Thigh muscle MRI in immune-mediated necrotising myopathy: extensive oedema, early muscle damage and role of anti-SRP autoantibodies as a marker of severity

Abstract

Objectives The aims of this study were to define the pattern of muscle involvement in patients with immune-mediated necrotising myopathy (IMNM) relative to those with other inflammatory myopathies and to compare patients with IMNM with different autoantibodies. Methods All Johns Hopkins Myositis Longitudinal Cohort subjects with a thigh MRI (tMRI) who fulfilled criteria for IMNM, dermatomyositis (DM), polymyositis (PM), inclusion body myositis (IBM) or clinically amyopathic DM (CADM) were included in the study. Muscles were assessed for intramuscular and fascial oedema, atrophy and fatty replacement. Disease subgroups were compared using univariate and multivariate analyses. Patients with IMNM with anti-signal recognition particle (SRP) autoantibodies were compared with those with IMNM with anti-HMG-CoA reductase (HMGCR) autoantibodies. Results The study included 666 subjects (101 IMNM, 176 PM, 219 DM, 17 CADM and 153 IBM). Compared with DM or PM, IMNM was characterised by a higher proportion of thigh muscles with oedema, atrophy and fatty replacement (p<0.01). Patients with IMNM with anti-SRP had more atrophy (19%, p=0.003) and fatty replacement (18%, p=0.04) than those with anti-HMGCR. In IMNM, muscle abnormalities were especially common in the lateral rotator and gluteal groups. Fascial involvement was most widespread in DM. Fatty replacement of muscle tissue began early during the course of disease in IMNM and the other groups. An optimal combination of tMRI features had only a 55% positive predictive value for diagnosing IMNM. Conclusions Compared with patients with DM or PM, IMNM is characterised by more widespread muscle involvement. Anti-SRP-positive patients have more severe muscle involvement than anti-HMGCR-positive patients.

Immune-Mediated Necrotizing Myopathy

“…Muscle MRI has only limited value in diagnosing patients with IMNM because it discriminates poorly between patients with IMNM compared to other types of myositis (e.g., dermatomyositis) [40••]. However, muscle MRI may be a useful tool to monitor the evolution of muscle disease over time.…”

“…In this regard, we have recently shown that in all types of myositis, including IMNM, fatty replacement begins very early after the onset of the disease. This suggests the possibility that early and intense induction therapy may help to decrease long-term disability in these patients [40••]. …”

“…Overall, the extent of muscle involvement as assessed by MRI is higher in IMNM patients than in those with DM or polymyositis (PM) and similar to IBM (Fig. 2) [40••, 42]. …”

“…Specifically, anti-SRP patients show more extensive edema, atrophy, and fatty replacement in the lateral rotator group; more atrophy and fatty replacement in the anterior compartment; and more atrophy in the medial compartment [40••]. To date, the muscle MRI characteristics of autoantibody-negative IMNM have not been described.…”

Autoimmune necrotising myopathy and HMGCR antibodies

“…Typically, patients have symmetrical and diffuse intramuscular and intrafascial oedema in affected muscles, in anterior, medial and posterior compartments 8. MRI is also useful for localising affected muscles, to guide muscle biopsy and to improve the diagnostic yield.…”