The reason why the defect does not appear in all women undergoing cesarean section is unknown, and the pathogenesis of the scar defect remains unknown. Different factors have been described as a possible cause of a cesarean scar defect (CSD). One possible factor related with the CSD is the difference in myometrial contraction between the thicker superior edge of the incision and the inferior one. This difference in thickness is usually more evident as the number of cesarean increases. The approximation of incision edges with different thickness can contribute to the development of the CSD [11].

Another possible factor suggested is the surgical technique used to close the hysterotomy; it is argued that the presence of a CSD can be in relation with the suture material used, with the suture technique, or both. Furthermore the combination of an ischemic suturing technique and a slow absorbable suture material can produce an abnormal healing [12]. Regarding the technique, Yazicioglu found that the frequency of incomplete healing was significantly lower in the group treated by full-thickness suturing [13]. A recently published meta-analysis found no significant difference in the risk of uterine scar defect with single-layer closure compared to double-layer closure [14].

Ofili-Yebovi found a relationship between multiple previous cesarean section and CSD and also noted that uterine retroflexion was another variable that was clearly associated (Fig. 19.2). In a retroflexed uterus, the lower segment is under a degree of tension, which may affect to the healing of the cesarean section scar [15].

There is an association between the degree of cervical dilatation and the duration of labor with an increase in the risk of CSD if the duration of labor is ≥5 h or the cervical dilatation is ≥5 cm [16]. In late labor, the modified cervix becomes part of the lower uterine segment. Low incisions are more common if cesarean section is performed late in labor and cervical tissue may be included in the closing sutures, interfering with the healing of the scar.

It is well documented that some late complications are present after a previous cesarean section. As well as the obstetrical complications, some gynecological disturbances have been described in patients who have a CSD. Postmenstrual abnormal bleeding, chronic pelvic pain, and secondary infertility are linked to this pathology.

The classic symptom in those patients is the presence of postmenstrual abnormal bleeding, of about 2–12 days of duration, usually scarce and dark in color. Morris [9] was the first to describe a relation between this postmenstrual bleeding and the presence of anatomic and histologic changes at the site of the cesarean scar.

Postmenstrual bleeding (PB) is estimated to occur in one in three (33.6 %) of women with a niche in the scar. There is a direct relation between the size of the defect and the quantity and duration of the bleeding, mainly in retroverted uteri. Probably a triple mechanism is involved in this postmenstrual bleeding. On the one hand, the disruption in the continuity of the endometrium acts as a reservoir pouch, in which some menstrual blood and debris are accumulated (Fig. 19.3); the slow outflow of this retained blood is linked to the PB. Another related mechanism is poor contractility of the uterine muscle around the scar, due to the existence of fibrotic tissue, which prevents normal myometrial contractions [10]. Last but not least, there is minimal production in situ due to local changes that take place in the niche as congested endometrium above the scar, lymphocytic infiltration, and the presence of small polyps [9] (Fig. 19.4).

The presence of a disruption in the myometrium at the site of the cesarean scar is associated with different clinical symptoms as dysmenorrhea, chronic pelvic pain, and dyspareunia. Among these symptoms, dysmenorrhea is the most common with an incidence of 53 %, followed by chronic pelvic pain in 39.6 % and dyspareunia in 18.3 % [17].

All those symptoms are probably caused by chronic inflammation and the lymphocytic infiltration present in the scar.

Secondary infertility has also been related to CSD. The accumulation of blood in the niche can affect the normal characteristics of the mucus and interfere with sperm transportation through this mucus. There is also minimal retrograde passage of blood, to the uterine cavity, especially in retroverted uteri, that can affect the quality of the endometrium with consequences during embryo implantation (Fig. 19.5).

The diagnosis of cesarean scar defect is based on a previous history of cesarean section, clinical symptoms, and diagnostic tools as ultrasound and hysteroscopy.

Currently, there is lack of consensus on the definition of cesarean scar defect. Ultrasound is usually the first diagnostic modality used in women with postmenstrual bleeding. The ultrasound study can be performed with conventional 2D ultrasound, 3D or saline infusion sonohysterogram (SIS), or gel (GIS) to fill the niche and to create a better image. Hysterosalpingography, hysteroscopy, and RMN can be also used to diagnose this defect.

