Navigating uterine niche 360 degree: a narrative review

Background The increasing prevalence of cesarean section (CS) deliveries globally has sparked apprehension regard‑ ing potential long‑term complications, notably the emergence of uterine niches. CS results in a scar that in certain patients, inadequate healing of that scar results in the development of a uterine niche. While most small niches show no symptoms, large cesarean scar niches in nonpregnant women can give rise to cesarean scar disorder syndrome. This syndrome is characterized by abnormal uterine bleeding, dysmenorrhea, and secondary infertility. In pregnant women, the presence of substantial niches may be linked to potentially life‑threatening complications, includ‑ ing cesarean scar dehiscence, uterine rupture, placenta accreta spectrum disorders, placenta previa, and cesarean scar ectopic pregnancy. Main


Background
The global cesarean section (CS) rate is rising, surpassing the WHO-recommended 10-15% of total births [1].Reportedly, Europe has a 25% CS rate, South America 42%, Latin America 40%, and Egypt 50%, raising concerns about potential long-term complications [2].One such consequence is the development of a uterine niche, also known as an isthmocele or CSD (cesarean scar defect), characterized by a discontinuity or defect in the myometrium at the site of a prior uterine scar [3].

Main text
Definition "Niche" as medical term describes a hypoechoic area within the myometrium at the site of a prior CS scar, indicating myometrial discontinuity [4].According to the European Niche Taskforce, it is defined as an indentation of at least 2 mm in the myometrium at the site of a previous CS scar, measured by transvaginal ultrasound (TV/US) [1].

Prevalence
The prevalence of uterine niches varies based on detection techniques and diagnostic criteria [5].Tulandi and Cohen's systematic review reported niche prevalence between 24 and 70% via TV/US and 56 and 84% through sono-hysterography in women with one or more CS.
In symptomatic women, niche prevalence tends to be higher.Armstrong et al. 's case-control study reported a 24% prevalence with transvaginal ultrasound in women with prior CS [6].Regnard et al. 's study, involving 33 women with prior CS, found a 60% prevalence using saline contrast sono-hysterography performed at least 3 months after the last CS [7].
In a prospective cohort study by Bij de Vaate involving 225 women with prior CS, examination by TV/US and gel instillation sonography (GIS) 6-12 months after the last CS revealed a uterine niche prevalence of approximately 24% with TVS and 56% with GIS [8].LF van der Voet and colleagues analyzed the incidence of uterine niche 6 to 12 weeks after CS using TV/US and GIS.TV/ US evaluation showed a niche in 49.6% of 263 women, while GIS evaluation demonstrated a niche in 64.5% [9].

