Our study prospectively analyzed the etiological distribution of BUS in 2532 consecutive outpatients at a major tertiary hospital. To our knowledge, this is the largest epidemiological analysis of the etiology of BUS, aiming to offer valuable references for physicians in diagnose and treatment.
In our current investigation, it was observed that BUS predominantly affects younger individuals, with a median age of 39 years, with slightly more common in females than males (51.62% vs 48.38%). Interestingly, the incidence of lesions was notably higher on the left side compared to the right side (46.01% vs 34.24%), with a considerable number of cases exhibiting bilateral lesions. This observation could potentially be attributed to a significant proportion of patients in our study having UPJO (25.2%) as the underlying cause. Prior research has suggested that approximately two-thirds of congenital UPJO cases affect the left kidney 9, which may contribute to this pattern. However, the mechanism still needs to be explored. Furthermore, a notable percentage of patients presented with bilateral lesions (19.75%), possibly due to factors such as radiotherapy-induced injury, pelvic lipomatosis, retroperitoneal fibrosis, and bilateral UPJO, as detailed in Table S1.
BUS encompasses a group of diseases characterized by hydronephrosis resulting from various external or internal factors via different mechanisms. In our study, ureteropelvic junction obstruction (25.2%), urological procedures (17%), and radiotherapy (7.3%) are the most common causes of BUS. We classified all etiologies into four categories according to the source: congenital malformation, iatrogenic injury, acquired non-iatrogenic factors and unknown factors. Our study revealed that congenital malformations were the leading cause of BUS (44.67%), followed by iatrogenic injury (30.33%). Some previous studies have highlighted iatrogenic injury as a significant factor 2, 10, 11. For instance, a systematic review by Yang et al 2, which included 538 BUS cases with robotic ureteral reconstruction, reported that iatrogenic injury accounted for up to 40%, compared to only 8% for UPJO. The disparity in findings could be attributed to our focus on outpatients, whereas previous analyses may have concentrated on inpatients requiring ureteral reconstruction. Congenital malformations like UPJO and megaureter are sometimes identified on imaging, presenting asymptomatically and manageable without intervention 12. Moreover, potential variations in the etiological epidemiology of BUS between Chinese and Western populations warrant further investigation.
Urologic procedures, radiotherapy, and obstetric and gynecologic surgery were identified as the top three sources of iatrogenic injury in our study. Previous literature has highlighted that gynecologic surgery accounts for the highest proportion of these injuries 13. We attribute this difference to the prevalence of ureteroscopy procedures for stone therapy 14. In our research, radiotherapy emerged as the second most common cause of iatrogenic injury. This type of injury is particularly prevalent in the treatment of gynecological tumors such as cervical and endometrial cancer in elderly women 7. Existing literature indicates that the risk of radiation-induced stricture escalates over time 15. Distinguishing between the causes of injury in patients undergoing radiotherapy and gynecologic surgery can be challenging. Symptoms that manifest immediately after surgery are typically attributed to the surgical procedure, whereas those appearing post-radiotherapy or during follow-up are generally linked to the radiotherapy. In contrast to the previous literature that highlighted gunshot as the main cause of trauma-related BUS 16, traffic accidents and knife injuries are the primary causes in the study.
The impact of sex and age on the etiological distribution in patients with BUS is significant. Our study found that male patients had a higher prevalence of congenital malformations (49.8% vs 39.86%) and a lower incidence of iatrogenic injuries (25.14% vs 35.2%) compared to female patients. Congenital malformations were more common in patients < 40 years old, while iatrogenic injuries were predominant in individuals ≥ 41 years old. The difference in etiology proportions is a key factor contributing to age and sex disparities. For instance, congenital malformations like UPJO were frequently seen in young individuals, whereas iatrogenic injuries and retroperitoneal fibrosis were more prevalent in the elder adults. Pelvic lipomatosis was notably common in men 17, while endometriosis and injuries from obstetric and gynecological surgery were exclusive to women. Additionally, injuries from radiotherapy were primarily associated with the treatment of gynecological tumors in women 7.
Patients with BUS may present a challenge if they have a history of ureteral balloon dilatation or reconstruction, as previous procedures can complicate re-repair efforts. In our study, we observed that 696 patients (27.49%) had undergone surgical intervention for BUS. This high prevalence could be attributed to our center's high volume in ureteral repair and the referral of patients with recurrent restenosis. Our analysis suggests that patients with a repair history have more congenital malformations and fewer iatrogenic injuries or acquired non-iatrogenic factors compared to those without a history. We speculate that BUS recurrence due to congenital malformations may be more common, while further research is necessary to validate this hypothesis. These results underscore the importance of vigilant monitoring in the care of BUS patients, particularly those with congenital malformations.
The study had several limitations. Firstly, it is not population-based and may not represent a random or unbiased selection of cases. The single-center study had a large number of patients with a history of ureteral balloon dilatation or reconstruction, potentially introducing bias in the distribution of causes. Secondly, determining the age of onset for BUS was challenging, so we examined the age of patients at the time of diagnosis in our outpatient department. Additionally, the complex nature of ureteral stricture made it difficult to classify them according to the criteria of the international classification of diseases.