Idiopathic inflammatory myopathies

Idiopathic inflammatory myopathies (IIMs) constitute a heterogeneous group of systemic autoimmune disorders that cause inflammation of skeletal muscles. Inflammation often results in muscle weakness, and may involve skin, joints, lungs, and heart.

Certain subtypes of IIMs are associated with cancer occurrence, especially during the 3 years before and after IIM diagnosis.

Presentation of IIM is subacute or chronic, and rarely acute. Patients with subacute disease present with typical dermatomyositis rash without skeletal muscle involvement (DM sine myopathy). Severe presentations include generalized weakness, dysphagia, respiratory failure, and cardiac involvement.

Diagnosis is confirmed with a thorough history and physical exam, followed by corroborative investigations which include serum muscle-derived enzymes and damage markers, myositis-specific and myositis-associated autoantibodies, electromyography, magnetic resonance imaging and biopsy of muscles.

Remission induction of IIM (except inclusion body myositis) is usually achieved with corticosteroids and early introduction of corticosteroid-sparing immunosuppressants. Intravenous immunoglobulin is an effective adjunct therapy and can be considered in patients with refractory IIM.

Maintenance of remission should be achieved with corticosteroid-sparing immunosuppressants with low-dose or no corticosteroid therapy.

No effective medical treatment for sporadic inclusion body myositis is available.

Specialist multidisciplinary team involvement is essential for optimal patient outcomes.

IIMs constitute a heterogeneous group of systemic autoimmune disorders that cause inflammation of skeletal muscles. Inflammation often results in muscle weakness, and may involve the skin, joints, lungs and heart.[1]Khoo T, Lilleker JB, Thong BY, et al. Epidemiology of the idiopathic inflammatory myopathies. Nat Rev Rheumatol. 2023 Nov;19(11):695-712. http://www.ncbi.nlm.nih.gov/pubmed/37803078?tool=bestpractice.com

Clinical subtypes of IIM include juvenile dermatomyositis, inclusion myositis, immune-mediated necrotizing myopathy, antisynthetase syndrome, and overlap myositis.[2]Ashton C, Paramalingam S, Stevenson B, et al. Idiopathic inflammatory myopathies: a review. Intern Med J. 2021 Jun;51(6):845-52. https://onlinelibrary.wiley.com/doi/10.1111/imj.15358 http://www.ncbi.nlm.nih.gov/pubmed/34155760?tool=bestpractice.com [3]Lundberg IE, Tjärnlund A, Bottai M, et al. 2017 European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies and their major subgroups. Ann Rheum Dis. 2017 Dec;76(12):1955-64. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5736307 http://www.ncbi.nlm.nih.gov/pubmed/29079590?tool=bestpractice.com

Each subtype is associated with a distinct clinical phenotype, and often myositis-specific and/or myositis-associated autoantibodies (MSAs/MAAs).[2]Ashton C, Paramalingam S, Stevenson B, et al. Idiopathic inflammatory myopathies: a review. Intern Med J. 2021 Jun;51(6):845-52. https://onlinelibrary.wiley.com/doi/10.1111/imj.15358 http://www.ncbi.nlm.nih.gov/pubmed/34155760?tool=bestpractice.com

This topic covers treatment options for adults.