**2.1 Methods**

Several methods exist to diagnose a niche: magnetic resonance imaging (MRI), hysteroscopy, two- or three-dimensional TVUS and SHG with the aid of saline or gel infusion.

By MRI, the thickness of the remaining myometrium can be measured quite exactly using T2 weighted views. As an advantage, the evaluation does not depend on the examiner's experience and consequently the measurements of RMT by MRI are more objective compared to those assessed by ultrasound [8]. On the other hand, MRI examinations are costly and not always available.

Hysteroscopy can simultaneously be used for diagnosis as well as for therapeutical interventions. As long as no mucus or residual blood obstruct the view, the potential defect can be visualized and treated in the same session. However, it is not possible to determine the RMT from inside the uterine cavity [9].

Ultrasound is widely available, harmless and rather unexpensive. However, diagnostic quality strongly depends on the examiner's experience. The exploration of a niche and the RMT can be facilitated by the use of saline or gel infusion enhancing the contrast. Several studies comparing ultrasound methods found that the use of contrast SHG is more sensitive and specific than simple TVUS for the identification of a niche [10]. In a prospective study of 371 patients, half of all myometrial defects remained undiagnosed with TVUS but could be seen with SHG, resulting in a higher prevalence of a niche using SHG (45,6% vs. 22,4%) [7] (**Figure 5**). A systematic review including 21 studies found a niche prevalence in up to 84% of women with a history of CS by using SHG for diagnosis [1]. SHG was considered as comparable to hysteroscopy in diagnosing defects of the myometrium with a sensitivity of 87%, specificity of 100%, a positive predictive value of 100%, a negative predictive value of 95% and an overall accuracy of 96% [1]. Because of the clear advantages of SHG, currently most authors consider it as a gold standard for diagnosing niches. If not available, simple TVUS with intrauterine fluid (e.g. blood during menstruation) to enhance the contrast [2, 7] constitutes a possible alternative.

**Figure 5.** *Diagnosis of a niche without (left) and with (right) contrast enhanced sonohysterography.*

#### **2.2 Timing**

For most exact results in measurement, optimal timing of the examination is necessary. This includes the day in the menstruation cycle as well as the interval to previous CS. Most authors agree that the best time for ultrasound is either during menstruation or in early follicular phase shortly after menstruation [2, 6]. The accumulation of blood in the isthmocele together with a thin endometrium facilitate the detection of the defect. Other authors prefer the time proximate to ovulation, when cervical mucus may distend a niche located within or near the cervix [11]. Using the follicular phase of menstrual cycle also is suitable to exclude the presence of an early pregnancy.

Unfortunately, data is limited concerning the best time interval to previous CS. The suggested time ranges from three to six months [2, 7]. Due to the fact that proper wound healing after CS takes up to six months [7], at least half a year interval before measuring the myometrial thickness is recommended.

#### **2.3 Procedure**

The assessment of the myometrium in search of a niche requires to follow a systematic workflow. A full bladder helps visualizing the lower uterine segment (LUS) but is not obligatory for the examination [2]. Variation in pressure with the probe is useful for an optimal image. Using doppler imaging, the differentiation between niche and myometrial vessels is possible. In 2019, Jordans et al. established a modified Delphi procedure for the sonographic examination of niches [2].

Briefly, the examination starts in the midsagittal plane including a proper visualization of the cervical canal. By panning the probe laterally left and right, the niche should become visible. Once the defect is identified measurements of length and depth are made. If the niche is complex with branches, both length and depth should be measured for every branch. Furthermore, the distance to the vesico-vaginal fold (VV) should be measured placing the caliper at the level where the residual myometrium is thinnest (**Figure 6**). Exactly at the same point measurement of the RMT is done. Next, adjacent myometrial thickness (AMT) is measured close to the niche. The last measurement in the midsagittal plane is the distance to the external os parallel to the cervical canal starting at the distal point of the niche (**Figure 7**).

Afterwards the niche is visualized in transverse plane by rotating the probe by 90 degree. Here, the niche's width can be measured. The transverse plane works best for detecting branches in case of complex niches.

Nevertheless, standardized cut off parameters for niche characterization, especially in the case of large niches, are not yet established. The following parameters

**Figure 7.** *Technique of niche measurement following the Delphi procedure [2].*

seem to be suitable for clinical practice: the ratios RMT/AMT, depth/RMT, depth/ AMT and RMT [1, 2, 7].

A further issue that needs to be considered in future studies is the clinical relevance of the size of the niche. To date, no study has clearly shown a direct correlation between the scar's thickness and the risk of e.g. uterine rupture [3, 5, 12, 13]. Therefore, further studies are needed to evaluate the association between a niche's size and clinical sequelae.