Risk factors and mechanism of niche formation
Vervoort has proposed hypotheses on niche development based on limited evidence.Surgical factors, such as the site uterine incision and technique of closure during CS, may contribute.Additionally, certain surgical habits, like not to close the peritoneum and inadequate hemostasis, could lead to adhesion development.Patient-related hypotheses focus on factors that may hinder normal wound healing and the angiogenesis that it necessitates [4].
The first hypothesis suggests that a low uterine incision within cervical tissue may impede wound healing due to mucus-producing glands in the cervical tissue.Local mucus production can lead to myometrial layer dehiscence, retention cyst formation, or enlargement of preexisting niches.Larger niches, frequently observed at the lower uterus during hysteroscopy and ultrasound, are associated with abundant mucus or Nabothian retention cysts [4].
Two prospective cohort studies support this hypothesis.One indicates that extremely low uterine incisions are a significant risk factor for impaired healing and the formation of large niches [10].Another study links CS during the active phase of labor, when the cervix is part of the birth canal, to an increased incidence of niche formation [11].
To further confirm the impact of uterine scar placement in the cervix on incision healing, additional evidence is needed.The routine creation of a bladder flap during CS, intended to move the bladder from the operative field, is a concern as it may affect the uterine incision level.Studies are beginning to explore whether this step can be safely omitted, but its impact on niche formation is yet to be investigated [12].However, existing studies did not specifically focus on niche formation, and studying the effect of incision site in the uterus can be challenging, particularly when the cervix is significantly effaced [4].
Table 1 illustrates some studies that discussed the relationship between the location of uterine incision and uterine niche (Appendix A).
The second hypothesis proposes that incomplete closure of the entire thickness of the uterine incision after CS may lead to niche formation.Failure to close the deep muscular layer, often due to tangential sutures and decidual-sparing techniques, can disturb the myometrium and contribute to niche development [4].
Guidelines recommend double-layer closure, as previous studies found the efficacy and safety of single-layer closure undetermined [13].However, in several countries, many gynecologists opt for single-layer closure over double-layer closure [4].Studies investigating the impact of repair techniques on uterine niche prevalence aimed to answer questions about differences between single and double-layer closure, the effect of locking sutures compared to non-locking sutures, and whole thickness versus split-thickness uterine closure.
A systematic review of these topics revealed variable and interlacing results.There was no statistically significant difference in niche incidence (p = 0.58), but longterm residual myometrium thickness was slightly thicker following double-layer repair (p= 0.06) [14].
Table 2 illustrates the studies that detected the effect of different repair techniques of uterine incision on Niche formation (Appendix A).
The third hypothesis suggests that certain surgical habits may lead to adhesion formation, delaying the healing of the uterine incision.Adhesions between the uterine scar site and the anterior abdominal wall can exert opposing forces on the edges of the uterine incision, pulling them away from the scar line.This counteracts the necessary retraction of uterine scar tissue for proper alignment of myometrial layers and scar healing.Gravity acting on a retroflexed uterus may intensify these opposing forces.Notably, individuals with large symptomatic niches undergoing laparoscopic correction often have retroflexed uteri [4].
The fourth hypothesis involves patient-related factors or underlying diseases that may delay wound healing.
Approximately 5% of patients may experience recurring niches even after laparoscopic surgical reconstruction, suggesting a genetic predisposition for poor wound healing influenced by unidentified variables.Animal models show genetic disposition can impact the histology and biomechanical healing of artificially induced myometrial defects.Some human trials note correlations between niche formation and BMI, and hypertensive disorders (including preeclampsia), but the exact mechanisms remain unknown [4].

Clinical presentation
The majority of uterine niches are asymptomatic and often discovered incidentally during ultrasound [2].While many women may be asymptomatic, uterine niche has been linked to various symptoms including bleeding (postmenstrual or intermenstrual), pain (dysmenorrhea or chronic pelvic pain), and reproductive issues (secondary infertility).Other reported symptoms include bladder dysfunction, scar abscess, and potential effects on in vitro fertilization (IVF) implantation [2,15].Niche-related obstetric consequences, such as CS scar pregnancy, abnormal placentation (previa-accreta), and scar dehiscence (partial or complete leading to rupture uterus), have also been described.Further research is needed for conclusive evidence on certain symptoms [2].

Gynecological issues
(1) Abnormal uterine bleeding in the following forms: (a) Post-menstrual spotting Postmenstrual spotting (PMS) is a characteristic symptom of uterine niche, occurring in approximately 30-55% of women 6 to 12 weeks after cesarean section (CS) delivery.PMS is defined as ≥ 2 days of spotting inter-menstrual or following menstruation.The cause is attributed to accumulated menstrual blood in the niche, where it hangs until gradually released due to restricted flow and weak contractility of fibrosed muscle [2].
In a prospective study 1 year post-CS, 20% of women with uterine niche experienced PMS, compared to 8.3% without, with a 3.34 odds ratio (OR) for larger niches [16].The size of the defect correlates with the duration of postmenstrual spotting.Pathological findings of free erythrocytes in scar tissue suggest local blood formation within the niche, causing occasional spotting.Regardless of the cause, blood pres-ence in the niche increases mucus secretion, contributing to PMS [2].
Uterine niche-related bleeding can be classified as AUB-N (abnormal uterine bleeding-niche) according to the PALM-COEIN terminology for abnormal uterine bleeding by FIGO, as it has not been specifically defined [15].

