Polymyositis is a rare autoimmune, at times considered paraneoplastic, inflammatory condition characterised by proximal muscle weakness. It is considered a form of idiopathic inflammatory myopathy (IIM). 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 recognised female predilection (~2F:M).

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 oedema

Other features, in a chronic phase, may include 3:

  • fatty infiltration of muscles
  • muscle calcification

This condition is usually progressive.

On MRI there is a broad differential:

Some differentials to consider include:

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Article information

rID: 29446
Synonyms or Alternate Spellings:
  • Polymyositis (PM)

Cases and figures

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    Case 1
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    Case 2: with lung involvement
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