Immune-mediated necrotizing myopathy (IMNM) is a subgroup of the idiopathic inflammatory myopathies. There are three major subtypes based on autoantibodies, of which statin-associated autoimmune myopathy has been increasing in recent years. It is characterized by myopathological findings of multiple necrotic and regenerating fibers with little infiltration of inflammatory cells.
As many as 40% of patients previously diagnosed with polymyositis and 19% of "possible myositis" would be classified as IMNM, but with the increase in the use of statins in 2010s, the number of cases of immune-mediated necrotizing myopathy has also increased. It is considered that 20-40% of myositis cases are immune-mediated necrotizing myopathy.
Depending on the autoantibody detected, it is classified into three major subtypes: anti-SRP antibody-positive immune-mediated necrotizing myopathy (30-40%), anti-HMGCR antibody-positive immune-mediated necrotizing myopathy (26-50%), and seronegative immune-mediated necrotizing myopathy (25-40%).