(b) Intermittent spotting
Predicting the causes of postmenstrual versus intermenstrual spotting in a uterine niche is challenging.The hypothesis suggests that blood production within the niche, indicated by the presence of free erythrocytes within the scar, may contribute to intermenstrual spotting [15].
(2) Pain Women with uterine niche may experience various forms of pain.The most common is cyclic pain (dysmenorrhea) affecting 40-50%, followed by noncyclic pelvic pain or chronic pelvic pain in 35%, and pain during sexual intercourse (dyspareunia) in 18%.Suprapubic pain is also reported [16].The size of the uterine niche correlates with pelvic discomfort and dysmenorrhea.Anatomical disturbances in the lower uterine section, fibrosis, and inflammatory infiltration are linked to these pain complaints [2].(3) Midcycle intrauterine fluid accumulation, abnormal vaginal discharge In about 45% of women, uterine niche-related pain may result from an excess of mucus formation due to retained blood [16].(4) Secondary infertility Jolijn Vissers proposed a hypothetical mechanism for impaired fertility in the uterine niche [15].
• Hypothesis 1: Intrauterine fluid buildup associated with the niche affects implantation.
In approximately 42% of patients with a large uterine niche, fluid accumulation is observed.This may impact pregnancy outcomes by reducing implantation rates and potentially being toxic for embryos.Further research is needed to assess the impact of intracavitary accumulated fluid on implantation, the potential embryotoxicity of intrauterine fluid, and the efficacy of surgical procedures on reproductive outcomes.It remains unknown whether promptly removing intracavitary fluid before embryo transfer in women with niche influences reproductive outcomes [15].
• Hypothesis 2: Alteration of the immunobiological environment in the presence of a niche (inflammatory environment) A prospective cohort study of 380 women's early pregnancies after CS identified that the presence of a uterine niche influences the implantation site.The risk of spontaneous abortion was associated with the distance between the implantation site and the scar.Seven out of eight pregnancies with embryo implantation near or in the niche resulted in miscarriage.Implantation was more common in the posterior uterine region, suggesting that the niche is not an optimal site for implantation [17].
• Hypothesis 3: Distorted uterine contractility produced by fibrosis or myometrial disruption at the site of the niche Vissers proposes that irregular or uncoordinated uterine contractions following CS compromise implantation.In a normal menstrual cycle, steroids regulate coordinated contractions from the sub-endometrial myometrium.Optimal reproductive outcomes are associated with coordinated endometrial wave patterns.The study suggests that the lack of coordination or efficiency in uterine contractions in women with a uterine niche may disrupt typical wave patterns, potentially decreasing the rate of implantation [15].
• Hypothesis 4: The collected old blood and mucus in the niche and uterine cavity can hinder penetration of sperms We propose that a lower incision during CS across cervical tissue, containing mucus-secreting glands, may lead to the formation of mucus-filled niches due to local mucus production.TV/US and hysteroscopy reveal that many niches, particularly in low positions, are associated with Nabothian cysts.The accumulation of mucus and blood in these large niches and the uterine cavity may inhibit sperm penetration and embryo implantation.Spotting during coitus, with blood in the cervical canal, could hinder the interaction between cervical mucus and sperm for immunological reasons [15].
• Hypothesis 5: Physical obstacle/barrier to the embryo transfer and implantation A large uterine niche, often associated with a severely retroverted and fixed (RVF) uterus, can create challenges in accessing the uterine cavity for embryo transfer.This anatomical distortion poses difficulties during assisted reproductive technolo-gies commonly used for infertility treatment.In some cases, the combination of a large uterine niche, a severely retroverted and fixed uterus, and a complicated niche may impede the insertion of a catheter for intrauterine insemination (IUI) or embryo transfer [15].
• Hypothesis 6: Psychological factors that lower the possibility of conception Symptoms of uterine niche, such as pain and spotting, can interfere with sexual activity.Medications prescribed for symptomatic relief may affect ovulation prospects during recovery.Sixty percent of women develop a uterine niche after CS, and 30% report postmenstrual spotting/bleeding.Unpredictable bleeding can occur at any menstrual cycle phase, causing anxiety, discomfort, and embarrassment, and impacting sexual behavior and arousal in affected women [15].
• Uterine niche effect on IVF/ICSI treatment Presence of uterine niche may have a detrimental effect on subsequent pregnancy of women undergoing IVF/ICSI treatment with lower pregnancy and livebirth rate [18].This may be due to the following: difficult embryo transfer is encountered in 20% women with niche undergoing IVF, due to distorted anatomy, especially in a retroverted uterus (Kulshrestha, Agarwal, and Kachhawa 2020) [16].Furthermore, decreased endometrial receptivity because of accumulated intrauterine fluid like hydrosalpinx or unfavorable microenvironment by changing endometrial microbiome or disrupted endometrial wave like activities needed for successful pregnancy [18].To date, no evidence supports niche repair to improve reproductive outcome in women undergoing IVF/ICSI treatment.Therefore, couples should be properly counselled about benefit/risks of niche repair prior to ICSI trial.
(5) Dysfunctional bladder Despite the proximity between the uterine niche and the urinary bladder, it was hypothesized that locally accumulating fluid and fibrosis at the niche site might lead to dysfunction.However, prospective trials did not provide support for this hypothesis [16].
(6) Abscess at the site of scar Abscess formation has been documented up to 6 years post-cesarean delivery, potentially due to infected remnants of blood and mucus, although this occurrence is uncommon [16].

