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Signs and symptoms
Symptoms include pain, with marked weakness and/or loss of muscle mass in the muscles of the head, neck, torso and upper arms and legs. The hip extensors are often severely affected, leading to particular difficulty in ascending stairs and rising from a seated position. Dysphagia (difficulty swallowing) or other problems with esophageal motility occur in as many as 1/3 of patients. Low grade fever and peripheral adenopathy may be present. Foot drop in one or both feet can be a symptom of advanced polymyositis and inclusion body myositis. Polymyositis is also associated with interstitial lung disease.
Polymyositis tends to become evident in adulthood, presenting with bilateral proximal muscle weakness often noted in the upper legs due to early fatigue while walking. Sometimes the weakness presents itself as an inability to rise from a seated position without help or an inability to raise one's arms above one's head. The weakness is generally progressive, accompanied by lymphocytic inflammation (mainly cytotoxic T lymphocytes).
Polymyositis, like dermatomyositis, strikes females with greater frequency than males. The skin involvement of dermatomyositis is absent in polymyositis.
The cause of polymyositis is unknown and may involve viruses and autoimmune factors. Cancer may trigger polymyositis and dermatomyositis, possibly through an immune reaction against cancer that also attacks a component of muscles.
Sporadic inclusion body myositis (sIBM): IBM is often confused with (misdiagnosed as) polymyositis and dermatomyositis that does not respond to treatment is likely IBM. sIBM comes on over months to years; polymyositis comes on over weeks to months. Polymyositis tends to respond well to treatment, at least initially; IBM does not.
Notable individuals with polymyositis
- Dan Christensen painter of abstract art. Died due to heart failure caused by polymyositis.
- Robert Erickson American composer and teacher who was a leading modernist exponent of "12-tone" composition. Died from the effects of polymyositis.
- David Lean film director. 
- Hill, C.L.; Zhang, Y.; Sigurgeirsson, B.; Pukkala, E.; Mellemkjaer, L.; Airio, A.; Evans, S.R.; Felson, D.T. (January 2001). "Frequency of specific cancer types in dermatomyositis and polymyositis: a population-based study". Lancet 357 (9250): 96–100. doi:10.1016/S0140-6736(00)03540-6. PMID 11197446.
- "Polymyositis and Dermatomyositis: Autoimmune Disorders of Connective Tissue". Merck Manual Home Edition. August 2013. Retrieved 1 January 2015.
- "Dan Christensen, 64, Painter of Abstract Art, Dies". New York Time. 27 January 2007. Retrieved 16 July 2012.
- "Obutiary -- Robert Erickson". New York Time. 29 April 1997. Retrieved 16 July 2012.
- Stevens, Jr., George (2006). Conversations with the Great Moviemakers of Hollywood's Golden Age at the American Film Institute. Knopf. p. 427. ISBN 978-1-4000-4054-4.
- Brownlow, Kevin (15 August 1996). David Lean: A Biography. Macmillan. pp. 1466–1467. ISBN 978-1-4668-3237-4. Retrieved 1 January 2015.
- The Myositis Association