MRI T2/STIR epaxial muscle hyperintensity in some dogs with intervertebral disc extrusion corresponds to histologic patterns of muscle degeneration and inflammation

MRI assessment of epaxial myosteatosis in dogs with intervertebral disc extrusions and associations with patient factors and outcome

Fat infiltration of skeletal muscle (myosteatosis) is a characteristic of reduced muscle quality. Objectives of this retrospective, observer agreement, methods comparison, cross-sectional study were to compare epaxial myosteatosis of dogs with surgically confirmed intervertebral disc extrusion (IVDE) using a visual semi-quantitative method and quantitative 2-point Dixon fat-water chemical-shift separation MRI method. Thoracolumbar MRI studies of 86 dogs were reviewed. Assessed locations were the site of extrusion and the adjacent intervertebral disc space immediately cranial. A visual fat score (VFS) was assigned to fat infiltration of the epaxial muscles. Quantitative assessment of fat infiltration was determined by calculating a muscle-fat fraction (MFF). There was moderate agreement between the two reviewers for VFS assessment (weighted Kappa 0.60, 95%CI 0.53-0.66) with moderate to strong correlation between scoring methods (P ≤ .001). Both VFS (P = .023, P = .02) and MFF (P = .041, P = .043) were significantly correlated with neurologic grade at presentation for the erector spinae muscle group at both sites. Higher VFS in the multifidus muscle increased the odds of a successful outcome when assessed at the extrusion site (odds ratio and 95%CI 4.3 (1.02-78.7), P = .0461). In summary, the visual and Dixon method of epaxial muscle-fat assessment are feasible in dogs with IVDE with moderate to strong correlation. Increased VFS of myosteatosis in the multifidus muscle at the extrusion site significantly increased the odds of a successful outcome. Further studies are indicated to assess the utility of myosteatosis as an MRI imaging biomarker in the preoperative imaging assessment of dogs with IVDE.