Obstetric complications of uterine niche in future pregnancy
Isthmocele is associated with a higher risk of pregnancyrelated complications, including CS scar pregnancy, abnormal placentation (previa-accreta-increta), and dehiscence at the scar site (partial or complete leading to ruptured uterus).The incidence of uterine rupture in posterior pregnancies is typically less than 2%, but it increases to 5% in the presence of large defects.Ultrasonography-measured scar thickness is not a reliable predictor of uterine rupture.Cesarean scar ectopic pregnancy, a rare obstetric issue where the embryo implants within the myometrium overlying the niche, has seen an increase in incidence in recent decades [2].

Uterine rupture or dehiscence risk in pregnancy with uterine niche according to RMT
Several studies have attempted to correlate RMT with the risk of uterine rupture during pregnancy.A cohort study had assessed 642 women with previous CS, reported a 4% rupture/dehiscence rate (15 ruptures, 10 dehiscence); moreover, the frequency increased as the myometrial thickness decreased.No cases of uterine rupture reported when RMT > 4.5 mm; on the other side, most cases of uterine rupture occurred when RMT was ≤ 2.5 mm.Cutoff > 3.5 mm of RMT, a sensitivity of 88.0%, a specificity of 73.2%, a positive predictivevalue of 11.8%, and a negative predictive value of 99.3% were used in estimating the risk of uterine rupture or dehiscence [19].

The emergent term CSDi
Trying to differentiate symptomatic niche from asymptomatic one: the term CSDi (caesarean section scar disorder) was emerged after Delphi study conducted 2023 by Klein Meuleman and his colleagues [20].They defined CSDi as myometrial defect more than 2 mm with one primary symptom or two secondary symptoms.Primary symptoms or problems include postmenstrual spotting, pain related to uterine bleeding, secondary unexplained infertility with intracavitary retained fluid, and technical difficulties of catheter insertion during embryo transfer.Secondary symptoms include dyspareunia, chronic pelvic pain, abnormal vaginal discharge, avoiding sexual intercourse, negative self-image, abnormal odor related to AUB, unexplained secondary infertility, secondary infertility despite assisted reproduction, and discomfort during leisure activities participation [20].
Obstetric issues related to niche are defined as complications of CSDi, not primary or secondary symptoms.These complications include Cs scar ectopic, placenta accreta, uterine scar dehiscence, or rupture [20].
The following conditions should be excluded before diagnosis of CSDi: cervical dysplasia, vaginal or uterine infections, other intracavitary pathologies, anovulation, hormonal therapy, IUD [20].
The following criteria needed to apply CSDi: premenopausal women, cycle regular at least 3 consecutive cycles, onset of symptoms after CS or worsened after CS, patient can be cured from a symptomatic niche [20].
Figure 9 summarizes criteria of diagnosis of uterine niche.

Diagnosis
Ultrasonography, sono-hysterography, MRI, and HSG are imaging techniques commonly used to evaluate cesarean section scars and diagnose uterine niche in addition to hysteroscopy [2].

