Idiopathic inflammatory myopathies - Polymyositis/Dermatomyositis

Systemic inflammatory disease of the skeletal muscle and skin with polymyositis (PM) and dermatomyositis (DM) being the most common types diagnosed. Typically symptoms can include progressive weakness which if not treated can eventually lead to muscle atrophy.

Pulmonary fibrosis can occur in subgroups of IIM while other subgroups are associated with malignancy. DM has a complement-dependent membranolysis of the intramuscular capillaries whereas PM is associated with cytotoxic T lymphocytes.
               
Multiple autoantibodies have been detected in IIM, some of which can be clinically useful in terms of sub-classifying the disease, determining response to treatment, associations with malignancy and response to treatment.

Current diagnostic markers include autoantibodies directed against several tRNA-synthetases (for example anti-Jo-1), signal recognition particles (SRP) and also Mi-2 of the nucleosome modelling complex. Jo-1 has a high specificity for IIM and is found in around 30% of adults exhibiting PM symptoms. Signal recognition particle antibodies mainly react with SRP54 component of the recognition particle and is associated with necrotising myositis. Anti-Mi-2 antibodies can be found more frequently in DM patients and around 95% of patients who test positive have DM.

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