Do I Look Fat on This? Early Fatty Replacement of Muscle on MRI in Immune Mediated Necrotizing Myopathy

These days, the concept of inflammatory myopathies is expanding. As our knowledge of inflammatory myopathies increases, so does the awareness that certain features, such as autoantibody type, muscle biopsy findings, laboratory findings, and imaging patterns, correspond with certain subtypes of inflammatory myopathy as well as phenotypes of muscle disease. One such subtype is immune-mediated necrotizing myopathy (IMNM). Patients with this phenotype of disease tend to have profound muscle weakness on presentation, high CK levels, as well as necrosis on muscle biopsy rather than inflammation. MRI is a modality that is utilized in the assessment of inflammatory myopathy, as it reveals inflammation in involved muscles. Interestingly, patients with IMNM may have a distinct pattern of muscle involvement on MRI.

Johns Hopkins sought to determine just that, by examining their cohort of patients with inflammatory myopathy who had available thigh MRI, and who fulfilled the criteria of IMNM, dermatomyositis (DM), polymyositis (PM), clinically amyopathic DM (CADM), or inclusion body myositis (IBM). I was privileged enough to be involved in this initiative led by Dr. I. Pinal-Fernandez. Fifteen muscles were assessed by MRI for features that are associated with inflammatory myopathy, such as presence or absence of edema, atrophy, fatty replacement, and fascial edema. Within the subgroup of IMNM, subsets of patients with anti-SRP and anti-HMGCR autoantibodies were analyzed, as these autoantibodies are associated with the necrotizing subtype of myositis. Six hundred sixty-six subjects were analyzed, including 101 patients with IMNM. These patients had a higher proportion of muscles with edema, atrophy and fatty replacement. Interestingly, anti-HMGCR patients showed a lesser proportion of muscles with atrophy compared to anti-SRP patients. Fatty replacement began early in the course of IMNM.

We as the authors concluded that, compared to patients with DM and PM, IMNM patients had more widespread muscle involvement and anti-SRP patients involvement was more severe. This corresponds with the clinical phenotype of profoundly weak patients with high CK levels at presentation.

It will be interesting to see going forward how MRI findings correlate with muscle biopsy pathology, and if classification criteria for myositis will evolve to include MRI findings.

You can read about our study in Abstract 2306 – "Thigh Muscle MRI Reveals Extensive Muscle Edema and Early Fatty Replacement in Patients with Immune-Mediated Necrotizing Myopathy".

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