LAUSR

OBJECTIVES To evaluate the relationship of thigh magnetic resonance imaging (t-MRI) with manual muscle testing-8 (MMT-8), muscle enzymes and autoantibodies. To determine the causal and mediating factors resulting in poor recovery of MMT-8 in inflammatory myositis (IIM). MATERIALS AND METHODS This was a single-center retrospective study in IIM patients. t-MRI was semi-quantitatively scored for muscle-oedema, fascial-oedema, muscle-atrophy, and fatty-infiltration. Spearman correlation of t-MRI scores was done with muscle enzymes at baseline, and MMT-8 at baseline and on follow-up. Causal mediation analysis was performed with age, sex, symptom duration, autoantibodies, diabetes, and BMI as independent variables, follow-up MMT-8 as dependent and t-MRI scores as mediating variables. RESULTS Baseline evaluation was done on 59 and follow-up on 38 patients. Median follow-up of the cohort was 31 (10-57) months. Baseline MMT-8 negatively correlated with muscle-oedema (r=-0755), fascial-oedema (r=-0.443), and muscle-atrophy (r=-0.343). Creatinine kinase (r = 0.422) and aspartate transaminase (r = 0.480) positively correlated with muscle- oedema. Follow-up MMT-8 correlated negatively with baseline atrophy (r=-0.497) and fatty-infiltration (r=-0.531). On follow-up, MMT-8 males had positive total effect (estimate [95%CI]) via atrophy (2.93 [0.44, 4.89]) and fatty-infiltration (2.08 [0.54, 3.71]). Antisynthetase antibody had a positive total effect via fatty-infiltration (4.50 [0.37, 7.59]). Age had a negative total effect via atrophy (-0.09 [0.19, -0.01]) and fatty-infiltration (-0.07 [-0.15, -0.01]). Disease duration had a negative total effect via fatty-infiltration (-0.18 [-0.27, - 0.02]). CONCLUSION Baseline fatty-infiltration and muscle-atrophy resulting from older age, female sex, longer disease duration and absent anti-synthetase antibodies, partly mediate muscle recovery in IIM.