the extensor aspects of knees and elbows). Sun exposure can exacerbate the skin lesions. Dilated cuticular telangiectasias at the base of the fingernails also are characteristic for dermatomyositis. Muscle biopsy, if obtained, detects degeneration and necrosis of muscle fibers that cause them to lose their staining characteristics so that the cytoplasm appears empty, resulting in the term ghost fibers. These changes combined with inflammatory cell infiltrates around blood vessels enforce the diagnosis of dermatomyositis. Four of the five following diagnostic criteria must be present for definite diagnosis: (1) cutaneous lesions typical for dermatomyositis; (2) elevated muscle enzyme levels (creatine kinase, transaminases, aldolase, lactate dehydrogenase); (3) progressive, proximal, symmetrical muscle weakness (shoulder and hip) developing over weeks to months; (4) pathologic electromyography (EMG), and (5) typical findings on muscle ...

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