MRI is a non-invasive method for the assessment of CS scar niche, scar volume, depth, and width represent good predictors of the development of post menstrual spotting.

Multiple previous studies reported that there is a relationship between CS scar niche and multiple gynecological symptoms [7, 10, 12,13,14,15]. The most common complaints related to CSD are prolonged menstrual bleeding and postmenstrual spotting (in up to three-quarters of women with CSD), followed by pelvic pain (39.6%), dysmenorrhea (53.1%), dyspareunia (18.3%), and secondary infertility [10]. The retention of the blood products inside the defect and poor contractility of the uterine wall related to decreased myometrial thickness and fibrosis explained the relation between cesarean scar defects and postmenstrual spotting [7].

MRI can easily define CSD and can also be reviewed retrospectively. Tang et al., has compared the use of TVUS with MRI and concluded that MRI is better than TVUS for the measurement of CSD which may help to improve the therapeutic strategy for CSD. Measurements by MRI showed a better prediction of the clinical symptoms of CSD, and more reflective of the severity of clinical manifestations [10]. However, MRI was not commonly used for CSD imaging due to its relatively high cost [14].

Multiple previous studies have postulated that there is a relation between CSD volume, residual adjacent myometrial thickness, and development of postmenstrual spotting based on the transvaginal ultrasound as a diagnostic method [7, 10, 16,17,18], however, a limited number of studies utilized MRI as a method for the evaluation of the CS scar defects [10, 14, 15].

Bij de Vate et al., reported that the semicircular scar defect shape is the most prevalent [7]. In this study, the most prevalent shape was droplet followed by semicircular defect shape in 43% and 40% of patients respectively. However, post menstrual spotting was significantly related to semicircular scar defect shape than other shapes which may be related to the higher volume of the defect in semicircular scar shape.

Previous studies concluded that residual myometrial thickness (RMT) at the cesarean section scar is one of the major parameters correlated with menstrual bleeding. They defined large niches as those with a residual myometrium thickness of < 50% of that of the adjacent myometrium. Reduced myometrial thickness in combination with lower contractility because of fibrosis would induce the development of postmenstrual spotting. Also, these publications reported required residual myometrium of 2–3 mm for hysteroscopic niche resection, given the risk of perforation and/or bladder injury [7, 10, 14, 18]. In the present study, women with post-menstrual spotting had significantly lower RMT than those without post-menstrual spotting. However, RMT was insignificantly correlated with the duration of postmenstrual spotting.

He et al., used posterior wall thickness at the same level of scar center to represent anterior wall thickness before the prior cesarean section because the normal anterior and posterior uterine wall thickness is similar to one another in the normal woman [19]. In the current study, the relative reduction in anterior wall myometrial thickness at the scar area in comparison to the posterior uterine wall was significantly higher in women with post-menstrual spotting with an average reduction of 79% in comparison to 57% without postmenstrual spotting.

Additionally, several previous studies concluded that postmenstrual spotting is related to niche volume [7, 10, 14]. The current work was in agreement with them regarding this point, also CSD volume (> 0.15 cm3) is the most important predictor for the development of postmenstrual spotting. with 97% sensitivity and about 100% specificity and 98.4% overall accuracy. In this study, scar depth and width represent other predictors for the development of postmenstrual spotting. These three parameters (depth, width, and volume) have a significant relationship with the duration of postmenstrual spotting as the increase in the defect depth, width and volume were accompanied by an increase in the duration of the postmenstrual spotting.

Limitations of this study included the cross-sectional nature of the study that jeopardize the external validity of the study and the possibility of selection bias in the control group i.e., women without postmenstrual bleeding as they are not selected from the normal population but women attending the gynecological outpatient clinic.

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