RESUMEN
OBJECTIVES: The aim of the present study was two-fold. Firstly, we aimed to develop and describe a technique for measurement of the transverse diameter (TD) of the levator ani muscle (LAM) hiatus in the coronal view using two-dimensional (2D) transperineal ultrasound (TPUS) in nulliparous women with a term pregnancy. Secondly, we aimed to evaluate the feasibility and reproducibility of 2D-TPUS assessment of LAM hiatal TD and assess intermethod agreement between 2D-TPUS and three-dimensional (3D) TPUS measurement of TD in the axial plane, which is considered the gold standard in nulliparous women with term pregnancy. METHODS: We recruited a group of nulliparous women with term pregnancy before the onset of labor. The study was conducted in two phases: Phase 1 involved developing and describing the 2D-TPUS technique for measuring LAM hiatal TD, and Phase 2 focused on assessing the technique's feasibility, reproducibility and intermethod agreement with 3D-TPUS measurement of LAM hiatal TD. In Phase 1, we enrolled 30 women. Each woman underwent acquisition of a 3D-TPUS volume, which was analyzed using multiplanar mode to identify and determine the appearance of the lateral borders of the LAM in the coronal plane, at the level of the plane of minimal hiatal dimensions. These borders were used as landmarks for TD measurement. Additionally, we measured the distance between the plane used for TD measurement and the center of the urethra in the axial view. In Phase 2, we recruited 100 women. Each woman underwent acquisition of three 2D-TPUS videoclips in the coronal plane, each encompassing a sweep of the entire LAM hiatus, and a 3D volume, all obtained during rest. On the 2D videoclips, TD was measured twice by one operator and once by another operator. In the 3D volume, TD was measured once, by one operator, in the axial plane; this measurement was considered the gold standard. Each operator was blinded to all other measurements during their assessments. We analyzed intraobserver and interobserver reproducibility and performed an intermethod (2D vs 3D) comparison. Bland-Altman analysis was conducted, and Levene's W0 test and Student's t-test were performed to explore clinical factors that might contribute to systematic differences. RESULTS: In Phase 1, we identified successfully the landmarks denoting the lateral borders of the LAM hiatal TD in the coronal view. These appeared as two symmetrical hypoechogenic indentations located at the inner border of the hyperechogenic structure of the LAM, at the point of maximum distance between the two sides of the LAM. The distance between the urethra and the plane where TD should be measured using 3D-TPUS in the axial plane had a median of 4 mm and varied from 0 to 9 mm. This enabled us to describe a method for assessing LAM hiatal TD in the coronal plane using 2D-TPUS. In Phase 2, LAM hiatal TD was measured successfully in all 2D and 3D acquisitions from the entire group of 100 women. The analyses for intraobserver and interobserver reproducibility and the intermethod comparison (2D vs 3D) revealed almost perfect agreement in TD measurements using 2D-TPUS, with intraclass correlation coefficients of 0.95 (95% CI, 0.92-0.96), 0.87 (95% CI, 0.78-0.92) and 0.85 (95% CI, 0.78-0.90), respectively. The average differences between measurements were 0.1 mm for intraobserver, 1.0 mm for interobserver and 0.2 mm for intermethod repeatability. No systematic differences were observed in any of the measurement sets, except in the interobserver analysis, although this difference was clinically not significant (38.2 vs 37.2 mm, P = 0.01). None of the examined clinical factors (maternal body mass index and maternal age) exhibited a statistically significant impact on intraobserver, interobserver or intermethod reliability. CONCLUSIONS: Utilizing our technique, described herein, to measure the LAM hiatal TD in the coronal view using 2D-TPUS is not only feasible but also highly reproducible and accurate in nulliparous women with term pregnancy. Moreover, it yields measurements that are comparable to those obtained in the reconstructed axial plane generated by 3D-TPUS. © 2024 International Society of Ultrasound in Obstetrics and Gynecology.