Classification of idiopathic inflammatory myopathies
Classification of idiopathic inflammatory myopathies is a classification of idiopathic inflammatory myopathies. Dermatomyositis, polymyositis, necrotising autoimmune myopathy (NAM) and overlap myositis usually have similar clinical presentations with respect to pattern of muscle weakness and elevated serum creatine kinase (CK) concentration.
Polymyositis as a separate clinical entity is now being questioned with increasing knowledge of the myositis-specific antibodies. Most cases of polymyositis are likely to be classified as a type of NAM.
Inclusion body myositis (IBM) differs from all the other idiopathic inflammatory myopathies in its pattern of muscle weakness, normal or mildly raised serum CK concentration, and poor response to treatment.
- Dermatomyositis, includes:
- Adult dermatomyositis with or without malignancy
- Juvenile dermatomyositis.
- Polymyositis is increasingly thought to be a type of necrotising autoimmune myopathy (NAM).
- Necrotising autoimmune myopathy (NAM) [NB1] includes:
- HMGCoA-R–antibody positive (statin-induced NAM and statin-naive NAM)
- SRP–antibody positive NAM
- autoantibody negative NAM.
- Overlap myositis is the most common type of idiopathic inflammatory myopathy and is associated with other inflammatory connective tissue diseases.
- Antisynthetase syndrome is the most common subtype of overlap myositis. Features include myositis, fever, arthralgia, Raynaud syndrome, severe interstitial lung disease and mechanic’s hands.
- Inclusion body myositis (IBM) has a more distal pattern of muscle weakness, a more progressive course, and is more treatment resistant than the other idiopathic inflammatory myopathies.
HMGCoA-R = 3-hydroxy-3-methyl-glutaryl-CoA reductase; SRP = signal recognition particle
NB1: necrotising autoimmune myopathy (NAM) is also known as immune necrotising myopathy.