Congenital middle ureteral stenosis is a rare cause of hydroureteronephrosis. To date, there have been few studies in the literature on the incidence of ureteral stenosis. Campell reported that in an autopsy of a sequence of 12080 children, 72 children had ureteral stenosis, 34% at the UVJ, 62% at the UPJ, and only 4% (3 children) had middle ureteral stenosis1. We reviewed a total of 1625 children who underwent surgical treatment due to hydroureter in the past 13 years, and 26 of them had congenital midureteral stenosis, accounting for approximately 1.6%, which was lower than that reported in the literature. However, our statistical data were based on actual clinical data and were more representative. The pathogenesis of this condition is unclear, and many theories attribute it to abnormal embryonic development, including abnormal fetal vessel compression, intrauterine inflammation, incomplete ureteral recanalization, ischemia due to abnormal branches of blood vessels and localized developmental arrest1–5. A review of the previous literature revealed that in children, congenital midureteral stenosis is often associated with urological anomalies, such as contralateral renal agenesis or atrophy, VUR, UPJO, crossed renal ectopia, solitary kidney and contralateral blind-ending ureter6–8, suggesting the possibility of bilateral aberrant renal and ureteral development and indicating that ureteral stenosis may be a mild manifestation of unilateral hypoplasia. In this study, there was one case of contralateral MCDK along with systemic dysplasia and two cases of contralateral renal axis malrotation.
Pathologically, ureteral stenosis is a mechanical obstruction due to structural abnormalities in the wall, which are distinct from the neurogenic and myogenic mechanisms of UPJO. There are two etiologies of ureteral stenosis: ureteral valves and true ureteral stenosis. Ureteral valves are anatomically demonstrable transverse folds of ureteral mucosa, potentially containing bundles of smooth muscle fiber, covered with normal urothelium9. In this study, no ureteral valves were found in any of our children after careful observation of pathological specimens. Studies on the ultrastructure of ureteral stenosis found that the stenotic ureter did not deviate fundamentally from its pattern, only quantitative changes in its composition were observed. These changes included lumen shrinkage and relative or absolute loss of smooth muscle with a normal, altered or disorganized arrangement, with or without connective tissue changes1–8. Our pathological specimens exhibited smooth muscle hyperplasia and fibrous tissue degeneration. A low level of chronic inflammatory cell infiltration was observed in the narrow segment, which is consistent with findings reported in the literature.
Midureteral stenosis tends to occur at the level of the bifurcation of the common iliac vessels, which is also a site of physiological ureteral narrowing. In our study, stenosis in this region accounted for 88% of cases. We found that 96% of the stenotic segments were within 1 cm in length and that upper ureteral stenosis was more common in patients with multiple stenotic sites and was rarely isolated. In addition, ureters with multiple ureteral stenotic sites were generally poorly developed and small in appearance. Previous literature has demonstrated that at sites of stenosis, the ureter lumen usually shrinks by approximately 60% and significantly impedes urine delivery6. However, clinically, we found that compared with UPJO, children with midureteral stenosis tend to have relatively mild hydronephrosis and a later onset of symptoms. In this study, 12 children were over 5 years of age when they developed symptoms. There have even been reports of midureteral stenosis diagnosed at 15 and 20 years of age10–11. The upper ureteral dilation in some children was not very severe, possibly due to the absorption of ureteral lymphoid tissue and the buffering effect of the ureter on the urine during low stenosis. Murnaghan speculated that the narrow segment could transmit peristaltic waves when the urine flow was low, allowing smooth passage, and that when the urine flow load exceeded a critical value, the narrow segment decompensated, resulting in clinical symptoms12.
Midureteral stenosis results in dilatation of the ureter above the stenotic site, usually with dilatation of the renal pelvis. The ureter also has a tendency to stretch, curl and droop in a manner clinically similar to that of UVJ malformation. However, these two diseases are very different. Midureteral stenosis consists of an obstruction at a certain distance from the bladder, while in intramural ureteral lesions, the ureter is usually dilated next to the bladder. Previous studies have shown that the preoperative diagnosis of midureteral stenosis is relatively difficult due to its symptomatic and radiographic similarity to intrinsic UVJO or UPJO. Most cases are diagnosed intraoperatively or by retrograde pyelography, and the rate of diagnosis on preoperative ultrasound is low. Many scholars consider ultrasound to be of limited value in distal ureteral obstruction localization. Hawang et al believed that children with hydronephrosis and megaureter should routinely undergo retrograde pyelography unless the distal ureter is well demonstrated on other tests1. It has also been recommended to routinely perform MRU in children with ureteral dilatation because MRU provides excellent anatomic and functional details of the collection system that allow the accurate diagnosis and treatment of ureteral stenosis13. However, routine retrograde pyelography in children with congenital hydronephrosis is controversial. Ruston pointed out that retrograde pyeloplasty or contrast visualization of the ureter before pyelography is rarely necessary in children14. Prior to this, Cockrell et al reported that the secondary abnormalities found by retrograde pyelography were in the surgical field where they would be detected and managed in the normal course of events14. MRU cannot be used to assess the condition of the distal ureter very well, and the examination is expensive; additionally, children need sedation for the examination. Thus, neither examination is recommended for routine use in children.
