Polymyositis (PM) 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.
Its 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 whose in their fourth decade. There is a recognoised female predilection (~2F:1M).
Muscle involvement is usually symmetric, but not necessarily evenly distributed.
- T2/STIR: affected regions during active disease have high signal intensity, reflecting muscle oedema
Other features include 3
- fatty infiltration of muscles
- muscle calcification
Treatment and prognosis
This condition usually tends to be progressive.
On MRI there is a broad differential
- 1. O'Connell MJ, Powell T, Brennan D et-al. Whole-body MR imaging in the diagnosis of polymyositis. AJR Am J Roentgenol. 2002;179 (4): 967-71. doi:10.2214/ajr.179.4.1790967 - Pubmed citation
- 2. Akira M, Hara H, Sakatani M. Interstitial lung disease in association with polymyositis-dermatomyositis: long-term follow-up CT evaluation in seven patients. Radiology. 1999;210 (2): 333-8. doi:10.1148/radiology.210.2.r99ja15333 - Pubmed citation
- 3. Tomasová Studynková J, Charvát F, Jarosová K et-al. The role of MRI in the assessment of polymyositis and dermatomyositis. Rheumatology (Oxford). 2007;46 (7): 1174-9. doi:10.1093/rheumatology/kem088 - Pubmed citation
- 4. Dion E, Cherin P, Payan C et-al. Magnetic resonance imaging criteria for distinguishing between inclusion body myositis and polymyositis. J. Rheumatol. 2002;29 (9): 1897-906. J. Rheumatol. (abstract) - Pubmed citation