Three-dimensional pelvic floor ultrasound measuring levator ani hiatus area and its association with pelvic organ prolapse: a population based retrospective cross-sectional study in Chinese women

Introduction

Several risk factors for pelvic organ prolapse (POP) have been identified, with an enlarged urogenital hiatus (UGH) being one of the most significant. Women exhibiting a genital hiatus (GH) size of ≥3.5 cm were nine times more likely to develop prolapse compared to those with a GH ≤2.5 cm (1). In a separate study (2) comparing Pelvic Organ Prolapse Quantification (POP-Q) scores between 100 women with and without apical prolapse, a GH greater than 4.5 cm was significantly associated with the presence of apical prolapse. Other studies (3,4) have also indicated that widened GH was associated with higher POP stages and risk of POP recurrence after surgical repair. Dietz et al. (5) retrospectively assessed the normal hiatus area in 544 Australian women using the three-dimensional/four-dimensional (3D/4D) pelvic floor ultrasound imaging and found that a hiatal area of >25 cm² during Valsalva should be defined as abnormal distensibility. A study (6) conducted in Shanghai, China, involving 258 women, found that the levator ani hiatus (LAH) of women with POP measured 22.34±5.01 cm² on the Valsalva maneuver, which is less than the Dietz’s result. Currently, there are no published data on the definition of normal hiatal dimensions in Chinese women. We aimed to conduct a retrospective cross-sectional study in the Sichuan province based on a large sample size to assess the normal hiatal area and the cut-off value for the hiatal area associated with objective POP occurrence. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1980/rc).

Methods

Study design and patients

This retrospective cross-sectional study was conducted at a tertiary hospital. Women were recruited from the Urogynecologic Department and the Postpartum Clinic and Rehabilitation Center for Pelvic Diseases between May 2017 and November 2022. Irrespective of the diagnoses, all the participants underwent clinical examination for prolapse using the POP-Q grading developed by the International Continence Society (ICS) and 3D pelvic floor ultrasound imaging. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the Institutional Review Board of West China Second University Hospital, Sichuan University, China, on March 7, 2022 (No. 2022-296) and individual consent for this retrospective analysis was waived.

Data collection and statistical analysis

A 3D transperineal ultrasound was performed after voiding with each patient positioned in the lithotomy posture. The Mindray Nuewa R8 system (Mindray Bio-Medical Electronics Co., Ltd., Shenzheng, China) equipped with a 4–9 MHz DE10-3WU transabdominal 3D ultrasound transducer was used. The transducer was gently placed between the external vaginal opening and the anus for exploration. The acquisition angle was set to a maximum transducer angle of 120° and positioned on the perineum with its main transducer axis oriented in the midsagittal plane. Key anatomical features, including the symphysis pubis, urethra, bladder neck, vagina, anal canal, rectum, and recto-anal canal angles, were clearly visualized in the same plane. Using 3D volumetric scanning, standard image of the LAH was obtained. The morphology and movement of the pelvic floor organs were observed in real-time in the median sagittal section of the pelvic floor. The plane of the LAH at rest and during the Valsalva and anal retraction maneuvers was also observed (Figure 1). Three ultrasonic images on Valsalva were acquired, and the image showing the most serious POP was used for the evaluation of the hiatal area on Valsalva maneuver. In the reconstructed 3D LAH plane, the area along the medial margin of the pubic symphysis, the medial margin of the bilateral pubic rami, and the medial margin of the bilateral puborectal muscle were defined as the area of the LAH. The hiatus area in the resting state and the maximum Valsalva state were measured and assessed in this study. All the measurement data was reviewed by a senior sonographer. If any data significantly deviates from the normal range, the ultrasound images would be reacquired and measured again.

Figure 1 The ultrasound images of the smallest levator ani hiatus planes in a 58-year-old woman without objective POP. (A) The area circled by green circle indicates the levator ani hiatus at rest. (B) The area circled by green circle indicates the levator ani hiatus during Valsalva state. POP, pelvic organ prolapse; R, rectus; U, urethral; V, vagina.

Each patient’s prolapse stage was graded according to the POP-Q scores for the anterior, middle, and posterior compartments. Stage 0 indicates no prolapse, defined by points Aa, Ap, Ba, and Bp all positioned at −3 cm, and point C or D lying between −total vaginal length (TVL) and −(TVL-2) cm. Stage 1 is assigned when the criteria for stage 0 is not met, but the most distal part of the prolapse remains more than 1 cm above the hymenal. Stage 2 is characterized by the most distal portion of the prolapse being ≤1 cm proximal to or distal to the level of the hymen. Stage 3 is defined as prolapse where the most distal portion descends more than 1 cm below the hymen, but does not protrude beyond 2 cm less than the TVL. Stage 4 represents the distal portion of the prolapse protruding to at least (TVL-2) cm. The anterior compartment was referred to as the anterior vaginal wall, the middle compartment indicated the uterus or vaginal fornix after hysterectomy, and the posterior compartment indicated the posterior vaginal wall. All the POP-Q scores were reviewed by the author, and if any data was illogic, this sample should be excluded.

