Transvaginal Ultrasonographic Visualization of Endometriosis as a Unified Peritoneal Thickening

Background and Objectives: This study characterized the peritoneal surface of the posterior cervix and vaginal fornix. Transvaginal ultrasonography (TVUS) was performed during laparoscopic surgery to visualize structures beyond the muscular and epithelial layers and to evaluate their association with endometriosis.

Pelvic pain was assessed using a 10 cm visual analog scale. During laparoscopic surgery, saline solution was infused into the pelvic cavity, after which sagittal TVUS images were obtained from the vaginal wall to the peritoneal surface, covering the region from the midline posterior fornix to the bilateral uterosacral ligaments. Fibrotic lesions within the trapezoidal area formed by the posterior cervix, posterior fornix, and uterosacral ligaments were dissected and excised for histopathological analysis. Vaginoperitoneal thickness (VPT)—the distance from the vaginal wall to the peritoneal surface—was measured before and after lesion removal.

TVUS revealed a thickened structure on the peritoneal surface of the posterior vaginal fornix. Uniform granular hyperechogenicity was observed in 65.4% of cases, while partial hypoechoic areas were noted in 34.6%. The structure appeared as a unified lesion extending from the midline to the lateral uterosacral ligament region. Histological examination confirmed endometrial glands and/or stroma in 60.4% of excised lesions and fibrosis in 31.7%. Postoperative VPT was significantly reduced compared with preoperative measurements, with improvement in pain scores observed.

TVUS visualized a continuous endometriotic lesion extending from the posterior cervix and fornix to the bilateral uterosacral ligaments. VPT measurement may provide a measurable parameter for describing peritoneal involvement in endometriosis.

Peritoneal endometriotic lesions, Transvaginal ultrasonography, Vaginoperitoneal thickness

Endometriosis is a chronic inflammatory disease characterized by the presence of endometrium-like tissue outside the uterus.1 It affects approximately 5–15% of sexually mature women2 and frequently causes severe dysmenorrhea and chronic pelvic pain (CPP), leading to substantial reductions in quality of life.3 The presence and distribution of endometriotic lesions are closely associated with pain symptoms.4 The revised American Society for Reproductive Medicine (r ASRM) classification is widely used to evaluate the extent of endometriosis.5 Lesions in the pouch of Douglas are assessed based on surface appearance, three-dimensional characteristics, and the degree of lesion spread. The Enzian classification was later proposed to evaluate deep infiltrating endometriosis, which may be difficult to detect by laparoscopy alone.6 This system categorizes deep pelvic lesions involving the vagina, rectum, uterosacral ligaments, and cardinal ligaments. Ultrasonography is an important tool for detecting endometriotic lesions that are difficult to identify macroscopically. These deep lesions typically appear as hypoechoic masses, sometimes with hyperechoic foci, and are recorded as hypoechoic areas on transvaginal ultrasonography (TVUS).7–12 Consistent with this, the Enzian classification recommends measuring and documenting lesions as hypoechoic areas visualized on TVUS. Surgical excision of endometriotic lesions effectively alleviates pain symptoms.13–17 Such procedures are often based on the assumption that lesions extend into the rectovaginal space and infiltrate the vaginal and rectal walls. However, Vercellini et al.18 proposed that lesions are located on the peritoneal surface, while Anaf et al.19 suggested that they do not infiltrate the rectal wall. Batt et al.20 reexamined the rectovaginal septum and noted that midline lesions are located in the retrocervical septum, above the rectovaginal space. Although opinions differ regarding the precise location and excision methods for endometriotic lesions, recent studies consistently demonstrate the effectiveness of lesion removal in reducing pelvic pain.21 We previously reported a laparoscopic technique for excising endometriotic lesions located on the peritoneal surface from the midline to the uterosacral ligament region.22,23 These lesions corresponded to the uterosacral ligament lesions described by Chapron et al.4 and the retrocervical lesions identified by Batt et al.20 Notably, they did not infiltrate the rectal or vaginal walls or extend into the lower rectovaginal space. We hypothesized that the lesions targeted in our surgical approach—located on the peritoneal surface adjacent to the vaginal wall—could be visualized by placing an ultrasound transducer against the posterior vaginal fornix. In this study, artificial ascites was infused into the pelvic cavity during laparoscopic surgery to enhance visualization of the peritoneal surface.24 Detailed TVUS images were then obtained from the vaginal wall to the peritoneal surface. Using this method, we aimed to identify and characterize structures located between the vaginal epithelial surface and the peritoneal surface. Furthermore, we sought to measure vaginoperitoneal thickness (VPT), defined as the distance from the vaginal epithelial surface to the peritoneal surface, before and after lesion excision. These techniques were used to determine whether the thickened peritoneal structure identified by TVUS decreased following laparoscopic resection and to clarify the histological composition of the excised specimens.

