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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.
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Approximately 5-10:100,000 in the general population.
Female preponderance (male-to-female ratio = 1:2) but no racial
predisposition.
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Both are autoimmune-mediated diseases, but the
mechanisms are different and the exact etiology is unknown.
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Primary idiopathic polymyositis (adult)
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Primary idiopathic dermatomyositis (adult)
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Childhood dermatomyositis or polymyositis with necrotizing vasculitis
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Polymyositis associated with connective tissue disorder (i.e., overlap
syndrome, mixed connective tissue syndrome)
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Polymyositis or dermatomyositis associated with malignant tumors
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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:
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- 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.
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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 ...