Data were analyzed with Python (Anaconda Distribution, version 3.7.4) and SciPy (version 1.6.3). Association analysis between the levator ani hiatal areas and the prolapse stages were performed using the univariant Chi-squared test and Wilcoxon rank sum test. The receiver operating characteristic (ROC) curve analysis was used to examine the relationship between the hiatal areas and prolapse stages across different compartments to obtain an estimate of normal hiatal areas. The ability of the hiatus areas to diagnose significant objective POP (defined as POP-Q stage 2 or higher) and severe POP (defined as POP-Q stage 3 or higher) was evaluated using ROC analysis. Optimal cut-off points, which maximized joint sensitivity and specificity (Youden index criteria), were estimated for the LAH area both at rest and during the maximal Valsalva maneuver from the ROC curve. A P value of <0.05 was considered statistically significant.

Results

In total, 1,633 women participated in the study, and the mean age of the study population was 39.7 years (range, 21–86 years) (Table 1). The prolapsed stages of the different pelvic compartments are shown in Table 2. Univariate analysis of variance revealed significant differences in the hiatal areas among different prolapse stages, irrespective of the anterior, middle, or posterior compartments (Table 3).

The Wilcoxon rank-sum test was performed to assess whether there was a statistically significant difference in the hiatal area between any two prolapse stages. For the anterior vaginal wall, the hiatal areas of prolapse stages 2, 3, and 4 were all significantly different from those of stage one in both the resting and Valsalva states. Additionally, the hiatal areas of prolapse stages 3 and 4 exhibited significant differences compared to stage 2, regardless of the resting or Valsalva states. However, no statistically significant difference was observed between the hiatal areas at stages 3 and 4 (Figure 2A,2B). Considering the uterus or vaginal fornix, the hiatal areas of prolapse stages 2, 3, and 4 all had significant differences with stage 1 both in resting and Valsalva states. Significant differences were found between the hiatal areas in stages 2 and 3 in the resting state; however, no statistical difference was observed during the Valsalva maneuver (Figure 2C,2D). Furthermore, considering the posterior vaginal wall, the hiatal areas of prolapsed stages 2, 3, and 4 all had significant differences with stage 1 at resting state. Additionally, the hiatal areas at prolapse stages 2 and 3 were significantly different at rest. In the Valsalva maneuver, only stages 2 and 3 had significantly different hiatal areas from stage one (Figure 2E,2F). These findings suggested that the hiatal area at rest was closely associated with the severity of the POP, especially in stages 1, 2, and 3. A larger hiatal area at rest corresponds to more advanced prolapse stages.

Figure 2 The boxplots of the Wilcoxon rank sum test analysing the association between hiatal area and different prolapsed stages. (A) The association between hiatal areas and different anterior vaginal wall prolapsing stages at rest. (B) The association between hiatal areas and different anterior vaginal wall prolapsing stages on Valsalva. (C) The association between hiatal areas and different uterus or vaginal fornix prolapsing stages at rest. (D) The association between hiatal areas and different uterus or vaginal fornix prolapsing stages on Valsalva. (E) The association between hiatal areas and different posterior vaginal wall prolapsing stages at rest. (F) The association between hiatal areas and different posterior vaginal wall prolapsing stages on Valsalva. The boxes in different colors represent the different stages. The upper and lower lines of the boxes represent the 75^th and 25^th percentile values of the hiatal area. The line in the box represents the median value of the hiatal area. *, P<0.05; **, P<0.01.

ROC statistics were obtained when significant objective prolapse (POP-Q stage 2 or higher) was tested against the hiatal area at rest and during the Valsalva maneuver. There was a fair relationship between the hiatal area at rest and the anterior vaginal wall prolapse [area under the curve (AUC): 0.65; 95% confidence interval (CI): 0.62–0.68], but the AUC on Valsalva was a little higher (AUC: 0.67; 95% CI: 0.64–0.70). A threshold of 20.02 cm² on Valsalva received a sensitivity of 0.58 and a specificity of 0.66. Regarding the uterus or vaginal fornix after hysterectomy, the AUC at rest was higher than that for the Valsalva maneuver. A cut-off of 16.30 cm² at rest had a sensitivity of 0.76 and a specificity of 0.77. Considering the posterior vaginal wall, the AUC at rest was also higher than that on Valsalva, and the cut-off of 15.41 cm² at rest exhibited a sensitivity of 0.67 and a specificity of 0.67 (Figure 3).

