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Anatomically Uterine niche is an iatrogenic pouch-like defect at the site of previous caesarean scar due to defective tissue healing. Radiologically niche should be defined as an indentation at the site of the CS scar with a depth of at least 2 mm. A niche can be subclassified in: (1) simple niche; (2) simple niche with one branch; (3) complex niche (with more than one branch). (1-3) Uterine niche occurs in up to 70% women with previous cesarean of whom 30% are symptomatic. Reported prevalence varies: 24-70% with transvaginal sonography (TVS) and 56-84% with gel/saline instillation sonohysterography (SHG).This may be an underestimation because many women are asymptomatic and also because clinicians may not recognize niche as a cause of symptoms due to unawareness. Prevalence of 45.6% was reported in a prospective observational study (n = 371) where sonohysterography was done six months post-cesarean. Prevalence increases with increasing number of previous cesareans. (1-5) Potential Risk Factors Niche forms due to poor healing of cesarean scar. Risk factors are: (5-8)
Factors Affecting Lower Uterine Segment:
Cervical dilatation of > 5 cm, > 5 h duration of labour and advanced fetal station predispose to large niche due to thinner or less vascularized myometrium resulting in inadequate healing(5,6)
Level of Uterine Incision Lower uterine incision towards the cervix results in poor healing, as mucus secreted by cervical glands interferes with myometrial approximation. Mucus accumulation gradually increases the niche size also (5,7) Cesarean section done in advanced labour after cervical effacement and also creation of uterovesical fold of peritoneum influence the level of uterine incision.
Uterine Closure Techniques Single-layer, decidua sparing closure technique predisposes to incomplete closure, compared to single full-thickness closure. A strong myometrial scar with proper anatomical approximation without tissue strangulation minimizes risk of niches (1,8) If muscular edges are thick, they are best approximated by including deeper part in the first layer and the remaining superficial cut edges in the second layer.
Non-perpendicular sutures leading to an irregular myometrial closure, locking sutures or very tight second layer leading to ischemic necrosis result in poorly healed scar predisposing to niche formation.
Thus, double-layer uterine closure using non-locking sutures is the optimal closure technique that results in thicker residual myometrium and hence potentially lower risk of niches.
Suboptimal surgical techniques: Inadequate haemostasis, tissue ischemia, devascularization and excessive tissue manipulation contribute to poor scar healing and adhesions, consequently forming niche.
Adhesions Adhesion formation with abdominal wall pulls the uterine scar towards abdominal wall, exerting counteracting force opposite to the direction of retracting uterine scar tissue and causing impaired wound healing. This mechanism is encountered in non closure of peritoneum and creation of bladder flap that is not sutured. (7)
Retroflexed Uterus Effect of gravity on uterine corpus also increases counteracting forces. Large niches are mostly found with retroflexed uterus. (6,7)
Patient Factors Genetic predisposition contributes to impaired healing, inflammation, or adhesion formation, post-operative infection. (7) Gestational diabetes (odds ratio, 1.73), previous caesarean (OR, 3.14) and advanced body mass index (OR, 1.06) are independent risk factors. Risk increases by 6% for every additional unit increase in body mass index. (8)
Diagnosis:
Niche can be visualized in non-pregnant state using TVS, SHG, 3-D ultrasound, magnetic resonance imaging or hysteroscopy. An anechoic space at least 2 mm deep in the myometrium at caesarean scar site clinches the diagnosis. Niche Size and Residual Myometrium thickness are measured.
Residual myometrial thickness (RMT) is the vertical distance between uterine serosa and apex of defect. Large niches are defined when RMT is < 50% of adjacent myometrium or ≤ 2.2 mm on TVS. Absent residual myometrium is termed a total defect. (9,10)
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