Immune-Mediated Necrotizing Myopathy (IMNM)

Diagnosis

People with immune-mediated necrotizing myopathy (IMNM) may exhibit some characteristic signs and symptoms, such as severe proximal muscle weakness. Given that IMNM presents similarly to other IIMs, however, diagnosis may require a combination of testing modalities:

Test	Characteristic findings
Clinical exam	Weakness in the proximal skeletal muscles
Blood tests	Highly elevated creatine kinase (CK) and other muscle enzymes such as aldolase
Myositis autoantibody testing	Presence of specific autoantibodies associated with IMNM (e.g. anti-SRP, anti-HMGCR)
Assessment of electrical activity in muscles/nerves	Distinctive pattern of electrical activity in affected muscles, as assessed by electromyogram
Muscle biopsy	Signs of muscle fiber necrosis and regeneration Little to no lymphocytic (white blood cell) infiltration into muscle tissues

New research and a better understanding of the IIMs has improved the ability to distinguish between and diagnose the different IIM subgroups. Many cases that were previously diagnosed as PM are now thought to be better classified as one of the other IIMs based on the above testing criteria.

References

1. Dalakas MC. Inflammatory Muscle Diseases. Longo DL, ed. N Engl J Med. 2015;372(18):1734-1747. doi:10.1056/NEJMra1402225
2. Pinal-Fernandez I, Casal-Dominguez M, Mammen AL. Immune-Mediated Necrotizing Myopathy. Curr Rheumatol Rep. 2018;20(4):21. doi:10.1007/s11926-018-0732-6
3. Allenbach Y, Mammen AL, Benveniste O, Stenzel W, Immune-Mediated Necrotizing Myopathies Working Group. 224th ENMC International Workshop:: Clinico-sero-pathological classification of immune-mediated necrotizing myopathies Zandvoort, The Netherlands, 14-16 October 2016. Neuromuscul. Disord. NMD 2018;28:87–99.

Last update: Feb 2023
Reviewed by Julie Paik, MD, MHS; Johns Hopkins University