Polymyositis

Not to be confused with poliomyelitis.

Polymyositis
Classification and external resources
ICD-10	M33.2
ICD-9	710.4
DiseasesDB	10343
MedlinePlus	000428
eMedicine	med/3441 emerg/474
MeSH	D017285

Polymyositis is a type of chronic 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 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.

Based on the conclusion of the paper "Interstitial lung disease (ILD) in Polymyositis and dermatomyositis" by Maryann Fathi and Ingrid E Lundberg published 12/13/2005:

Investigations to detect interstitial lung disease should be performed during the initial evaluation as well as during follow-up of patients with myositis, because ILD is a frequent manifestation in patients with polymyositis or dermatomyositis and because ILD is associated with increased morbidity and mortality. This evaluation should include chest radiograph, HRCT of lungs, pulmonary function tests including diffusing capacity, and serum levels of anti-Jo1 antibodies. In the patients with ILD, clinical or subclinical, treatment with high doses of corticosteroids in combination with other immunosuppressive therapy should be initiated. Some histopathologic features including DAD, UIP, neutrophil alveolitis, digital infarcts showing microangiopathy in dermatomyositis, and amyopathic dermatomyositis have all been reported as risk factors for poor outcome. Presence of these factors is an indication for the use of aggressive immunosuppressive therapy (e.g., Methotrexate) as well as careful monitoring of lung function.

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.

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.

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(PM) 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 ovarian, lung, pancreatic, stomach, and colorectal cancers, and non-Hodgkin lymphoma. Patients suffering from this disease should ensure that they are suitably monitored and look for tell-tale signs which might indicate the presence of these, and other, cancers.

Laboratory findings

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

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 other immunosuppressive medications. IVIG has also shown to be a beneficial treatment. Specialized exercise therapy may supplement treatment to enhance quality of life.

External links

• The Myositis Association [1]

de:Polymyositis tr:Polimiyozit