Figure 3 ROC curves and the performance of the proposed cut-off hiatal area for significant objective prolapse (POP-Q stage 2 or higher). (A) ROC curve and cut-off hiatal area at rest for the diagnosis of significant objective prolapse in the anterior pelvic zone. (B) ROC curve and cut-off hiatal area on Valsalva for the diagnosis of significant objective prolapse in the anterior pelvic zone. (C) ROC curve and cut-off hiatal area at rest for the diagnosis of significant objective prolapse in the middle pelvic zone. (D) ROC curve and cut-off hiatal area on Valsalva for the diagnosis of significant objective prolapse in the middle pelvic zone. (E) ROC curve and cut-off hiatal area at rest for the diagnosis of significant objective prolapse in the posterior pelvic zone. (F) ROC curve and cut-off hiatal area on Valsalva for the diagnosis of significant objective prolapse in the posterior pelvic zone. AUC, area under the curve; CI, confidence interval; POP-Q, Pelvic Organ Prolapse Quantification; ROC, receiver operating characteristic.

When severe prolapse (POP-Q stages 3 and 4) was tested against the hiatal area, there was also a fair relationship between the area at rest and the anterior vaginal wall prolapse (AUC: 0.81; 95% CI: 0.78–0.84). A threshold of 16.02 cm² at rest exhibited a sensitivity of 0.80 and a specificity of 0.68. Concerning the middle zone, a cut-off of 20.30 cm² at rest got a sensitivity of 0.39 and a specificity of 0.75 (AUC: 0.57; 95% CI: 0.50–0.64). For the posterior zone, the cut-off of 17.83 cm² at rest yielded a sensitivity of 0.70 and a specificity of 0.61 (AUC: 0.66; 95% CI: 0.56–0.75) (Figure 4).

Figure 4 ROC curves and the performance of the proposed cut-off hiatal area for severe prolapse (POP-Q stage 3 or 4). (A) ROC curve and cut-off hiatal area at rest for the diagnosis of severe prolapse in the anterior pelvic zone. (B) ROC curve and cut-off hiatal area on Valsalva for the diagnosis of severe prolapse in the anterior pelvic zone. (C) ROC curve and cut-off hiatal area at rest for the diagnosis of severe prolapse in the middle pelvic zone. (D) ROC curve and cut-off hiatal area on Valsalva for the diagnosis of severe prolapse in the middle pelvic zone. (E) ROC curve and cut-off hiatal area at rest for the diagnosis of severe prolapse in the posterior pelvic zone. (F) ROC curve and cut-off hiatal area on Valsalva for the diagnosis of severe prolapse in the posterior pelvic zone. AUC, area under the curve; CI, confidence interval; POP-Q, Pelvic Organ Prolapse Quantification; ROC, receiver operating characteristic.

Based on our findings, we propose that when the hiatal area of <14.45 cm² at rest and <20.02 cm² during Valsalva indicates a low correlation with significant objective POP. Conversely, a hiatal area of ≥20.30 cm² at rest is highly indicative of the possibility of severe POP.

Discussion

This study is the first to investigate the relationship between the LAH area and the POP in Chinese women using pelvic floor 3D ultrasound. This study found that both at rest and during the Valsalva maneuver, larger hiatal area was associated with a more advanced POP stage. A hiatal area of less than 14.45 cm² at rest may be the optimal size for less correlated with POP, while a hiatal area ≥20.30 cm² at rest indicates a high possibility of severe prolapse. This finding aligns with the data in the research conducted by Zhou et al. (6), who recruited 258 women categorized as nulliparous, parous women without prolapse, POP, and postpartum groups. It found that the hiatal areas at rest of the nulliparous and parous without prolapse groups were 10.85±1.64 and 11.43±2.00 cm², respectively. The average hiatal area at rest in the POP group was 13.47±3.47 cm².

