Polymyositis (PM)

Diagnosis

People with polymyositis (PM) may exhibit some characteristic signs and symptoms, such as chronic muscle weakness. Given that PM presents similarly to other idiopathic inflammatory myopathies (IIMs), however, diagnosis of PM is often achieved by exclusion of other IIM subgroups. Diagnosis may require a combination of testing modalities.

Test	Characteristic findings
Clinical exam	Weakness in the proximal skeletal muscles Lack of skin abnormalities often found in dermatomyositis
Blood tests	Elevated creatine kinase (CK) and other muscle enzymes such as aldolase
Myositis specific or associated autoantibodies (MSA, MAA)	These myositis autoantibodies can be associated with PM and when associated with anti-synthetase antibodies it is called anti-synthetase syndrome.
Assessment of electrical activity in muscles/nerves	Distinctive pattern of electrical activity in affected muscles, as assessed by electromyogram Distinctive pattern of electrical activity in affected nerves, as assessed by a nerve conduction velocity test
Muscle biopsy	Distinctive pattern of electrical activity in affected muscles, as assessed by electromyogram Distinctive pattern of electrical activity in affected nerves, as assessed by a nerve conduction velocity test
Magnetic resonance imaging (MRI)	Infiltration of muscles tissue by cytotoxic T cells Lack of muscle biopsy findings associated with other IIMs (e.g., inclusion bodies in inclusion body myositis, blood vessel inflammation in dermatomyositis, etc.)

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. Findlay AR, Goyal NA, Mozaffar T. An overview of polymyositis and dermatomyositis. Muscle Nerve. 2015;51(5):638-656. doi:10.1002/MUS.24566
2. Baig S, Paik JJ. Inflammatory muscle disease – An update. Best Pract Res Clin Rheumatol. 2020;34(1):101484. doi:10.1016/j.berh.2019.101484

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