The significance of muscle echo intensity on ultrasound for focal neuropathy: The median- to ulnar-innervated muscle echo intensity ratio in carpal tunnel syndrome

Objective: The objective of this study was to investigate the usefulness of muscle ultrasound for evaluating muscle changes caused by denervation in carpal tunnel syndrome (CTS), which is a focal neuropathy. Methods: The mean and standard deviation (SD) of echo intensity (EI) in the thenar and hypothenar muscles were calculated in 35 patients with CTS and 11 healthy subjects. Patients were assigned to three subgroups based on CTS severity as determined by electrodiagnostic tests. The ratio of thenar muscle pixel brightness to hypothenar muscle pixel brightness was used in statistical analyses. The ratio of mean pixel brightness was termed the EI ratio, and the ratio of the SD of pixel brightness was termed the inhomogeneity ratio. Results: Both the EI ratio and the inhomogeneity ratio were significantly higher in the patient group than in the control group. In a comparison of the three patient subgroups, the severe group showed significant differences in both the EI and inhomogeneity ratios compared to the other two groups. Subjects with denervation potential in the abductor pollicis brevis (APB) had higher EI and inhomogeneity ratios than subjects without denervation potential in the APB. Conclusion: The EI ratio and inhomogeneity ratio are useful variables with which to evaluate disease severity and the presence of denervation in patients with CTS. Significance: Muscle ultrasound has clinical significance in the detection of muscle changes that result from neuropathy.