Background/Purpose: The aim of this study was to define the pattern of muscle involvement in patients with immune-mediated necrotizing myopathy (IMNM).

Methods: All Johns Hopkins Myositis Longitudinal Cohort subjects with an available thigh magnetic resonance imaging (tMRI) who fulfilled criteria for IMNM, dermatomyositis (DM), polymyositis (PM), clinically amyopathic DM (CADM) or inclusion body myositis (IBM) were included in the study. Fifteen muscles were assessed by tMRI for the presence or absence of edema, atrophy, fatty replacement, and fascial edema. Disease subgroups were compared using univariate and multivariate analysis. Within IMNM, patients with anti-SRP and anti-HMGCR were also compared.

Results: The study included 666 subjects (101 IMNM, 176 PM, 219 DM, 17 CADM and 153 IBM). Compared to patients with DM or PM, IMNM was characterized by a higher proportion of thigh muscles with edema, atrophy, and fatty replacement (p<0.01), independent of other confounding variables. Compared with anti-SRP, anti-HMGCR patients showed a lesser proportion of muscles with atrophy (-9%, p=0.04). In IMNM, muscle abnormalities were especially common in the lateral rotator and gluteal groups. Fascial involvement was more widespread in DM. Interestingly, fatty replacement of muscle tissue began early during the course of IMNM as well as the other muscle diseases. Although it had a negative predictive value of 93%, an optimal combination of tMRI features had only a 55% positive predictive value for diagnosing IMNM.

Conclusion: Compared to patients with DM or PM, IMNM is characterized by more widespread muscle involvement. Compared with anti-HMGCR, anti-SRP patients seem to have more severe muscle involvement.