With the development of ultrasound techniques for ureteral examination and improvements in the diagnostic rate, ultrasound can serve as a noninvasive and repeatable effective method for the diagnosis of ureteral stenosis or stricture. Ultrasound can be used to determine the location of a ureteral obstruction according to the degree of ureteral dilation and morphological changes, as well as to determine the etiology and extent of stenosis according to differences in the acoustic images of the stenosis site. In our center, the diagnosis of ureteral stenosis mainly relies on ultrasound, IVU and CTU, while MRU and retrograde pyelography are performed irregularly. The accuracy of the ultrasound examination is highly dependent on the personal experience of the examiner. Notably, our rate of preoperative midureteral stenosis diagnosis is higher than that reported in the literature, with the diagnostic rate of ultrasound reaching 96%. As early as 10 years ago, there were related reports in the domestic literature demonstrating a rate of coincident lesion location diagnosis and quantitative ureteral stricture diagnosis by ultrasound of 96.6% and 91.5%, respectively16. There are some differences in the operation of Chinese and Western ultrasound examinations. The ultrasound examination and diagnosis at our center were completed by a sonographer with certain clinical experience after real-time examination. Unlike in Western countries, where ultrasound technicians operate, take pictures and keep files first, and then the sonographer writes the diagnosis report. Our examination is a dynamic examination. During the process, if the ureter is not clear, the child can be allowed to drink a large amount of water. In the state of holding urine, the ureter can be repeatedly observed across the iliac blood vessels; then, after letting the child urinate, another examination can be performed and compared with the first examination. Due to the sound window of the kidney and bladder, the UPJ and the terminal ureter adjacent to the bladder were well displayed; however, the ureter in the middle segment was deep, and in many cases, the ureter did not dilate behind the bladder. Therefore, some scholars believed that ultrasound could easily miss the diagnosis or misdiagnose the dilated ureter as the intestinal loop. To prevent this, we prepared the bowel sufficiently to reduce interference with intestinal contents and gases. We believe that the false-negative rate is due to limited experience; an experienced sonographer can diagnose most cases of midureteral stenosis.
IVU is a common and accurate visual examination for the diagnosis of ureteral stenosis. If children have good renal function, IVU can often clearly show the degree of ureteral hydronephrosis and the shape and location of ureteral stenosis, and the accuracy of ureteral stenosis localization is high. Long-term obstruction causes poor renal function, which needs to be extended to approximately 1 hour to improve the positive rate of diagnosis. Therefore we do not recommended preoperative MRU or retrograde pyelography in children with suspected midureteral stenosis but instead propose ultrasound and IVU for routine preoperative evaluation. When ureteral dilatation is not obvious, the distal ureter is not clearly displayed, or the diagnosis is not clear, such as in suspected cases of multiple stenotic sites, we will perform MRU, CTU or retrograde pyelography to confirm the diagnosis.
Compared with the diagnosis of midureteral stenosis, the treatment of this condition is not difficult. The surgical method is greatly affected by the location and number of stenotic segments. The majority of single stenotic segments are short, so resection of the stenotic segment and ureteroureterostomy are feasible. For upper ureteral stenosis combined with UPJO, pyeloplasty can be performed directly. In two children with multiple ureteral stenoses, because the stenosis was relatively dispersed and the distance was longer, we adopted ileal ureteral replacement. Both of these children had left ureteral defects, the operation of left ureteral replacement with the appendix was difficult, and there were more postoperative complications. We did not use the appendix for left ureteral replacement. Laparoscopic and robotic technology has been successfully used in the treatment of ureteral stenosis; however, due to the long time span of this study, we mainly used open surgery in the early stage and only recently started to use laparoscopic surgery to treat ureteral stenosis, with a small number of cases. In the future, more children will undergo treatment with laparoscopic and robotic surgery. The prognosis of these patients was very good; the symptoms resolved on long-term follow-up, ultrasound examination suggested that hydronephrosis was relieved or resolved, the urinary tract was unobstructed, and IVU examination showed no obstruction.