A) Imaging
(1) Ultrasound Transvaginal ultrasonography (TV/US), being noninvasive and widely available, is the primary tool for assessing women with abnormal uterine bleeding (AUB) and a history of CS delivery.TV/US can detect uterine niche in 50% of cases, but its effectiveness depends on operator skill and hormonal influences [21].Before the European Niche Taskforce's 2019 recommendations, there were no standardized criteria for assessing CS niche.
A landmark article by the Taskforce established a standardized method for identifying and assessing uterine niche using ultrasonography in non-pregnant women.Gel instillation sonography remains the standard method, but ultrasonography, performed during specific cycle days (7)(8)(9)(10)(11)(12)(13)(14), may provide sufficient information without the need for gel instillation [21].
Table 3 summarizes the findings of a literature search that yielded articles that defined research questions related to ultrasonographic assessment of uterine niche (Appendix A).

• Ultrasonographic standard criteria for detection and diagnosis of the uterine niche (by European Niche Taskforce)
Sonographically, the uterine niche can be classified as simple, simple with one branch, or complex (main niche with more than one branch).A branch is a smaller part at the serosal side with a width less than that of the main niche (Fig. 1) [22].For surgical planning, it is crucial to estimate distances between the niche and the external os, as well as between the niche and the vesicovaginal fold, although not essential for basic evaluation.Depth, width, length, residual myometrial thickness (RMT), anterior myometrial thickness (AMT), and distances to the vesicovaginal fold and external os are clinically significant measurements [23].
For a simple niche, measurements can be performed in a single plane, while complex niches may require multiple planes.Length and width measurements are preferred at the greatest width or length.Endometrium documentation is considered irrelevant to niche assessment; therefore, calipers should be placed on the myometrium boundary [23].

• Tips for the visualization of niches
Experts propose various recommendations and techniques to enhance niche visualization, emphasizing the importance of obtaining a clear view of the lower uterine section.In transvaginal ultrasound, the position of the probe (anterior vs. posterior fornix) and the pressure applied can significantly impact niche visualization.Greater pressure may be needed for more proximally positioned niches, while less pressure is required for those located distally or for assessing the vesicovaginal fold [1].
A full bladder is not necessary for visualizing the vesicovaginal fold.While Doppler ultrasound can be valuable for distinguishing a niche from other uterine wall abnormalities, it is not essential for niche assessment.The main key is to achieve a clear view of the lower uterine segment [1].
Table 4 summarizes the international guidelines for ultrasound evaluation of the niche [21,24] (Appendix A).Fig. 2 Sagittal view of uterus measuring depth and length of uterine niche [22] in subsequent pregnancies.The volume can be calculated automatically, manually using VOCAL, or manually based on niche dimensions (depth × width × length × 0.52) [25] (Table 5, Appendix A).
(2) Other imaging ➢HSG Hysterosalpingography (HSG) was among the initial modalities used to diagnose uterine niche (Fig. 5).A drawback of HSG is its inability to assess niche size and residual myometrial thickness, parameters easily determined by sono-hysterography [24].
➢ MRI Wu Shun Felix Wong et al. explored the use of MRI in assessing cesarean scar defects (CSD).Notably, all sonographic parameters, such as depth, width, length, RMT, and AMT, can be measured using MRI as illustrated in Fig. 6 [26].The limited documentation of CSD incidence by MRI suggests its underutilization as a diagnostic tool for patients with post-CS bleeding or infertility complaints mainly due to its cost.It may benefit in the presence of huge niches or suspicion of other pathology [26].

B) Role of endoscopy in the evaluation of niche
Hysteroscopy and laparoscopy provide direct visualization and identification of the Caesarean section scar niche.Hysteroscopic observations include indentation Fig. 3 Sagittal view of uterus to assess RMT and AMT [22] or cavity within the isthmic myometrium, a prominent distal ridge, breach of the lining mucosal layer, presence of lateral branches, aberrant vascular patterns at the base, and cyst-like or polypoid formations [21].
Hysteroscopy allows both visualization and treatment of the niche but cannot assess residual myometrial thickness [2].In laparoscopy, the niche may manifest as ballooning of the lower segment, often accompanied by adhesions to the bladder or anterior abdominal wall.Recognizing these patterns is crucial for accurate documentation in women undergoing gynecological examinations [21].See laparoscopic (Fig. 7a,b) and hysteroscopic (Fig. 8a,b) images for illustration.Fig. 5 Forty-year-old woman's hysterosalpingogram reveals a significant CS niche near isthmic part (arrows) [24] Fig. 6 Cesarean scar Niche measured by magnetic resonance imaging: D stands for the defect's depth, w for its width, t for the scar myometrium's thickness, and T for the adjacent myometrial thickness [26] Fig. 7 Laparoscopic view of uterine niche after bladder dissection, concomitant hystroscopic transillumination

