Estimation of net muscle volume in patients with muscular dystrophy using muscle CT for prospective muscle volume analysis: an observational study

OBJECTIVES Muscle volume in patients with muscle disease is an index of disease progression. The aim of this study was to demonstrate a new method of muscle volumetry using CT of thigh muscles. DESIGN Observational study. PARTICIPANTS (1) For muscle volumetry using CT and dual-energy X-ray absorptiometry (DXA), 13 patients with muscle disease participated. (2) For prospective CT volumetry, 12 patients participated over 4 years. PRIMARY AND SECONDARY OUTCOME MEASURES To establish the new CT volumetry, the results of which were correlated with the muscle mass calculated from DXA (primary outcome). To show the decrease in muscle volume using our method over 4 years (secondary outcome) METHODS (1) Helical CT imaging of the thigh was performed. CT images were analysed by applying estimated functions, and the accumulation of outcomes resulted in muscle volumes. We refer to this method as 'net muscle volumetry'. Simultaneously, DXA was performed in these patients, and the muscle mass of the thigh was calculated with vendor-provided software. (2) To evaluate longitudinal changes in muscle volume, net muscle volumetry at the 14 cm section of the middle part of the thigh was performed repeatedly over a 4-year period. RESULTS (1) Volumes of the thigh muscle on one side were calculated to be 300-3400 cm(3) using CT. Muscle mass of the thigh was calculated from DXA to be 1100-5000 g. These results correspond closely, with a Pearson correlation coefficient of 0.993. (2) Thigh net muscle volumes of seven patients with gait disturbance decreased over the 4-year study period (p<0.01). CONCLUSIONS A method of measuring net muscle volume using CT, which was validated by muscle mass calculated from DXA, was developed. Decrements in net muscle volume over 4 years support the reliability of this method. This less arbitrary method is suitable for assessment of muscle volume in patients with muscular dystrophy.