Transvaginal ultrasound is accurate in detecting cesarean scar defects. The niche is defined by the presence of an anechoic area at the site of a previous cesarean section (Fig. 19.6). This niche is usually triangular in shape with the vertex toward the isthmus. Another proposed diagnostic criterion is the presence of fluid within the incision site [18]. The prevalence of a niche on evaluation with conventional 2D ultrasound is 24 % [16]. The best time to perform ultrasonography diagnosis of CSD is during the late proliferative phase in which the cervical mucus can fill the niche. The use of 3D ultrasound facilitates the study of the defect in multiple planes and offers more information than conventional ultrasonography (Fig. 19.7).

Cesarean scar defects can also be diagnosed by hysterosalpingography, usually as an incidental finding. The presence of anatomic defect as a diverticulum or thin linear defects at the lower uterine cavity is a common finding in patients with a previous cesarean section, and these defects can be found at around 60 % of patients [19].

The use of SIS or GIS provides a clear visualization of the CSD due to the filling of the niche with liquid, facilitating the diagnosis. Moreover, more defects are detected using sonohysterography and more defects are classified as large than with the use of conventional 2D ultrasound [20]. The instillation of liquid inside this defect allows us to find different shapes and sizes. The prevalence of a niche on evaluation with gel is around 56 % [16]. The main advantage of the use of gel is that remains longer time filling the disruption; this allows performing a better evaluation of the defect.

Hysteroscopy allows a direct visualization of the scar defect. During hysteroscopy, a pseudo-cavity is visualized in the anterior wall of the low uterine segment or in the upper third of the cervical canal. Hysteroscopically, a double arch of fibrous tissue is identified and a dome between those archs (Fig. 19.8). The dome of the isthmocele is covered by a congestive endometrium with different grades of inflammation. In the early proliferative phase, blood and some clots are usually visualized filling the anatomical defect and the cervical canal.

Magnetic resonance imaging (MRI) can also detect myometrial defect located at the lower uterine segment. The MRI displays a linear low signal niche, sometimes filled with fluid (Fig. 19.9). The use of MRI can be useful to planning the corrective surgery and to rule out other conditions.

There are two main classifications used for the CSD. The one proposed by Gubbini [21] in which the depth and the base of the isthmocele are measured and the surface of the isthmocele is calculated. According to the result of the surface, the isthmocele is classified into three grades: grade 1 with less than 15 mm³, grade 2 with a surface between 16 and 25 mm³, and grade 3 with more than 26 mm³. In his review, he found that more than 55 % of cases were grade 1.

Yebovi focused the other classification of the CSD on the measurement of the endometrial thinning at the cesarean defect; he defined the degree of thickness by the ratio between the myometrial thickness at the level of the defect and the thickness of the adjacent myometrium and defined a severe defect a ratio >50 % [14] and dehiscence a ratio equal or superior to 80 %.

Other authors have defined CSD as severe when the remaining myometrium at the level of the niche is less than 2.2 mm visualized with ultrasound examination or 2.5 mm in women who underwent hydrosonography for the diagnosis of the CSD [22] (Fig. 19.10).

Various surgical options have been proposed to treat the CSD: on one hand, a reparative treatment with laparoscopic repair of the dehiscence and, on the other, the resectoscopy correction in order to improve the symptoms. Other alternatives are the vaginal repair of the CSD and the use of oral contraceptives to reduce menstrual blood. The surgical treatment should only be reserved for symptomatic patients with postmenstrual bleeding, chronic pelvic pain, or secondary infertility. The first two options are the commonly used, and the election of any of them is usually related with anatomical conditions of the CSD.

The first reference about the use of the resectoscope in the treatment of a CSD was made by Fernandez [23] who performed the resection of the fibrotic tissue of the inferior part of the scar to facilitate the drainage of the menstrual blood collected in the scar, improving the postmenstrual bleeding. Since then, multiple articles have been published, and the resectoscopy has become the most reported approach for the treatment of symptomatics CSD. Fabres in addition to the resection of the fibrotic tissue underneath the pouch defect used the local fulguration of the dilated blood vessels and endometrial glands in the CSD, responsible of the in situ production [24] (Fig. 19.11). The main risk associated with the resectoscopy surgery is the possibility of uterine perforation and secondary bladder injury; in order to prevent this complication, some authors recommend to avoid the resectoscopic surgery if the remaining myometrium at the level of the niche is less than 2 mm [25].