Management of uterine niche
The decision to treat uterine niche depends on factors like symptoms, secondary infertility, pregnancy plans, and defect size.For asymptomatic cases with no pregnancy plans, conservative management through clinical observation is often recommended.Management options include: (1) Clinical management • Expectant management: Monitoring without intervention.
• Pharmacological treatment: Medications for symptom relief.
▪ Hysteroscopic approach ▪ Laparoscopic approach ▪ Vaginal approach ▪ Laparotomy • Radical surgery: Reconstructive surgery is commonly preferred, but each approach carries specific risks like infections, bowel/bladder injury, and hemorrhage.The choice of treatment depends on individual circumstances and preferences [27].

(A) Medical management
Oral contraceptives show efficacy in reducing bleeding issues associated with isthmocele.A study demonstrated that a three-cycle course of combined oral contraceptives (COCs) led to a reduction and cessation of spotting, Florio et al. compared hormonal therapy with hysteroscopic repair for alleviating uterine niche symptoms, revealing that hysteroscopic correction was more successful in reducing the duration of postmenstrual spotting and pelvic pain [27].
Various management options, including laparoscopic and vaginal repair, hysteroscopic ablation, COCs, and levonorgestrel intrauterine system, were evaluated, indicating effectiveness in shortening the menstrual cycle in symptomatic patients, except for levonorgestrel intrauterine system [28].However, cyclic oral COCs had inferior results compared to hysteroscopic niche resection according to a systematic review by Vander voet et al. [29].

(B) Surgical management
Indications for niche surgery include persistent menstrual disturbances (postmenstrual and intermenstrual spotting) and infertility attributed to the uterine niche.Surgical options encompass radical measures like hysterectomy and reconstructive techniques employing minimally invasive approaches.Commonly discussed in the literature are laparoscopic niche repair (with or without hysteroscopic guidance), hysteroscopic approaches, (1) Hystroscopic approach Hysteroscopic repair of uterine niche is a minimally invasive and low-risk procedure involving diagnostic and operative hysteroscopy, known as isthmoplasty [2].The procedure is focused on addressing RMT, a critical consideration [27].
The literature emphasizes RMT as the primary factor for choosing hysteroscopic repair.Potential risks include bladder injury and uterine perforation, especially when RMT over the niche is ≤ 3 mm.Some authors recommend hysteroscopy for women with RMT greater than 2 to 2.5 mm or a niche depth to myometrial thickness ratio of less than 50%, even if not desiring future pregnancy, but caution is needed due to the risk of bladder injury and perforation [27].
Isthmoplasty methods vary, with common approaches involving resection of proximal and distal niche edges using a 9-mm resectoscope and unipolar electrical current.Electrocauterization of the niche base is commonly performed.Other techniques include shaving the niche edges, connecting it to the cervical canal [30], or removing fibrous tissue beneath the niche [31].Sonographic guidance may be used but does not necessarily reduce morbidity [32].
A 2015 multicenter randomized controlled trial in the Netherlands showed significant improvement in the postmenstrual bleeding duration and associated pain after hysteroscopic isthmoplasty in patients with at least 3-mm RMT uterine niche [33].
Bladder injury risk makes hysteroscopic resection unsuitable for women with RMT ≤ 3 mm, recommending laparoscopic or vaginal procedures.Resection of the caudal ridge poses a theoretical risk of cervical incompetence, leading some authors to discourage it [21].
Table 6 illustrates the studies that evaluated the effect of hysteroscopic repair of uterine niche (Appendix A). (

