Characterized by proximal muscle weakness (muscles closest to the body’s midline), IMNM can exist in three different subtypes. Depending on the presence of different autoantibodies in the blood, the subtypes include:
Weakness in the proximal muscles, difficulty climbing stairs and standing up from a chair, difficulty lifting arms over the head, falling, and fatigue.
Symptoms differ in severity with each subtype. Anti-SRP myopathy typically has the most severe weakness and most prevalent extra-muscular features along with cardiac involvement.
Malignancies (autoantibody-negative IMNM), myocarditis (anti-SRP myopathy), and interstitial lung disease (anti-SRP myopathy).
Exposure to statin (a cholesterol-lowering drug) is thought to be a risk for anti-HMGCR myopathy. Tumors have also been identified as a potential trigger for developing autoantibody-negative IMNM.
Allenbach, Y., Benveniste, O., Stenzel, W., & Boyer, O. (2020). Immune-mediated necrotizing myopathy: Clinical features and pathogenesis. Nature Reviews Rheumatology, 16(12), 689–701. https://doi.org/10.1038/s41584-020-00515-9
The Myositis Association. (2019, May 15). Necrotizing myopathy. The Myositis Association. https://www.myositis.org/about-myositis/types-of-myositis/necrotizing-myopathy/
Pinal-Fernandez, I., Casal-Dominguez, M., & Mammen, A. L. (2018). Immune-Mediated Necrotizing Myopathy. Current rheumatology reports, 20(4), 21. https://doi.org/10.1007/s11926-018-0732-6