The purpose of the laparoscopic management is to restore the myometrial continuity at the site of the CSD which leads to a reduction of the niche and consequently to an improvement of the related symptoms. The main advantage of the laparoscopic approach is that we can consider this as a reparative surgery which leads to an increase in the thickness of the uterine wall, something that can’t be done with the hysteroscopic approach [26]. Klemm firstly used a combined laparoscopic-vaginal approach to repair the defect [27]. Donnez described a totally laparoscopic approach with excision of the fibrotic tissue around the scar and laparoscopic suture to approximate the healthy myometrium of each side of the opened scar [28]. The laparoscopy surgery offers a clear visualization of the surgical area after the dissection of the bladder with low risk of damage (Fig. 19.12).

The vaginal approach of the cesarean section defect is also considered a reparative surgery, which corrects the defect and increases the thickness of the uterine wall. As we referred before, this was firstly used in combination with laparoscopy approach. A new vaginal repair technique has been recently proposed in which after the opening of the cervico-vesical space and the dissection of the bladder, the scar is opened and the fibrotic tissue removed. The opened scar is secondary closed with two layers of suture [29]. The approach that uses the vaginal route is a minimally invasive way of repairing the myometrial continuity.

The use of oral contraceptives can be a conservative alternative for the management of the postmenstrual bleeding. The published results on effectiveness are conflicting. While different studies have concluded that the medical therapy fails to eliminate the bleeding [10], others support the use of oral contraceptives for treating intermenstrual bleeding in patients with defects at the previous cesarean uterine to reduce the menstrual blood [30]. There are no consistent studies about the use of the hormonal intrauterine device.

The surgical outcomes are different depending on the surgical procedure. After hysteroscopy surgery, between 59.6 [8] and 64 % [25] of patients reported a postoperative improvement of postmenstrual bleeding. This improvement was more evident in patients with anteflexed uterus.

Very few of these pregnancies reported in the literature progressed beyond the first trimester [36, 41] as almost all are terminated during this period. It is likely that if a developing pregnancy in a cesarean section scar were to continue to the second or third trimesters, there would be a substantial risk of uterine rupture with catastrophic hemorrhage, with a high risk of hysterectomy causing serious maternal morbidity and loss of future fertility. There is also a danger of invasion of the bladder by the growing placenta. A pregnancy that protrudes through the scar, if viable, can implant on other abdominal organs and continue to grow as a secondary abdominal pregnancy [32, 42].

However, if the pregnancy continues within the uterus, the risk of placenta accreta is significantly increased, up to three- to fivefold [43, 44]. CSP progressing to 35 weeks of gestation has been described, but this case was complicated by massive hemorrhage and disseminated intravascular coagulopathy at CS, requiring a lifesaving hysterectomy [41]. There are very few cases reported in the literature of ectopic pregnancy within a cesarean scar resulting in live birth [45].

CSP may present from as early as 5–6 weeks [40] to as late as 16 weeks [46]. A light, painless vaginal bleeding is usually the early presenting symptom in 39 %. Approximately 16 % of women complain of accompanying mild to moderate pain and 9 % complain of only abdominal pain [46]. It can be an incidental finding in an asymptomatic woman (37 %). Severe acute pain with profuse bleeding implies an impending rupture.

Collapse or hemodynamic instability strongly indicates a ruptured CSP. Clinical examination in stable women is usually unremarkable. The uterus may be tender if the CSP is in the process of rupture.

Transvaginal ultrasound (TVUS): TVUS on its own has a diagnostic sensitivity of 86.4 % (95 % CI 0.763–0.9050) [48]. TVUS is the first line to diagnosis or to confirm CSP (Fig. 19.14). The criteria are:

The thickness of the intervening myometrium between the gestation sac and the bladder has been shown to be less than 5 mm in two-thirds of the cases [47].

In order to reduce the risk of a false diagnosis, a combined approach is recommended: a TVUS to obtain the fine details of the gestation sac and its relation to the scar followed by a meticulous abdominal scan with a full bladder [34, 48]. The abdominal scan provides a panoramic view of the uterus and an accurate measurement of the distance between the gestation sac and the bladder.

The color flow Doppler shows a circular peritrophoblastic perfusion surrounding the gestational sac that helps to reach a diagnosis [49] and to delineate the CSP sac location of the placenta in relation to the scar and the bladder [36] (Fig. 19.15).

3D US has been used to enhance the diagnostic accuracy of a CSP [23, 24, 50, 51] (Fig. 19.16).

