Polymyositis

Polymyositis (PM)("inflammation of many muscles")is a type of chronic inflammation of the muscles (inflammatory myopathy) related to dermatomyositis and inclusion body myositis.

Signs and symptoms
Symptoms include pain, with 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 this is non-specific and occurs in other autoimmune connective tissue disorders. Dysphagia (difficulty swallowing) and/or other aspects of oesophageal dysmotility 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 diseases.

Polymyositis is linked to an increase in the occurrence of certain cancers (particularly Non-Hodgkin lymphoma, lung and bladder cancers), but the overall association is weaker than in the related condition Dermatomyositis. The latter is linked to a significant increase in the risk of a variety of malignances, including ovarian, lung, and pancreatic cancer.

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.

Another concern is Interstitial lung disease (ILD).

Causes
The etiology of polymyositis is unknown and may be multifactorial, perhaps related to autoimmune factors, genetics, and viruses. In rare cases, the cause is known to be infectious, associated with the pathogens that cause Lyme disease, toxoplasmosis, and other infectious agents. Polymyositis usually is considered non-fatal in the absence of ILD.

It is hypothesized that an initial injury causes release of muscle auto antigen, which is subsequently taken up by macrophages and presented to CD4+ TH cells. Activated TH cells synthesize IFN-γ that stimulate further macrophages and further inflammatory mediator release like IL-1 and TNF-α

Another important event in the pathogenesis of Polymyositis is the increased expression of MHC proteins by m/s cells. Auto-Ag is presented in association with MHC-I molecules on the surface of Myocytes and is recognized by CD8 cytotoxic T cells that subsequently initiate m/s destruction.

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 dermatomyositis 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.

Laboratory findings
Presence of Anti Jo antibodies in >65% of patients. Elevated serum creatine kinase is characteristic, but not specific to polymyositis.

Anti-signal recognition particle antibodies (anti-SRP antibodies) are associated with polymyositis, but are not very specific for it. For individuals with polymyositis, the presence of anti-SRP antibodies are associated with more prominent muscle weakness and atrophy.

Treatment
Typically, high-dose steroids are the treatment of choice. Generally, muscle strength will improve within 4–6 weeks (useful to distinguish from inclusion body myositis). Unresponsive patients may be tried on other immunosuppressive medications. IVIG has also shown to be a beneficial treatment. Specialized exercise therapy may supplement treatment to enhance quality of life.