Pre- and Post-Surgical MRI Analysis of Levator Ani in Pelvic Organ Prolapse Patients: A Single-Center Study

Introduction

POP is a multifactorial disorder, influenced by anatomy, physiology, genetics, lifestyle, and reproductive factors, contributing to pelvic dysfunction in women [1]. Numerous studies have indicated a strong association between parity and the prevalence of POP. POP significantly impacts women’s quality of life [2]. Approximately 30–50% of women experience POP, with a rising incidence. Although 41–50% of women show POP symptoms, only 3% in the United States report them [3]. The projected increase in POP prevalence in the United States is 46%, anticipating 490 000 cases by 2050, with 11–19% potentially needing surgery [4–6]. Surgical intervention is effective for moderate to severe POP, but with high reoperation rates of up to 30% [7]. Clinical treatment is primarily decided by the patient’s chief concerns and physical examination, crucial for diagnosis and classification. Established in 1996 by The International Continence Society (ICS), the POP-Q system is a widely accepted method for classifying POP [8,9]. This system measures compartment prolapses in relation to anatomical indicators. Prolapse near the hymen is considered negative, while protrusion beyond the hymen is positive. The POP-Q system identifies specific anatomical points: Aa and Ba on the anterior vaginal wall, C and D at the vaginal apex, and Ap and Bp on the posterior wall. Additionally, it includes Gh, Pb, and TVL points, which represent the genital hiatus, perineal body, and total vaginal length, respectively [10].

The levator ani muscle (LAM), a key stress-bearing tissue of the pelvic floor, plays a pivotal role in supporting it. The levator hiatus, another significant anatomical feature of the pelvic floor, has significant physiological and clinical importance [11]. The area of the levator hiatus (LHA) is a vital indicator of overall pelvic health [12,13]. Diseases such as bladder, uterus, and rectal prolapse often occur in the levator hiatus [14]. Clinically, the distance between the LAM and the subpubic bone is significant in the diagnosis of stress urinary incontinence [15]. MRI can measure the anteroposterior and transverse diameters of the pelvic floor, evaluating anatomical and functional abnormalities, such as symmetry in bilateral LAMs and muscle ruptures. In women with POP, changes in the shape, anteroposterior diameter, transverse diameter, and LHA occur, along with an increased distance between the LAM and the lower border of the pubic symphysis, all of which are related to POP.

In our earlier study, MRI based on the uterine-vaginal axis quantitatively assessed the morphology of the uterus and vagina, demonstrating the effectiveness of MRI in evaluating the structure of pelvic floor organs and tissues. Combining MRI with computer processing technology enables dynamic evaluation of the LAM’s anatomical morphology and structure, objectively reflecting pelvic floor function and assessing treatment effects. The present study assessed pelvic floor MRI parameters in patients with moderate to severe POP, conducting a dynamic comparison these parameters before and after POP surgery. One study showed that the use of a horizontally oriented 1.5 T MR scanner in the absence of interference or proximity of the examiner provided objective evidence of minimal pelvic organ descent and hiatus enlargement, which was not possible with pelvic examination and transperineal ultrasonography [16]. An earlier study reported that MRI with vaginal coils revealed morphological alterations in the urethra and supporting structures in patients with stress urinary incontinence [17]. In a study of the same period, it was concluded that there was considerable variation in the size and configuration of urethral support structures in nonparous asymptomatic women, but the variation was not related to MRI measurement techniques[18].

This single-center study, involving 33 women diagnosed with POP, employed MRI to compare preoperative and postoperative parameters of the LAM and the levator hiatus. The MRI evaluations identified structural changes in the LAM among women with POP, suggesting potential correlations with improvements in clinical function.

Material and Methods

PATIENT CHARACTERISTICS:

This study was conducted in accordance with the principles outlined in the 2013 amendment of the Declaration of Helsinki. The Ethics Committee of the Kunming Medical University approved the study following a preliminary evaluation. All participants gave their written informed consent. A retrospective cohort study was conducted, analyzing MRI scans of 33 patients, selected between October 2019 and March 2022, at the Kunming Medical University. The inclusion criteria were: (1) postpartum female patients with POP with a clinical diagnosis of POP-Q stage II–IV; (2) showing anterior, apical, or posterior support ≥1 cm below the hymen; (3) ≤6 births; and (4) postoperative follow-up for 6 months.

Exclusion criteria were: (1) women with a BMI of ≥30, (2) patients with symptoms of sexual dysfunction and defecation disorders, and (3) no previous total pelvic floor reconstruction surgery. The patient cohort consisted of 10 (30.30%) cases of stage II, 5 (15.15%) cases of stage III, and 18 (54.55%) cases of stage IV. All patients underwent pelvic floor reconstruction surgery.