This study was approved by the Ethics Committee of the Hospital and conducted in accordance with the Declaration of Helsinki. The study population consisted of 108 patients who, prior to surgery, exhibited at least one physical finding suggestive of pelvic endometriosis—namely, tenderness in the pouch of Douglas, induration in the pouch of Douglas, or restricted uterine mobility—or were found to have an ovarian endometriotic cyst. Table 1 summarizes the clinical details of 108 patients who underwent TVUS during laparoscopic surgery, and these data provide the basis for subsequent analyses of diagnostic accuracy and clinical correlations. Preoperative treatments, which were administered according to individual case profiles, included relugolix,25 dienogest,26 GnRH analogs,27 and levonorgestrel intrauterine system.28 Table 1. | Op | No. of Patients | Age (Years) | BMI | Gravida | Para | |---|---|---|---|---|---| | R | 2 | 32.6 ± 13.7 | 20.9 ± 2.0 | 0 | 0 | | RO | 11 | 37.1 ± 8.3 | 22.5 ± 4.6 | 0.8 (0–2) | 0.9 (0–2) | | RM | 17 | 37.2 ± 4.4 | 22.6 ± 3.2 | 0 | 0 | | ROM | 8 | 36.8 ± 7.1 | 20.8 ± 3.0 | 1.1 (0–3) | 0.7 (0–2) | | RH | 47 | 46.6 ± 2.9 | 22.6 ± 3.6 | 1.2 (0–4) | 1.1 (0–4) | | ROH | 23 | 46.5 ± 3.3 | 23.0 ± 4.9 | 1.1 (0–4) | 0.8 (0–4) | Abbreviations: R, RMUE; O, ovarian cystectomy/salpingo-oophorectomy; M, myomectomy; H, supracervical hysterectomy; BMI, body mass index; RMUE, resection of the median and uterosacral endometriosis. Dysmenorrhea and CPP were assessed using a 10-cm visual analog scale (VAS). Table 2 presents the classifications of patients according to the VAS scores. The classifications were mild (<4 for dysmenorrhea, <2 for CPP), severe (≥8 for dysmenorrhea or ≥6 for CPP), and moderate for intermediate scores. Table 2. | Mild | Dysm < 4 and CPP < 2 | |---|---| | Moderate | Other than Mild and Severe | | Severe | Dysm ≥8 or CPP ≥6 | Abbreviations: Dysm, dysmenorrhea; CPP, chronic pelvic pain. TVUS was performed using an ultrasonographic system equipped with a 9-MHz convex transducer (Sonovista GX30; Konica Minolta, Tokyo, Japan). Major blood vessels in hypoechoic areas were identified and excluded using color Doppler imaging. The patient was positioned with the pelvis tilted to approximately 12°F. Approximately 100 mL normal saline was then instilled into the pouch of Douglas. After obtaining sagittal scans of the posterior cervix and posterior vaginal fornix at three sites, namely, midline (m), right (r), and left (l), vaginoperitoneal thickness (VPT) was measured. The ultrasonographic transducer was placed in the posterior fornix, and the pelvic cavity was observed with laparoscopy (Figure 1). This figure illustrates findings after supracervical hysterectomy and lesion excision. Figure 2 presents a schematic finding of resection of the median and uterosacral endometriosis (RMUE): (A) in the pelvic lesion, (B) sagittal view of the lesion in the median pelvis and (C) removal of the lesion. Figure 3 illustrates the surgical technique. The peritoneum of the pouch of Douglas was incised within a trapezoidal area defined by the posterior cervix, posterior fornix, and uterosacral ligaments. Subsequently, fibrotic lesions were dissected and excised. Residual lesions were coagulated with bipolar forceps and removed using Cooper scissors. When the Douglas pouch was obliterated, adhesiolysis was performed, the ureter was identified lateral to the uterosacral ligament, and the rectum was mobilized posteriorly to expose the inner uterosacral ligament and posterior fornix. RMUE was performed according to previously reported methods.22,23 Excised specimens were histologically examined for endometrial glands, stroma, and fibrosis. Statistical analyses included Student’s t test, Mann–Whitney U test, analysis of variance, and χ2 tests.

