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Polymyositis is a rare autoimmune, at times considered paraneoplastic, inflammatory condition characterized by proximal muscle weakness (myositis). It is considered a form of idiopathic inflammatory myopathy. The condition is closely related to dermatomyositis, and the term “polymyositis” is applied when the condition spares the skin.
It principally affects the skeletal system but can also affect other organ systems, such as the lungs (see polymyositis - lung involvement).
There are several forms of the disease. The most common form affects those in their fourth decade. There is a recognized female predilection (F:M = ~2:1).
Polymyositis usually presents insidiously over a few months with symmetrical painless muscular weakness with no cutaneous manifestations (cf. dermatomyositis). Myalgia has been reported in less than one-third of the patients.
Muscle involvement is usually symmetric, but not necessarily evenly distributed.
MRI has been used for diagnosis, follow-up, and to identify the best site for muscle biopsy.
T2/STIR: affected regions during active disease have high signal intensity, reflecting muscle edema
Other features, in a chronic phase, may include 3:
fatty infiltration of muscles
Treatment and prognosis
This condition is usually progressive.
On MRI there is a broad differential:
Some differentials to consider include:
usually, will not involve all the muscular compartments and will not present with a relatively symmetrical bilateral pattern
usually involving patients over 50 years
has a more focal, sometimes nodular, myositis
drug-induced myositis (e.g. statins, antiretroviral drugs used in HIV treatment)
reversible after discontinuation of the medication
muscles evolve from an edematous phase to become atrophic
there is no associated skin and subcutaneous edema
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