MRI PARAMETERS:

All participants underwent supine multi-plane proton density MRI using a 3T superconducting magnet (GE Discovery MR750w, USA). Participants were instructed to empty their bladders 1 hour before the examination and to drink 300–500 ml of water 30 minutes prior, ensuring adequate bladder filling. A single physician trained the patients in the Valsalva manoeuvre and supervised their breath-hold technique. For imaging, patients were positioned supine, with their knees bent and in valgus, simulating the lithotomy position. Axial plane images were collected before and after surgery in resting and Valsalva states. For resting-state imaging, T2-weighted fast spin-echo (T2FSE) MRI on the axial plane was conducted, with parameters TR/TE 4,200/90, slice thickness 4.0 mm, and slice spacing 0.4 mm. During the Valsalva state, T2-weighted single-shot fast spin-echo (T2-SSFSE) MRI was performed on the axial planes, with parameters TR/TE 708/68, interleaved slice thickness 4.0 mm, and slice spacing 1.0 mm.

MRI files were saved in Digital Imaging and Communications in Medicine (DICOM) format and imported into Mimics medical image processing software (Materialise NV, Belgium, version 21.0). The contours of the LAM and the levator hiatus were delineated on the axial plane at the level of the inferior point of the pubic symphysis. Two experienced radiologists, working on the same workstation, analyzed the patients’ MRI images on sagittal and axial planes, which were then reviewed by a senior urologist.

SHAPE OF THE LEVATOR HIATUS:

The shape of the levator hiatus was observed on MRI at the axial plane corresponding to the inferior point of the pubic symphysis. The shapes of the levator hiatus were categorized into 4 types: V, U, O, and irregular. The morphology of levator hiatus in both static and Valsalva states was evaluated before and after POP surgery (Figure 1A–1D).

MEASUREMENT PARAMETERS OF THE LAM:

The parameters of the LAM were measured using MRI at the axial plane of the pubic symphysis. Under static and Valsalva conditions, the LAM was analyzed using Mimics software, incorporating the following evaluation indices: (1) Length and width of levator hiatus (LHL and LHW): LHL refers to the distance from the inferior point of the pubic symphysis to the posterior border of the levator hiatus. LHW denotes the maximum lateral distance between the left and right medial sides of the levator hiatus (Figure 2). (2) LHA: This represents the area enclosed by the LAM’s medial border and the pubic symphysis’s posterior border at the inferior point level (Figure 3). (3) Distance between LAMs and the midpoint of lower margin of pubic symphysis (LSG-L and LSG-R): These denote the distance from the front of the left and right LAMs to the inferior point, respectively (Figure 4).

STATISTICAL ANALYSES:

Measurement data are expressed as the mean±standard deviation (SD), and the categorical data as percentages. Fisher’s exact test was utilized to identify pre- and post-surgery shape differences in the levator hiatus. To compare pre- and post-surgery mean values in paired samples, either a paired t test or nonparametric Wilcoxon signed-rank tests was used. P values of less than 0.05 (P<0.05) were considered statistically significant. All tests were two-sided with a significance level set at P<0.05. Statistical analyses were performed using SPSS version 22.0 (IBM Corp Armonk, NY).

Results

FOLLOW-UP OF SURGICAL EFFICACY:

After surgery, 31 patients (93.94%) showed no anatomical recurrence of POP. In 2 cases (6.06%), the anterior vaginal wall was classified at stage II in the Valsalva state, although it remained within normal range in the resting state. These patients experienced only mild urinary incontinence symptoms and did not require further intervention.

MORPHOLOGY OF LEVATOR HIATUS:

MRI in both static and Valsalva states revealed the morphology of the levator hiatus at the axial plane of the inferior point of the pubic symphysis. Postoperatively, the prevalence of “V”- and “U”-shaped levator hiatus increased, while “O” and “irregular” shapes decreased, compared to preoperative observations. Statistically significant differences in the shape of the levator hiatus were noted before and after POP surgery in both static and Valsalva states (χ2=12.734, P<0.05 and χ2=13.686, P<0.05, respectively) (Table 1).

LAM PARAMETERS IN STATIC STATES:

In static states after surgery, LHL (95% CI: 2.70 to 7.78, P<0.05), LHW (Z=−4.494, P<0.05), LHA (95% CI: 457.38–743.32, P<0.05), LSG-L (95% CI: 6.03–11.55, P<0.05), and LSG-R (95% CI: 5.65 to 8.85, P<0.05) demonstrated statistically significant reductions (Table 2).

LAM PARAMETERS IN VALSALVA STATES:

During Valsalva states, post-surgery measurements indicated significant reductions in several LAM parameters compared to pre-surgery values. These included LHL (95% CI: 4.93–9.44, P<0.05), LHW (95% CI: 4.17–11.65, P<0.05), LHA (95% CI: 518.76–992.21, P<0.05), LSG-L (95% CI: 10.79 to 17.31, P<0.05), and LSG-R (95% CI: 10.07–16.19, P<0.05) (Table 3).

Discussion

POP is a common pelvic floor dysfunction among elderly women. It involves the loss of support of the anterior/posterior vaginal wall or the vaginal apex, leading to the protrusion of the bladder, rectum, and uterus or out of the vagina. POP can result in urinary, sexual, and defecation dysfunction, among other symptoms, significantly impacting patients’ quality of life [19,20]. Pelvic floor reconstruction is an effective treatment for POP, aiming to restore prolapsed organs to their normal or near-normal positions. Broker et al [21] conducted dynamic pre- and post-surgery scans on patients undergoing anterior and posterior pelvic floor reconstruction, noting substantial improvements in clinical symptoms and organ positioning during exertion. The evaluation of surgical efficacy predominantly relies on symptoms and physical examination, with limited research focusing on MRI-evaluated structural changes in the LAM in women with POP.

