Clinical features of necrotising autoimmune myopathy

Allenbach, 2020Ashton, 2021Schmidt, 2018

Necrotising autoimmune myopathy (NAM), also known as immune necrotising myopathy, is a group of inflammatory myopathiesAllenbach, 2020 responsible for approximately one-fifth of all cases of idiopathic inflammatory myopathySchmidt, 2018. Polymyositis is a rare condition that usually affects muscles only and is thought to be a type of NAM.

NAM is recently described, though increasingly prevalent, and characterised by rapidly progressive, severe proximal muscle weakness in association with a very high serum creatine kinase (CK) concentration (20 to 50 times normal)Schmidt, 2018. Cardiorespiratory features can be present (eg interstitial lung disease, myocarditis).

NAM can occur concomitantly with malignancy (including lung, breast and ovarian carcinoma and lymphomaSchmidt, 2018), other inflammatory connective tissue diseases and viral infections (including human immunodeficiency virus [HIV] and hepatitis C). Rarely, it is associated with use of statins; see statin-induced NAM.

NAM is diagnosed on clinical features and typical muscle biopsy findings. On autoantibody testing, it can be associated with 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGCoA-R) antibodies (possibly associated with statin exposure) and signal recognition particle (SRP) antibodies. Approximately 20% of people with NAM are autoantibody negative.

People with NAM and positive SRP antibodies are more likely to develop interstitial lung disease or myocarditis than those with positive HMGCoA-R antibodiesAllenbach, 2020.

Management for NAM is outlined in Management overview for idiopathic inflammatory myopathies.