Besides the LAH area, many other indices have been used to indicate the size of the GH, such as the length of the GH in the POP-Q system, anteroposterior hiatal diameter (AP), and left-right hiatal diameter (LR) under pelvic floor ultrasound. Guanzon et al. performed a retrospective cohort study and found that GH widening during the Valsalva maneuver was associated with greater baseline leading-edge descent and higher POP stage even after adjusting for TVL (3). Liu et al. assessed the relationship between the AP and POP stages using two-dimensional pelvic floor ultrasound and found that AP ≥6.0 cm indicated an enlarged levator hiatus, which correlated with an increased likelihood for POP (7). Although these findings are similar to our study, the GH in the POP-Q system is the surface diameter between the middle of the external urethral meatus and the posterior midline of the hymen (8), which does not provide a true picture of the GH surrounded by the deep levator ani and the perineal body. According to the perineal complex triad hypothesis (9), the levator ani muscles, perineal membrane, perineal body, and vaginal fascia collectively form a functional perineal complex surrounding the UGH. However, failure of one or more of these structures could result in larger AP and LR simultaneously. Neither the AP nor LR diameter could represent the real state and change of the UGH alone; therefore, we used the LAH area measured by 3D pelvic floor ultrasound as a comprehensive indicator.

In the study conducted by Dietz et al. (5), a threshold of 25 cm² during Valsalva exhibited a sensitivity of 0.52 and a specificity of 0.83 for identifying significant prolapse, as defined by clinical examination. In our study, for detecting significant prolapse of the anterior vaginal wall, a threshold of 20.02 cm² on Valsalva had a sensitivity of 0.58 and a specificity of 0.66. For the middle compartment, a cut-off of 16.30 cm² at rest had a higher AUC with a sensitivity of 0.76 and a specificity of 0.77. For the posterior vaginal wall, the cut-off of 15.41 cm² at rest exhibited a sensitivity of 0.67 and a specificity of 0.67. This suggested that the hiatal area at rest had a better indicative performance than the hiatal area during the Valsalva maneuver. The measurement of the hiatal area at rest is more repeatable than measuring the dimensions at the Valsalva maneuver, as most patients are unable to achieve maximum hiatal area when Valsalva movements are performed during ultrasound examinations. Additionally, the cut-off value for predicting significant objective POP was 20.02 cm² on the Valsalva maneuver, while that of severe POP was 20.99 cm² following our study; both were smaller than Dietz’s findings. This may possibly be due to racial differences. Like a study compared the pelvic floor anatomy between Chinese and American white women based on the magnetic resonance imaging (MRI) imagings, finding that the anteroposterior diameter of the UGH at Valsalva was 0.4 cm smaller in Chinese women than in American white women on average (10).

Some researchers have found that comparing with the supine position, in the standing position can achieve larger hiatal dimensions (11-13). However, the supine position is the most commonly adopted posture during the transperineal ultrasound test and we performed the transperineal pelvic floor ultrasound in the supine position in our study, so the achieved cut-offs may be suitable to evaluate Chinese women at risk of POP in supine.

Notably, it was found that the cut-off value of the hiatal area for significant objective POP in the anterior vaginal wall was the smallest both at resting state (14.45 cm²) and during Valsalva (20.02 cm²). This finding may help explain why the anterior vaginal wall prolapse accounted for the highest proportion of all POPs (14,15).

The strengths of this study are the inclusion of a large sample size and the separate analysis of the correlation between the LAH area and the descent of different pelvic compartments. In this study, the results revealed that the hiatal area of patients with POP stage four was not significantly different from that of patients with POP stage two or three, particularly in the middle and posterior compartments. This finding may be possibly due to the small sample size of POP stage four patients included in our study. Another limitation of our study is that all the samples were women at risk of pelvic floor dysfunction and the truly normal hiatal was deficient though some of them were only having stage one POP. So there may be potential of selection bias.The results of the study suggest that in terms of diagnosis, we need to explore a standardized process for pelvic floor ultrasound testing, and in terms of treatment, we need to explore techniques and methods that can effectively reduce the LAH area.

Conclusions

In conclusion, using ROC analysis based on data from Chinese women, we have established a normative cut-off value of 14.45 cm² for the LAH area in this population at rest. When the hiatal area is ≥20.30 cm² at rest, it has an obvious correlation with severe POP. Based on the limitation of the study, the future prospective investigations are crucial to verify these findings.

Acknowledgments

The abstract has been presented in the 50^th IUGA meeting as the abstract poster in 2025.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1980/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1980/dss

Funding: This work was supported by the National Key Research and Development Program of China (grant No. 2023YFC3606001), and the Key Research and Development Program of Science & Technology Department of Sichuan Province (grant No. 23ZDYF2049).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1980/coif). F.Z. is from Guangzhou AID Cloud Technology Co., Ltd. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the Institutional Review Board of West China Second University Hospital, Sichuan University, China, on March 7, 2022 (No. 2022-296) and individual consent for this retrospective analysis was waived.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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