A thickened structure on the peritoneal surface of the pouch of Douglas was clearly visualized following saline infusion into the pelvic cavity. This structure appeared as a unified lesion extending from the posterior cervix through the midline posterior fornix to the uterosacral ligament region, with variable thickness. Figure 4 shows laparoscopic findings following supracervical hysterectomy and RMUE. By compressing the posterior fornix using the transducer, the vaginal wall protruded into the pelvic cavity, revealing a broad area from the midline to the lateral uterosacral regions. Figure 4A shows the vaginal vault when no pressure was applied, whereas Figures 4B–E depicts compression of the left (C), central (D), and right (E) sagittal pelvic planes by the transducer following supracervical hysterectomy and RMUE. Figure 5 presents TVUS images of the posterior fornix. The vaginal wall appeared slightly hypoechoic, while the peritoneal surface was visualized up to the point where echo brightness disappeared due to artificial ascites. Image patterns were classified as granular hyperechoic (h) in 65.4% and low echoic (l) or mixed in 34.6%. Patterns varied by case and scan site. Figure 6 illustrates posterior fornix deformation under compression. While the vaginal fornix protruded and curved into the pelvic cavity, the retrocervical lesion remained stable. The anterior peritoneal surface on the retrocervix became indistinct, whereas the posterior surface remained clearly visible. The measurement method of VPT is shown in Figure 7. Compression of the posterior vaginal fornix created a vertex point at the junction of the posterior cervix and vaginal fornix. VPT was measured at the thickest point in each sagittal plane (m, r, l) on the vaginal fornix, posterior to the vertex point. VPT tended to decrease posteriorly, with no thickened structure observed at the rectovaginal septum. This study excluded 11 cases of poor fluid delineation caused by uterine retroflexion adhesions. Figure 8 compares pre- and post-RMUE TVUS images in the same patient. VPT decreased postoperatively across all scan sites. Moreover, Figure 9 and Table 3 show significant postoperative reductions in VPT at all scan sites. Following RMUE, pain symptoms improved. Figure 10 demonstrates marked improvement in dysmenorrhea and CPP. In cases where supracervical hysterectomy was performed, we excluded postoperative dysmenorrhea data from the analysis. Table 3. | Before RMUE | After RMUE | | |---|---|---| | m-VPT | 6.8 ± 1.9 | 4.5 ± 1.1 | | r-VPT | 6.7 ± 1.6 | 4.4 ± 1.1 | | l-VPT | 6.5 ± 1.7 | 4.3 ± 1.0 | Abbreviations: VPT, vaginoperitoneal thickness; SD, standard deviation; RMUE, resection of the median and uterosacral endometriosis. As shown in Table 4, histological analysis identified endometrial glands and/or stroma in 60.4% of cases and fibrosis in 31.7%. Although no statistically significant correlation was found, gland/stroma components were more frequent in patients experiencing severe pain, while fibrosis was more common in mild cases. Table 4. | Endometriosis | Fibrosis | No Em nor Fibrosis | | |---|---|---|---| | Mild | 14 | 13 | 2 | | Moderate | 27 | 14 | 5 | | Severe | 20 | 5 | 1 | Abbreviation: Em, Endometrial Gland and/or Endometrial Stroma.