Recent advancements in the study of pelvic floor anatomy and pathophysiology have shown that POP often involves multiple parts of the pelvic floor, with severe cases affecting the entire area [22–24]. MRI has proven to be highly effective in displaying the anatomical structure of pelvic floor and identifying various types of lesions. With recent technological progress, MRI now allows visualization of pelvic floor organs from axial, coronal, and sagittal perspectives in both static and Valsalva states. This enhancement in imaging capability is crucial for accurately delineating the location and extent of pelvic floor injuries, as well as monitoring recovery postoperatively. The importance of measures made at rest should be noted. During Valsalva, there are 2 features that cause the hiatus to enlarge: the weakness of the levator and the connective tissues uniting the perineal membranes through the perineal body and the dilating effect of the prolapse itself, which can stretch even a normal hiatus open during straining. Resting images are important because they are not affected by the prolapse. Dynamic MRI imaging is increasingly being utilized to evaluate surgical outcomes following pelvic floor reconstruction [25,26].

The LAM, along with connective tissue, plays a crucial role in supporting pelvic organs and maintains pelvic floor stability. Various factors can lead to LAM injury, often resulting in incomplete repair of the pelvic floor muscles. Increased abdominal pressure can displace pelvic organs and alter their normal morphology, contributing to POP. Dietz et al [27] identified an enlarged levator hiatus and ruptured LAM as independent risk factors for POP. Numerous studies [15] have indicated that in women with POP, the anteroposterior and transverse diameters of the levator hiatus increase, along with the distance between the LAM and the lower point of the pubic symphysis. Typically, the levator hiatus appears “U”- or “V”-shaped on axial MRI images. However, in patients with pelvic floor disorders such as cystocele, uterine prolapse, rectal prolapse, pelvic floor intestinal hernia, or peritoneal hernia, the morphological of the levator hiatus may change [28]. In this study, pre-surgery levator hiatus shapes were predominantly “O” and “irregular”, especially under Valsalva conditions. After POP surgery, most levator hiatus shapes shifted towards the normal “V” and “U” configurations. This study suggests that structural changes in the morphology of the LAM may be associated with improved clinical function.

Encircled by the medial edge of the LAM, the levator hiatus is a crucial anatomical unit of the pelvic floor [11,29]. Previous research [30] has shown a correlation between LHA and the descending distance of pelvic organs in both resting and Valsalva states. Specifically, LHA in the resting state is associated with the extent of pelvic floor organ descent during Valsalva maneuvers. Dietz et al [31] definedan area of the levator hiatus greater than 25 cm2 as indicative of abnormal expansion during maximum Valsalva maneuvers, suggesting that a LHA less than 25 cm2 is associated with a reduced risk of pelvic organ prolapse. Moreover, a larger LHA is linked to more significant symptoms. In this study, post-POP surgery LHA was consistently less than 25 cm2 in both resting and Valsalva states. Additionally, LHL, LHW, LSG-L, and LSG-R were significantly reduced compared to preoperative measurements. These results underscore the effectiveness of pelvic floor reconstructive surgery. Postoperative improvements in symptoms and quality of life indicate enhanced support from the LAM and stabilized morphology under stress in POP patients. Dynamic MRI has proven effective in demonstrating anatomical recovery following pelvic floor reconstruction surgery and correlates with subjective symptoms improvement.

The primary objective of pelvic floor reconstruction surgery is to reduce the size of the levator hiatus, rather than directly repairing fractures or altering the thickness of the LAM, with the aim of restoring the anatomical position and support of pelvic organs [32]. Although surgery may not fully restore the levator ani structure, mesh placement promotes collagen deposition, forming scar tissue. This scar tissue is strong enough to function as a substitute for damaged ligaments, effectively reducing the LHW’s increased abdominal pressure and thus preventing pelvic organ descent of pelvic organs [33,34]. Evaluating parameters of the LAM, including the LHW, LHL, and LHA, is crucial for assessing pre- and post-surgery morphological and structural changes in women with POP.

This study has some limitations. First, all MRI scans were conducted with patients in a supine position, which may reduce the impact of gravity on the pelvic floor and potentially affect the assessment of pelvic organ descent. Second, the study’s reliance on MRI, an expensive imaging technique, limited the sample size and presents a constraint in terms of accessibility and cost. Third, the levator muscle space was not measured in this study, which was measured in the plane from the subpubic side to the anorectal angle. Finally, levator ani defects were not considered in the evaluation of scan results.

Conclusions

MRI effectively captures and illustrates the morphology of the LAM shape through both static and Valsalva scans, enabling quantitative assessment of the pelvic floor’s levator ani structure. Moreover, MRI evaluations of the LAM in women with POP indicate structural changes that are likely correlated with improved clinical function.