In the present study, the incidence of POUR after anorectal surgery for benign anorectal diseases was shown to be 19.05%. The independent risk factors for POUR were: female, male with BPH, postoperative VAS score > 6, PCEA and a surgery time > 30 min. We developed and validated an easy-to-use nomogram for the preoperative prediction of POUR after anorectal surgery for benign anorectal diseases, and we found that BPH had the largest weight, followed by severe postoperative pain, PCEA and surgery time > 30 min, sex had the least influence.

The incidence of POUR in this study was 19.05%, lower than reported in previous studies (22–52%) [7, 8, 10, 12, 13]. However, the etiology of POUR is multifactorial and may be influenced by sex, age, medical history, surgery-related factors, postoperative analgesia and the intraoperative infusion volume. Thus, the above factors may influence its incidence [15, 27]. In the present study, there were fewer elderly patients, less intraoperative infusion, and better postoperative pain control, all of which may have reduced the incidence of POUR after anorectal surgery for benign anorectal diseases.

Whether sex is a risk factor for POUR remains controversial [26, 28,29,30,31,32,33,34]. This diversity may be related to different types of operations. Our study showed that the incidence of POUR after anorectal surgery for benign anorectal diseases in female patients was 24.6%, which was significantly higher than for male patients (14.41%). Multivariate logistic regression analysis revealed that sex was an independent risk factor for POUR and the risk of POUR in women was 1.973 times higher than that in men. This result is consistent with a previous study [26]. The possible reason is that the urethra of female patients is short and close to the anus, so they are more likely to be irritated during a perianal operation, which can easily lead to POUR.

In addition, we found that men with BPH is an independent risk factor for POUR (OR = 4.913). BPH is clearly a risk factor for lower urinary tract dysfunction and retention [27], and the first symptom for most BPH patients is acute urinary retention [35]. A meta-analysis (3,821 patients) revealed that BPH was significantly associated with an increased risk of POUR (OR = 2.83) [31]. And in the nomogram model, BPH was found to be the most important factor affecting the occurrence of POUR.

Numerous studies have identified diabetes as a risk factor for POUR after anorectal surgery for benign anorectal diseases [26, 36]. The proposed mechanism involved autonomic neuropathy affecting the bladder's autonomic nerves, leading to sensory loss and subsequent increased residual urine or urinary retention [37]. However, we did not find diabetes to be a significant risk factor for POUR. This discrepancy might be attributed to the relatively short duration of diabetes and good glycemic control among the participants in this study. Additionally, the number of patients with a history of diabetes in the cohort was low, comprising only 11 cases (1.81%). Therefore, further large-scale studies are necessary to elucidate the relationship between diabetes and POUR after anorectal surgery for benign anorectal diseases.

Pain is perhaps the most common complication after anorectal surgery, which not only makes patients suffer, but also increases complications, delays the recovery and increases medical costs. Consisted with a previous study [15], our findings clearly showed that postoperative VAS score > 6 was an independent risk factor for POUR, suggesting that effective postoperative analgesia can reduce the incidence of POUR. Severe postoperative pain was a significant predictor of POUR, second only to BPH. Previous studies have reported that long-acting local anesthetics, high doses of a local anesthetic, continuous epidural infusion and PCEA are risk factors for POUR [15]. In the present study, we found that PCEA was an independent risk factor for POUR. Therefore, when PCEA is used in patients undergoing anorectal surgery, we can take some preventive measures to prevent POUR, to achieve satisfactory analgesic effect and reduce the risk of POUR.

Since fluid administration is usually constant during an operation, longer procedures tend to require a greater intraoperative infusion volume. However, it remains controversial whether the increased incidence of POUR caused by a prolonged surgery time is related to the increased volume of intraoperative infusion [27, 38]. In our study, there was a significant difference in surgery time between the POUR and non-POUR groups, but there was no difference in the intraoperative infusion volume between the two groups, possibly because a preventive strategy was adopted in our study. The intraoperative infusion volume mostly ranged from 200 to 400 mL, and no correlation between injection volume and POUR was found. The findings suggested that the increase in the incidence of POUR caused by prolonged surgery time may be related to the type of surgery and the intraoperative operation procedures. We found that more than 3 incisions and the types of surgery increased the incidence of POUR. A previous study reported that the number of hemorrhoids > 3 and the severity of hemorrhoids > 4 are independent risk factor for POUR [30].

With the development of comfort medicine and ERAS, more and more patients are used CEB + IA for anorectal surgery in China. Compared with GA, CEB + IA has the advantages of less effects on general physiological functions, better postoperative analgesia and faster recovery. In addition, ultrasound-guided caudal epidural puncture greatly improves the success rate, which is favored by many anesthesiologists. Many previous studies have suggested that spinal anesthesia leads to a higher incidence of POUR than general anesthesia [34, 39]. However, some studies suggested the opposite [15, 40]. In the present study, there was no correlation between the types of anesthesia and POUR, but further studies are warranted.

To the best of our knowledge, we have developed the first predictive model for POUR after anorectal surgery for benign anorectal diseases. We evaluated the nomogram by C-index, a calibration plot and DCA, and found that it was a good evaluation model for POUR. Using the nomogram, we found that BPH had the largest weight for the occurrence of POUR after anorectal surgery for benign anorectal diseases, followed by severe postoperative pain, PCEA and surgery time > 30 min, sex had the least influence. Therefore, we recommend that physicians and anesthesiologists develop individualized treatment plans to prevent POUR in the patients who are male with BPH, have severe postoperative pain and/or PCEA. It can promote ERAS to some extent and improve patient satisfaction.

There are a number of limitations to our study. First, we only collected the patient's preoperative history of BPH, but no formal urinary system examination was conducted, so we could not evaluate the degree of preoperative urethral obstruction. Second, the study was a single center nested case–control study, and no preventive measures for POUR of anorectal surgery for benign anorectal diseases have been investigated. Third, we did not perform postoperative bladder ultrasound. Therefore, further prospective randomized controlled studies are needed to find measures to prevent and manage POUR after anorectal surgery for benign anorectal diseases.