Congenital UPJO commonly arises from an aperistaltic ureter segment, which hinders the formation of efficient peristaltic waves. This abnormality leads to the replacement of the spiral musculature with longitudinal muscle or fibrous tissue in that specific area. These causes are thought to result from inadequate recanalization in utero at 10 to 12 weeks of gestation. Additionally, conditions such as abnormal secretion of compounds like transforming growth factor-β, epidermal growth factor, cytokines, nitric oxide, and neuropeptide Y have been identified to be critical in developing UPJO [3]. The significance of interstitial Cajal cells in UPJO remains controversial. Studies examining the distributional variance of Cajal cells in the obstructed and unobstructed ureteropelvic junction have yielded conflicting results across various publications [4].

The involvement of crossing vessels in the development of the UPJO remains a contentious issue. Although these vessels are found in approximately 20% of the general population and in up to 38%–71% of UPJO cases [5,6] the exact relationship between crossing vessels and UPJO remains uncertain. Some cases suggest that crossing vessels pose a purely mechanical obstacle owing to the absence of histopathological changes [7,8]. In contrast, in other instances, crossing vessels trigger inflammation, fibrosis, and smooth muscle hypertrophy at the ureteropelvic junction, leading to obstruction. Not all UPJO cases are linked to silent crossing vessels. Instead, they may result from different tissue pathologies. Other extrinsic causes include congenital kidney abnormalities such as horseshoe kidneys, duplex kidneys, and fibroepithelial polyps [9,10].

POM is characterized by functional obstruction due to an aperistaltic segment near the bladder, hindering normal urine flow. Histological studies revealed elevated collagen levels (predominantly type I) in this segment, contributing to the disruption of communication between cells, ureteroarrhythmias, and obstruction [11-14]. Other theories propose that factors such as muscle atrophy of the inner longitudinal layer, hypertrophy of the outer layer, and compressive circular muscles lead to obstruction [15,16]. Additionally, histologic evidence points to different structural abnormalities like fibrotic terminal ends or excessive muscle responsiveness causing contraction [17,18]. Another hypothesis proposes that obstruction reflects the developmental progression of the distal ureter from a single circular muscle layer to the child's dual-layer (circular and longitudinal) structure [19,20]. The dilated proximal ureteral segment is observed to contain altered connective tissue, contributing to fibrosis, dilation, and potential ureteroarrhythmias with impaired peristaltic wave transmission [19-21].

Prenatal US allows earlier detection of the megaureter, with many cases remaining symptom-free and resolving without surgery [27]. Spontaneous resolution typically occurs within the first 2 years of life but can extend to young adulthood [27,28]. In literature, spontaneous resolution rates vary between 34% and 88% [29-31], with approximately 24% of megaureter cases requiring surgery, especially those with larger ureteral diameters [27]. Factors like baseline ureteral dilation, washout pattern, and timing of presentation can influence the need for intervention or potential resolution, emphasizing the importance of long-term ultrasound monitoring until puberty [32].

Endoscopic endopyelotomy, introduced by Ramsay et al. in 1984 [38], offers benefits such as shorter hospital stays and quicker recovery. Endopyelotomy is based on the principle of Davis intubated ureterotomy, where a full-thickness incision through a strictured segment of the ureter heals over a stent to a larger caliber within weeks [39]. This procedure involves a full-thickness incision on the lateral side to avoid crossing the vessel on the ureteral lumen using a cold or hot knife or holmium lasers, followed by ureteral stenting. However, the success rate of endopyelotomy is reportedly worse than that of open, laparoscopic, or robotic pyeloplasty. The available studies on antegrade endopyelotomy have shown success rates of 65% to 93% [40]. Therefore, to increase the success rate of the procedure, careful patient selection is warranted, and patients with a less severe grade of hydronephrosis, better renal function, and short stricture length may be good candidates for this procedure [41]. In the pediatric population, the median success rate was 71% (46%–100%) in the primary group and 75% (25%–100%) in the secondary group which failed in primary procedures, while complications were reported in 14% of both primary and secondary procedures [42]. Considering these results, careful patient selection is important for successful procedures, and endopyelotomy can be considered a secondary rather than a primary operation.

Pyeloplasty is considered the gold standard for UPJO treatment and can be divided into dismembered and flap procedures. Dismembered pyeloplasty, also referred to as the Anderson-Hynes operation, is a versatile surgical technique characterized by complete disconnection of the ureter and removal of the affected segment. This procedure allows for the correction of issues such as redundant pelvis and transposition of the UPJ in cases where crossing vessels obstruct urinary flow. Flap procedures such as Foley Y-V plasty present benefits such as reduced operative time and decreased risk of devascularization of the UPJ, making them suitable for addressing long ureteral strictures. Comparative studies investigating surgeries, such as Anderson-Hynes and Y-V plasty, have revealed variations in success rates and ease of surgery [43,44]. These findings underscore the importance of tailoring the choice of procedure to the specific characteristics of each case, ensuring the best possible outcomes for patients undergoing pyeloplasty.

Laparoscopic treatment of UPJO, pioneered in 1993 by Schuessler et al. [45] and Kavoussi and Peters [46], has become the standard for UPJO management because of its superior outcomes compared to endourologic techniques. Laparoscopic pyeloplasty (LP) offers excellent success rates ranging from 85% to 100% and is suitable for various patient groups, including infants and older individuals [47,48]. Although LP requires advanced skills for intracorporeal suturing, its benefits include reduced pain, decreased blood loss, and improved aesthetics. Due to the larger working space and more familiar anatomy, retrocolic and transmesenteric transperitoneal routes are more frequently chosen. However, the choice between transperitoneal, anterior extraperitoneal, or retroperitoneal approaches can be made by considering factors such as operative time and anatomy familiarity. The success rates of LP through the retroperitoneal and transperitoneal approaches were not significantly different; however, the operative time for the retroperitoneal approach was longer. Novel approaches, such as mini-laparoscopy and one-trocar-assisted pyeloplasty, provide safe and effective alternatives to traditional LP [49,50].

Robotic systems enhance laparoscopy by using a three-dimensional magnified view, instrument mobility, and tremor elimination. However, they lack tactile feedback, have higher costs, and require longer procedure times than traditional laparoscopy, offering no significant clinical advantage to experienced laparoscopic surgeons [51]. Despite this, robotic pyeloplasty is likely to be favored when technology is available. Recent meta-analyses comparing robotic, laparoscopic, and open pyeloplasty in the pediatric population have shown that robotic pyeloplasty reduces analgesia requirements, estimated blood loss, and hospital stay compared with LP, with similar success and complication rates [52]. Recently, single-port robot-assisted LP using the da Vinci SP system has been introduced, showing a success rate similar to that of conventional robotic systems with fewer incision sites, and increasing application is expected [53].