Background/Purpose: Immune-mediated necrotizing myopathies (IMNM) are a severe condition with early muscle damage attested by MRI of thigh muscles. Presence of damage in the other muscle groups, as well as the pattern and the severity of muscle damage in IMNM according to clinical and serological features remain to be clarified.

Methods: IMNM patients with a whole-body MRI (WB-MRI) (n=42) (anti-HMGCR, anti-SRP and seronegative)were included as well as 60 sIBM patients (as controls). Each muscle groups (n=55) were evaluated and fat replacement was estimated in T1 sequence using the Mercuri score (1= normal; 2= mild involvement, fat replacement <30%, 3=moderate involvement, 30-60%; 4=severe involvement, >60%). Overall lesion load was defined as the sum of all abnormal Mercuri scores (≥2) evaluated in each muscle groups (reported in percentage from minimum 0% to maximum 100%) and lesion load quotient was defined as the overall lesion load divided by disease duration (years) at WB-MRI. Multivariate analysis including MRI data but also age at first symptom, disease duration and treatment initiation delay were performed by principal component analysis (PCA) and hierarchical clustering analysis (HCA).

Results: IMNM patients (anti-HMGCR, n=25; anti-SRP, n=12 and seronegative, n=5) were aged 48.1 ±15.8 years at WB-MRI and had a disease duration of 9.8±8.1 years. Most severely affected muscle groups (mean Mercuri±SD) were located in the pelvifemoral (gluteus minimus: 2.71±1.15, gluteus medius: 2.6±1.17, great adductors: 2.55±1.27 and perineal: 2.43±1.42), lumbar (lumbar extensors: 2.19±1.11) and to a lesser extent in the scapular region (subscapularis: 2.73±1.16).

Unsupervised analysis showed two subgroups of patients: one with a mild lesion load (15±10%, n=32/42) and another with a severe lesion load (60±10%, n=10/42: p<0.001). In the first group, the mean disease duration before WB-MRI was 6.8±6.0 years compare to 19.52±5.7 years in the second (p<0.0001). Correlational studies confirmed that disease duration was the most important predictor of muscle damage (e.g lumbar r=0.76, and pelvifemoral muscle groups r ranging from 0.55-0.73; p<0.01).

Nonetheless, multivariate analyses – adjusted for disease duration, age at first symptoms and treatment initiation delay – demonstrated a more severe involvement of the gluteus maximus (p=0.04) and the great adductor (p=0. 04) in seropositive vs seronegative patients.

Overall lesion load quotient was similar in IMNM compared to IBM (14±14.9 vs 9.4±8, p=0.3), but muscle involvement pattern was different. Lesion load quotient was more severe in the trapezius (p=0.05), infraspinatus (p=0.03), psoas (p=0.0008), iliac (p=0.0002), longus adductor (p=0.01) and pectinus (p=0.0003) in IMNM compared to IBM.

Conclusion: IMNM is associated with a severe fatty replacement in the axial and pelvifemoral muscle groups. Disease duration and serological status are important predictor of muscle damage. IMNM and IBM patients seem to have comparable overall lesion load, but progression was more severe in some muscle groups.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/severe-axial-and-pelvifemoral-muscle-damage-in-immune-mediated-necrotizing-myopathy-evaluated-by-whole-body-mri/