Combination of the multiplanar views and surface-rendered images helps identify subtle anatomical details of a well-developed trophoblastic shell around the gestational sac [50]. The thin myometrium between the gestational sac and the bladder wall can be recognized with confidence. Furthermore, peritrophoblastic flow surrounding the CSP may be illustrated by 3D power Doppler.

The superior soft tissue characterization and anatomical information provided by MRI allows patients and clinicians to consider conservative management as initial therapy, especially with the increasing availability of minimally invasive uterine artery embolization [52]. MRI can accurately detect the exact location of pregnancy, thus confirming the diagnosis [53, 54].

Huang et al. [55] performed a study regarding the use of intravenous contrast in MRI done for patients with CSP; this study concluded that contrast-enhanced MRI could be used as a reliable adjunct and initial imaging modality for diagnosing CSP in selected cases. The imaging features of contrast-enhanced MRI may result in a more accurate diagnosis before specific treatment for CSP.

Diagnostic hysteroscopy only helps to confirm the finding of a normal and empty uterine cavity together with the pregnancy tissues at the lower corpus [56].

Hysteroscopic removal of conceptive tissues implanted in a cesarean section scar seems to be a feasible and safe procedure that might be considered as a treatment option [57].

Another diagnostic option for CSP is laparoscopy [58–60]. The uterus size is usually seen normal or bulky (depending on the gestation age) with the CSP arising as a hillock with a “salmon red” ecchymotic aspect, bulging the uterine serosa from the previous cesarean section scar behind the bladder [61].

The fallopian tubes and the ovaries are seen normal.

Generally, termination of pregnancy in the first trimester is strongly recommended, as there a high risk of subsequent uterine rupture, massive bleeding, and life-threatening complications as with any ectopic pregnancy.

Treatment objectives should be to perform feticide prior to rupture, to remove the gestational sac, and to retain patient’s future fertility.

Treatment can be divided to medical or surgical approach or an association between both.

The administration of methotrexate (MTX) is a standard treatment for tubal ectopic pregnancy, and it is also effective with CSP. The administration can be systemic or local.

CSPs have been shown to respond well to it (dose of 50 mg/m2), especially in those with b-hCG levels <5,000 miu/ml [62]. Conservative medical treatment is appropriate for a woman who is pain-free and hemodynamically stable with an unruptured CSP of <8 weeks of gestation and a myometrial thickness <2 mm between the CSP and the bladder. All women considered suitable for MTX treatment should have prior baseline full blood count and liver and renal function tests performed. They must be agreeable to surgery if medical treatment fails or if the CSP ruptures.

Systemic treatment alone is not the best treatment option due to 62 % complication rate. IM MTX injection has a slow action and the pregnancy continues to grow. It is recommended to use more than one injection and to associate it with other treatments.

MTX can be injected locally with ultrasound guidance, to the gestational sac via transabdominal or via transvaginal route. Transabdominal route requires a longer needle, used with caution not to penetrate the bladder wall, and does not require any anesthesia. The transvaginal approach allows for a shorter distance to the gestation sac with minimal risk of bladder injury.

A review of the literature by Arslan et al. [64] shows that uterine curettage was either unsuccessful or caused complications in eight out of nine women, requiring surgical treatment, and in a case series of eight CSPs, Wang et al. [59] had four secondary referrals after failed curettage, thus indicating a failure rate of 70 % [12, 17].

The gestation sac of a CSP is not actually within the uterine cavity and the chorionic villi implant into the cesarean section scar of the lower segment. Therefore, not only the trophoblastic tissue is unreachable by the curette but also such attempts can potentially rupture the uterine scar leading to severe hemorrhage and cause more harm. Profuse bleeding during the procedure and absence of chorionic villi in the specimen obtained by curettage must prompt immediate laparoscopy/laparotomy.

Operative laparoscopy should be performed only after a prior TVS confirms the diagnosis (Fig. 19.17). The CSP mass is incised and the pregnancy tissue removed in an endo-bag. Bleeding can be minimized by local injection of vasopressin (1 unit/ml, 5–10 ml), hemostasis achieved by bipolar diathermy and the uterine defect closed with endoscopic suturing (Fig. 19.18).

Laparotomy followed by wedge resection of the lesion (hysterotomy) should be considered in women who do not respond to conservative medical and/or surgical treatments, present too late or if facilities and expertise for operative endoscopy are not available. Laparotomy is mandatory when uterine rupture is confirmed or strongly suspected (Fig. 19.19).