Feasibility and reproducibility of a novel introital exo‐anal ultrasound approach for assessment of the external anal sphincter

INTRODUCTION: Obstetric anal sphincter injuries are a major cause of anal incontinence and are frequently underdiagnosed at delivery. Imaging techniques that are feasible, reproducible, and applicable in obstetric practice may be useful to support assessment in selected clinical situations. This study evaluated a three-dimensional exo-anal introital ultrasound technique combining linear freehand reconstruction (Polyline OmniView) and contrast enhancement using Volume Contrast Imaging (VCI) to enable coronal reconstruction and measurement of the anterior external anal sphincter (EAS). MATERIAL AND METHODS: This was a prospective feasibility and reproducibility study conducted at Sant'Orsola University Hospital, Bologna, Italy. Nulliparous women with singleton pregnancies were recruited between September 2023 and September 2024. 3D introital exo-anal ultrasound volumes were acquired using a high-frequency transvaginal probe. Volumes were reconstructed using Polyline OmniView combined with VCI to obtain a coronal view of the anterior EAS. The primary outcome was the feasibility of measuring the coronal length of the anterior portion of the EAS. The secondary outcome was intra- and interobserver reproducibility of this measurement, obtained after standardized offline volume analysis. RESULTS: Seventy women were included. Measurement of the anterior EAS length was feasible in all cases (100%). Reproducibility was excellent, with minimal mean differences, intraclass correlation coefficient values of 0.96 (95% CI, 0.94-0.98) for intraobserver agreement and 0.97 (95% CI, 0.96-0.99) for interobserver agreement, and narrow limits of agreement on Bland-Altman analysis. The mean length of the anterior EAS was 14.4 ± 1.5 mm for the first measurement by Operator 1, 14.4 ± 1.6 mm for the second measurement by Operator 1, and 14.6 ± 1.6 mm for measurements by Operator 2. CONCLUSIONS: Three-dimensional exo-anal introital ultrasound incorporating linear freehand reconstruction and contrast enhancement allows feasible and highly reproducible measurement of the anterior external anal sphincter. This approach may represent a feasible imaging option for anal sphincter assessment and warrants further evaluation not only in women with suspected obstetric anal sphincter injuries but also in the longitudinal follow-up of women with OASIS and in the assessment of patients presenting with anal incontinence.