2) Transvaginal approach
The transvaginal approach for repairing uterine niche is a minimally invasive and effective method [2].The procedure, as described by Chen et al., involves placing patients in the dorsal lithotomy position, evacuating the bladder with a catheter, and using vaginal retractors to expose the cervix.Adrenaline is administered for hydro dissection and hemostasis.An incision is made at the cervicovaginal junction, and dissection proceeds until the peritoneum of the vesico-uterine pouch is identified.The retractor is positioned to provide a clear view, and a transverse incision is made at the identified niche level.The uterine defect is then repaired by removing the niche and suturing the incision edges [34].This transvaginal approach is comparable to laparoscopic repair in efficacy but offers shorter surgical time and lower hospital costs [27].
Table 7 summarizes the studies that evaluated the effect of vaginal repair on uterine niche (Appendix A).
(3) Laparoscopic approach The laparoscopic approach for repairing large uterine niche defects (RMT ≥ 3 mm) in symptomatic women with a desire for future pregnancy involves excising niche edges to remove scar tissue and closing the defect with two-layer sutures [2].Introduced in 2003 by Jacobson, this approach has been adopted by various authors.It requires a trained laparoscopic surgeon proficient in conventional laparoscopy or robotic surgery [27].

Identifying the niche correctly is crucial and can be achieved through various techniques
• Hysteroscopic transillumination: Concurrent hysteroscopy during laparoscopy enhances niche identification as the hysteroscope light is more evident at the thinner endometrium of the niche.• Concurrent TV/US: Transvaginal sonography can be employed if the scar is not directly discernible following uterovesical space dissection.• Foley catheter at LUS: Inserting a Foley catheter through the cervical canal into the uterine cavity and filling it at the lower part helps identify the niche.• Slip and hook technique: If direct identification is challenging, a Hegar dilator is slid forward blindly against the anterior uterine wall at the isthmus, creating a "hooking effect" for niche perforation with laparoscopic guidance.
Table 8 detects the studies that evaluated the effect of Laparoscopic repair of uterine niche (Appendix A).
Vitale et al. conducted another systematic review and meta-analysis, analyzing approximately 33 papers.
There is sufficient evidence to justify surgical intervention for symptomatic niche management, with intervention reported to reduce bleeding symptoms in over 80% of patients.However, evidence supporting surgery to improve reproductive outcomes or reduce obstetric complications in subsequent pregnancies is not as established [35].
Laparoscopic and transvaginal approaches are preferred for patients with a thin remaining myometrium above the niche (≤ 2.5 mm) when the hysteroscopic approach fails.Hysteroscopic isthmoplasty may be the safest and most effective in patients with sufficient RMT above the niche, while laparoscopic and transvaginal surgeries are preferable for women with a thinner RMT above the niche (≤ 2.5 mm) [35].
Table 9 summarizes the studies that evaluated the effect of different approaches for the repair of uterine niche (Appendix A).

Reproductive outcome after niche management
In a systematic review by Harjee, comprising 13 studies (1 RCT, 6 prospective case series, and 6 retrospective case series) on 234 patients with uterine niche and secondary infertility, surgical intervention was performed in 188 patients via hysteroscopy, 36 via laparoscopy, 7 via Fig. 9 Diagnostic criteria of uterine niche and CSDi laparotomy, and 3 vaginally.Overall, 65.4% of patients became pregnant, with a live birth rate of 87.1%.The reoperation risk was 2%.Hysteroscopy showed better pregnancy outcomes compared to untreated cases in the RCT [36].
Verberkt et al. 's systematic review included 21 papers (1 RCT, 1 case series, 5 prospective, and 14 retrospective cohort studies) with 648 women experiencing secondary infertility and 237 without infertility.The live birth rates were higher in infertile women (54%) than those without infertility (36%).Different surgical approaches (hysteroscopic, laparoscopic, and transvaginal repair) showed varying live birth rates in infertile women, with hysteroscopic isthmoplasty demonstrating better outcomes 55%, 60% after vaginal repair, and 42% after laparoscopic repair.Versus 52%, 25%, and 36% respectively in women without infertility the pregnancy rate following hysteroscopic isthmoplasty was significantly higher than with expectant care (RR 2.41), as reported in the single RCT.Scar dehiscence was documented in 2.8% of deliveries following hysteroscopic repair, while laparoscopic and transvaginal approaches did not show scar dehiscence.The overall quality of the analyzed trials ranged from moderate to poor, with a notable risk of bias.Many included studies did not evaluate or document evaluation of other factors of infertility including semen analysis, tubal pathology, endometriosis [37].
Table 10 summarizes the studies reporting the pregnancy rate after management of uterine niche for women complaining of infertility (Appendix A).

