While its etiology remains uncertain, cesarean scar niche (CSN) is well publicized, as are its pathological clinical manifestations. In a future pregnancy, they include cesarean scar pregnancy (CSP), which in turn can lead to placenta accreta spectrum, and possible uterine rupture/dehiscence of a residual thin myometrial layer. CSP refers to the implantation of an early pregnancy on the scar or in the niche at the site of a prior cesarean delivery (CD); it has an incidence of 1 per 1,000 pregnancies. An estimated 52% of CSPs occur after even just one CD.1 CSP has been linked to placenta accreta spectrum and has been shown to be its precursor.2 Both CSP and placenta accreta spectrum can be consequences of CD and share a common histology of villous or placental attachment/invasion into the cesarean scar.3 The incidence of placenta accreta spectrum has risen from about 1 in 4,000 live births in the 1970s to 1 in 2,500 in the 1980s; in 2016, the incidence of placenta accreta spectrum was reported as 1 per 272 live births.4
Placenta accreta spectrum denotes the attachment of the placenta into and through the myometrium,5 and it can result in severe complications, including hemorrhage, hysterectomy, and intensive care treatment. The increasing rate of placenta accreta spectrum parallels the increasing CD rate, which rose from 5.8% in 1970 to 31.9% in 2016.6 Multiple repeat CDs are increasing in frequency as well. At the beginning of the century, placenta accreta spectrum mainly occurred after manual removal of the placenta, uterine curettage, or endometritis. Recently, experts are in agreement that the main determinant of placenta accreta spectrum is the uterine scar and niche formation after a previous CD.5 Larger niches are associated with an increased incidence of uterine rupture or dehiscence in a subsequent pregnancy.7
In the nonpregnant state, such niches are associated with intermenstrual bleeding, pelvic pain, painful intercourse, painful menses, and subfertility, becoming increasingly more severe in women with greater numbers of CDs.8-10 Conception rate with assisted reproductive treatment is notably reduced.11
Understanding its etiology
Monteagudo and colleagues first described a “niche” in 100% of 44 women evaluated for postmenopausal bleeding who had a prior CD.12 CSN has been the subject of well over 3,000 publications over the past 30 years. While the topic generates much interest among researchers, it is garnering little traction among practicing obstetricians. Such “niches,” also referred to as isthmocele, cesarean scar defect, or a diverticulum, was first described in 196113 and later defined on ultrasonography as a hypoechoic triangular-shaped uterine defect outlined by saline instillation sonohysterogram (SIS), reflecting a discontinuation of the myometrium at the site of a previous CD.12 In 2019, a European task force further defined a CSN as an “indentation at the site in the cesarean section scar with a depth of at least 2 mm” and extended the classification to include branches as extensions toward the anterior uterine serosa.14 Using this criterion, sonographic postoperative evaluation after one CD revealed a CSN in 68.9% of women with one single-layer uterine closure and in 73.6% of women after a double-layer closure.15 Larger niche sizes with thinner residual myometrial thickness appeared more frequently when a single-layer closure technique was used, without closure of the peritoneum. Its prevalence varies from 56% to 84%.16,17
Etiology of CSN formation: Our hypotheses
The precise pathophysiology of CSN remains elusive. Speculations attributed niche formation to numerous factors: timing of surgery, cervical incision, incomplete closure of the uterine incision, adhesion formation between the CD scar and the abdominal wall, and inherent maternal conditions which may impair healing, such as smoking, obesity, diabetes, maternal age, and labor status.18-20 Retroflexion of the uterus is reportedly associated with increased incidence and size of the niche, with CSN 50% more likely to develop in women with a retroflexed versus an anteverted uterus.21 We demonstrated the origin of niche formation in real-time from the start to the completion of uterine closure by a video capture of a single-layer closure followed by an immediate SIS of the ex vivo hysterectomized uterus, and histopathologic proof of the presence of endometrial cells defining the “niche.”22 This case exposes the misalignment of the uterine wall, while including the endometrium in the closure (FIGURE 1). Similarly, pathologic studies of hysteroscopy-resected isthmocele ridges of symptomatic women with niche-related subfertility revealed the tissue edges lined by endocervical, endometrial, or isthmic mucosa either combined or isolated in the scar.23 The presence of endometrial/cervical tissue in the myometrial closure has been debated for over a century.24,25
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