Ultrasonographic visualization revealed that the markedly thickened structure between the vaginal wall and the peritoneal surface consisted of endometriotic lesions. These lesions were distributed within a trapezoidal area extending from the posterior cervix and posterior vaginal fornix at the midline to the uterosacral ligaments laterally, consistent with previous descriptions of posterior compartment disease.22,23 No lesions were identified in the rectovaginal septum below the midline. The close correlation between preoperative ultrasonographic findings and histological confirmation in most excised specimens supports the utility of ultrasonography for characterizing peritoneal thickening. However, small or predominantly fibrotic lesions may be more difficult to detect. The obliteration of the pouch of Douglas appears to result from adhesions formed through posterior displacement of the uterus and anterior displacement of the rectum. These adhesions likely reflect the fusion of lesions originally located on the posterior cervix and vaginal fornix. Once the pouch becomes shallow,18 midline lesions are less accessible from the peritoneal surface, suggesting that some lesions visualized as peritoneal thickening may have originated on the retrocervix or vaginal fornix surface before obliteration occurred. The distribution of lesions can be interpreted in the context of menstrual debris dynamics. Retrograde menstrual blood disperses into the pouch of Douglas and relocates with postural changes and bowel movements. Debris tends to accumulate in the cul-de-sac in the upright position and around the liver in the supine position. These patterns, together with peristalsis, immune responses, and hormonal influences, shape lesion persistence. This framework aligns with established concepts of retrograde menstruation and peritoneal fluid circulation and provides a plausible explanation for the concentration of lesions within the trapezoidal region bounded by the posterior cervix, fornix, and uterosacral ligaments—a region previously shown to be prone to adhesion formation.22 Following RMUE, VPT decreased and pain symptoms improved. These observations indicate that the thickened peritoneal tissue identified preoperatively was successfully excised, which may have contributed to symptom improvement. However, the cause of pelvic pain is multifactorial, and causal inference cannot be drawn from observational data alone. In this cohort, greater residual thickness tended to accompany persistent chronic pelvic pain, suggesting a potential relationship between the extent of remaining peritoneal thickening and ongoing symptoms. Further investigation is needed to clarify the clinical significance of this association. Endometriotic lesions typically appear as hypoechoic masses on ultrasound.7–12 However, prior studies have described hyperechoic components on lesion surfaces or peripheries,8,9,24 indicating that echogenicity varies with lesion composition. In this study, approximately 60% of excised lesions exhibited uniform hyperechogenicity, whereas 40% contained hypoechoic regions. Histological study29 has shown that red and black lesions—rich in blood content and cellular activity—are more likely to yield histological confirmation of endometriosis, whereas white lesions are considered less likely to demonstrate histopathological features consistent with endometriosis. Red lesions in the r-ASRM classification are often observed to exhibit hyperechoic points.24 Recent study30 has identified fibrosis as an important pathological component of endometriotic lesions, although its contribution to lesion morphology remains incompletely understood. Fibrotic remodeling may partially account for the characteristic appearance of white lesions, including their tendency to be detected as hypoechoic structures. Superficial peritoneal findings in this cohort were rarely uniform; white lesions frequently coexisted with punctate red or black lesions. However, these mixed superficial appearances were not clearly correlated with deeper peritoneal thickening. Although the association between VPT and echogenicity was not statistically significant, a trend toward a relationship was observed. Although some portions of the measured thickness contained hypoechoic areas, no discrete nodules typical of Enzian-classified deep lesions were identified. Whether such hypoechoic areas indicate early forms of deep infiltrating disease remains unclear. Collectively, these observations indicate that echogenicity reflects a combination of cellular activity, blood content, fibrosis, and three-dimensional structures. Therefore, ultrasonography provides valuable information but may over-or underestimate disease burden when fibrosis predominates. A nuanced interpretation of echogenicity is essential for accurate assessment. In this study, intraoperative ultrasonography of artificial ascites enabled the precise visualization of the peritoneal surface. This controlled environment enabled a detailed assessment of peritoneal thickness; however, it does not replicate routine outpatient imaging conditions. The present work should therefore be interpreted as establishing a conceptual and methodological basis for evaluating peritoneal thickening. Whether this approach can be adapted for standard preoperative ultrasonography warrants further investigation. Even without artificial ascites, physiological fluid, bowel movement, ovarian contact, or cyst boundaries may assist in delineating the peritoneal surface, suggesting that its applicability in routine clinical practice warrants further investigation. The scope of this study should be interpreted in light of its design. The investigation was conducted at a single center and focused on a specific surgical and imaging environment, enabling a detailed characterization of peritoneal thickening but limiting broader generalization. Establishing a conventional control group for endometriosis research is inherently challenging, as subtle peritoneal changes may occur even in asymptomatic women, and residual alterations may persist after menopause. Premenarchal individuals would represent the most definitive controls, given the hormone-dependent nature of endometriosis. These considerations outline the framework within which the present findings should be understood and highlight directions for future refinement, including larger-scale studies and standardized imaging protocols.

The present study characterized pelvic endometriotic lesions as thickened structures distributed continuously from the midline to the uterosacral ligaments along the posterior vaginal fornix. Ultrasonography demonstrated predominantly uniform hyperechogenicity within these lesions. Resection of the thickened peritoneal surface in the trapezoidal region of the cul-de-sac measurably reduced peritoneal thickness. Histological examination identified endometriosis in 60% of resected specimens and fibrosis in an additional 30%, underscoring the heterogeneous nature of peritoneal disease. By introducing the concept of VPT, this study establishes a methodological foundation for quantifying peritoneal involvement in the posterior compartment. Although VPT has not yet been validated as a diagnostic metric, its potential to reflect structural alterations associated with symptomatology warrants further exploration. Variability related to age, menstrual cycle, and individual anatomical differences should be incorporated into future investigations to refine its clinical applicability. These findings highlight the value of ultrasonography as a noninvasive modality for delineating peritoneal thickening and mapping lesion distribution in the cul-de-sac. As methodological refinements and broader validation efforts progress, ultrasonographic assessment of peritoneal thickness may enhance the precision of preoperative evaluation and refine individualized management strategies for patients with endometriosis. Footnotes Conflict of interests: none. Funding sources: none. Disclosure: none. Acknowledgments: We would like to thank Natsuyuki Fujisawa, MD, for his assistance in the laparoscopic surgery and the staff of the Tsujinaka Hospital Kashiwanoha for their assistance during the clinical phase of this study.

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