Conclusion
In conclusion, pathogenesis of niche formation is still unclear with different hypotheses.Ultrasound assessment of RMT is essential to determine surgical repair approach.Repair of uterine niche will improve symptoms (bleeding, pain), with no evidence to improve reproductive outcomes/obstetric complications in following pregnancies.Optimal timing for niche measurement Immediate after the end of menstruation, when the endometrium is the thinnest, is the most optimal time for a US assessment.
2 Allison [57] Effective method for measurement (TVS or contrast) Saline infusion is a valuable complement to Transvaginal sonography, particularly for endometrial and adjacent lesion assessment.
3 Baranov [58] Cohort study Niche detected in 46.4% of cases by TVS, VS 69.1% of cases on saline contrast, 4 Vikhareva Osser [10] Cohort study • The shape of the niche did not alter at saline sonohystrography; however, it is simpler to distinguish the scar defects' borders at sonohystrography than at non-contrast ultrasonography.
• The depth and length of the niche at saline sonohystrography were more than they were previously.The difference was 2 mm and 1 mm in women with the previous 1 CS, while the difference was 4 mm and 2 mm in women with the previous 2 CSs.Retrospective study Niche prevalence by 3D-TVS is 50%, but by 2D saline infusion sonography, it is 86%.Sonohysterography with saline contrast is more sensitive than 3D transvaginal ultrasonography for assessing cesarean scar defects.Moreover, it examines the size and shape of the defect more precisely, hence estimating its severity.

Study author Study design Research question Results
11 Alalfy [63] Observational cross-sectional comparative study 3D ultrasonography with SHG is superior to 2D SHG in evaluating the niche width and RMT before ICSI, hence offering a more accurate assessment of the scar niche.
12 Naji [64] Niche specified measures RMT-depth, length and width of niche must be assessed sagittal and transverse planes.
13 Tower [59] RMT measured from the defect's apex to the myometrium's edge.
14 Fabres [65] Retrospective study The ideal time of the menstrual cycle to assess uterine niche.
Sonography is most effective for identifying uterine niche during the bleeding phase, typically a few days following menses.

•
Role of 3D ultrasound in niche assessment LUDWIN et al. introduced the VTS scoring method for categorizing uterine niches using 3D-SHG.A score of ≤ 2 suggests clinical irrelevance and further specific features should be assessed.A score > 2 indicates a clinically relevant uterine niche, necessitating close follow-up

Fig. 1
Fig.1Red area representing the main niche, while the blue-colored area represents the branch[22]

Fig. 8 a
Fig. 8 a, b illustrate hysteroscopic view of uterine niche 34 years, precious 2 CS, complaining of postmenstrual spotting, chronic pelvic pain Figures 9 and 10 summarize algorithms for diagnosis and management of uterine niche.

Table 1
Studies evaluating the relationship between CS type and uterine niche formation

Table 2
Studies evaluating the relationship between cesarean section closure technique and niche formation

Table 3
Two-and three-dimensional ultrasonography with or without SIS for the diagnosis of uterine niche

Table 4
International guidelines for the ultrasonographic evaluation of uterine niche

Table 5
Correlation between imaging criteria and clinical significance of uterine niche -

Table 6
Studies evaluated the effect of hysteroscopic repair of uterine niche

Table 7
Studies evaluated the effect of vaginal repair on uterine niche

Table 8
Studies evaluated the effect of laparoscopic repair of uterine niche

Table 9
Studies that evaluated the effect of different approaches for repair of uterine niche

Table 10
Studies reporting the pregnancy rate after management of uterine niche for women complaining of infertility