External anal sphincter volume measurements using 3-dimensional endoanal ultrasound

Objective Significant nerve injury to a muscle can be associated with muscle atrophy and volume loss. Three-dimensional (3D) ultrasound can measure muscle volume, but the reproducibility of the technique has not been established for the anal sphincter. Study design Using a 10 MHz 360-degree rotating endoanal probe, we performed 3D endoanal ultrasounds on 9 nulliparous and 23 asymptomatic primiparous subjects at 12 weeksʹ postpartum. Two blinded examiners measured the length of the external anal sphincter (EAS) from a midsagittal image, and the width of the EAS and internal anal sphincter (IAS) from axial images at mid anal canal. The EAS volume was calculated by repetitively outlining only the EAS in each sequential axial view. Both examiners measured the EAS volumes twice, blinded to previous calculations. Results The intrarater reliability for EAS volume was 0.79 to 0.89 (intraclass coefficient). The mean difference of the EAS volume between the 2 examiners was 0.5 mL (P = .3, t test). Correlation between the 2 examiners for measuring EAS volume was r = 0.77 (P< .001, Pearsonʹs). The “limits of agreement” (between 2 examiners) varied by as much as 40% of the mean volume. Conclusion Quantitative 3D ultrasound of the anal sphincter is moderately reproducible.