Diagnosis of patients with suspected immune-mediated necrotizing myopathy

IMNM: immune-mediated necrotizing myopathy; CK: creatine kinase; DM: dermatomyositis; HMGCR: 3-hydroxy-3-methylglutaryl coenzyme A reductase; EMG: electromyography; MRI: magnetic resonance imaging; AMA: antimitochondrial antibody; SRP: signal-recognition particle; EKG: electrocardiogram; TTE: transthoracic echocardiogram.

* Refer to UpToDate content on cutaneous findings consistent with DM and antisynthetase syndrome.

¶ Serologic tests for myositis-specific autoantibodies are often available as a panel to eliminate the need to order autoantibody tests individually. However, the results are often not available for several weeks, and treatment decisions may be required before the specific serologic profile is known. In some clinical settings, specific autoantibody tests can be ordered individually and may be available more quickly (eg, anti-Jo-1, anti-HMGCR).

Δ Some clinicians forgo a muscle biopsy when clinical phenotype consistent with IMNM and anti-HMGCR autoantibody testing is unequivocally positive.

◊ Muscle biopsy is often obtained to verify diagnosis.

§ Perform age- and sex-appropriate malignancy workup.

¥ For patients with overlap features suggestive of another systemic rheumatic disease (eg, systemic sclerosis, Sjögren's disease), additional testing for myositis-associated autoantibodies may be appropriate. Refer to UpToDate content on myositis-associated autoantibodies. If AMA is positive and if clinically indicated, cardiac workup should include EKG and TTE.