Idiopathic inflammatory myopathy is applied to a group of autoimmune inflammatory muscle disorders (myositis). Which specific disorders are included in this group has evolved, particularly with advances in the discovery of myositis-specific antibodies 1,2.
Diagnosis
The diagnosis of idiopathic inflammatory myopathies relies on symptoms, clinical examination, MRI findings, and biochemical results (e.g. muscle enzymes, autoantibodies) 5,6, although the gold standard is considered a histological diagnosis on muscle biopsy 6.
Pathology
There are several distinct conditions considered to be idiopathic inflammatory myopathies 1,2:
sporadic inclusion body myositis
Additional suggested subtypes include 5
cancer-associated myositis
Polymyositis was historically classified as an idiopathic inflammatory myopathy 3, however, many patients given this diagnosis occurred before many myositis-specific antibodies were discovered, and subsequent research of these historical cases has shown that many cases would be re-classified as other idiopathic inflammatory myopathies 1. Thus, the term polymyositis is generally discouraged, and the existence of polymyositis as a standalone entity is a subject of conjecture 2,4.
Markers
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myositis-associated autoantibodies
e.g. anti-Ro52, anti-PMScl, anti-Ku, anti-U1RNP 5
positive in up to 60% of patients 5
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