Polymyositis is an idiopathic inflammatory myopathy.
As a standalone entity, this condition is exceedingly rare with reported prevalence rates of .
5 to 8.
4 per million populations.
Polymyositis (PM), Dermatomyositis and Inclusion Body Myositis are often grouped together although they do have different pathogenesis.
Polymyositis tends to affect predominantly adults and the Male to Female prevalence is 1:2.
Children are not affected unlike Dermatomyositis which affects children.
Pathophysiology The exact mode of disease causation is not known, but it is clear that the immune system plays an important role in the pathogenesis of PM.
Although the exact sequence of events is not clear, it is postulated that viral infections (HIV, HTLV-1, Simian retrovirus and the coxsackievirus B), toxins or hereto unknown factors cause damage to the vascular endothelium and leads to release of muscle auto antigens.
These auto antigens are in turn presented to the T cells which secrete pro inflammatory cytokines which further increase the influx of inflammatory cells.
The inflammatory cells active in PM belong to the mononuclear type and consist of CD8+ T cells and macrophages.
The activated T cells and macrophages produce many pro inflammatory cytokines, important among which are TNF, Interferon gamma, and Interleukin -2.
These cytokines increase permeability, cause endothelial damage and finally cause muscle damage by a common pathway of complement activation.
The pro inflammatory cytokines also induce aberrant expression of the antigens of the Major histocompatibility complex.
Aberrant expression of this leads to stimulation of CD8+ T cells which cause muscle damage by their direct cytotoxic effect.
Antibodies in Polymyositis A number of antibodies have been detected in PM.
Up to 50% of patients suffering from PM have auto antibodies in their blood.
These antibodies are classifies as Myositis associated antibodies (MAA) and Myositis Specific Antibodies (MSA).
MAA is present in 20-50% of patients and MSA is present in 20-40% of the patients suffering from Myositis.
Myositis Specific Antibodies Myositis Specific Antibodies (MSA) are specific for PM and DM and is seen in 20-40% of patients.
The three main target antigens are:- · tRNA (Anti Synthetase): These are most specific for PM.
It is detected in around 23% of patients suffering from PM.
This antibody is directed against the tRNA of various amino acids.
These anti synthetase have been detected for 6 out of the 20 amino acids.
The commonest of these anti synthetase is directed against histadyl tRNA.
This antibody is called Anti Jo-1.
Antibodies specific for other amino acids like Glycine (anti EJ), alanine (anti PL12),isoleucine are seen in less than 1% patients.
The anti Jo-1 antibody is more specific for PM than DM and is rare in children.
· Nuclear Mi2 protein: The target antigen is a protein complex comprised of seven proteins involved in transcription.
This is a specific serologic marker for DM.
This antibody is usually associated with an acute onset, good response to immunosuppressant medications and a good prognosis.
· Signal Related Polypeptide: The target antigen is a protein RNA complex comprising six proteins and a 300 nucleotide RNA.
It is usually associated with acute PM, cardiac involvement, poor response to therapy and a generally poor prognosis.