Polymyositis is a type of inflammatory myopathy, related to dermatomyositis and inclusion body myositis. Polymyositis means 'many muscle inflammation'.

Presentation

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 by the person being unable to rise from a seated position without help, or inability to raise their arms above their head. The weakness is generally progressive, accompanied by lymphocytic inflammation (mainly cytotoxic T8 lymphocytes).

Polymyositis, like dermatomyositis, strikes females with greater frequency than males. The skin involvement of dermatomyositis is absent in polymyositis.

Causes

The cause is unknown, but seems to be related to autoimmune factors, genetics, and perhaps viruses. In rare cases, the cause is known to be infectious, associated with the pathogens that cause Lyme disease, toxoplasmosis, and others.

Diagnosis

Diagnosis is fourfold, including elevation of creatine kinase,history and physical examination, electromyograph (EMG) alteration, and a positive muscle biopsy.

Sporadic inclusion body myositis (sIBM): IBM is often confused with (misdiagnosed as) polymyositis and polymyositis that does not respond to treatment is likely IBM. sIBM comes on over months to years, polymyositis comes on over weeks to months. It appears that sIBM and polymyositis share some common features, especially the initial sequence of immune system activation, however, polymyositis does not display the subsequent muscle degeneration and protein abnormalities as seen in IBM. As well, polymyositis tends to respond well to treatments, IBM does not. IBM and polymyositis apparently involve different disease mechanisms than are seen in dermatomyositis.

Symptoms

Marked weakness and/or loss of muscle mass in the proximal musculature, particularly in the shoulder and pelvic girdle. The hip extensors are often severely affected, leading to particular difficulty in ascending stairs and rising from a seated position.Thickening of the skin on the fingers and hands (sclerodactyly) is a frequent feature, although it is non-specific and occurs in other autoimmune disorders of the connective tissues. Dysphagia (difficulty swallowing) and/or other aspects of oesophageal dysmotility occur in as much as 1/3 of patients. Low grade fever and peripheral adenopathy may be present.

Lab

Presence of Anti Jo antibodies in >65% of patients.

References

• The Myositis Association
• Kasper DL, Braunwald E, Fauci AS, Hauser SL, Longo DL, Jameson JL. Harrison's Principles of Internal Medicine. New York: McGraw-Hill, 2005. ISBN 0-07-139140-1