Mixed urinary incontinence (MUI) is defined as a disease co-occurring with both urge urinary incontinence (UUI), in which involuntary urine leakage takes place as the result of urgency, and stress urinary incontinence (SUI), in which involuntary urine leakage takes place as abdominal pressure increases as the result of coughing or sneezing [1]. As women age, the prevalence rate of MUI or UUI gets higher, and MUI accounts for 30% to 50% of all incontinence [2,3]. The midurethral sling has been a main method in the treatment of SUI since tension-free vaginal tape (TVT) surgery was first introduced as a surgical treatment for that type of incontinence owing to its simpler and less invasive process. The transobturator tape (TOT) procedure, which was introduced to supplement defects in the TVT operation, has been widely used as well [4,5]. It is often reported that the TOT procedure has a high rate of success for SUI [4]. Although the success rate of operation for MUI varies according to the report, it is mostly reported as being lower than the success rate of surgery for SUI [6]. Also, there have been suggestions that surgical treatment for patients with MUI is not appropriate because of the possibility that UUI will continue despite surgery and that conservative treatment should therefore be the primary treatment for mixed incontinence [7].

Interestingly, the mechanism through which the sling procedure improves urge incontinence is not fully known, but many studies have reported that the urgency symptoms of patients with MUI improve when the sling procedure is performed. Koonings et al suggested that the mechanism through which urinary urgency takes place in MUI is stimulation by urine leakage into the proximal urethra by increasing abdominal pressure [8]. Those authors suggested that urgency and UUI could be improved at the same time if urine leakage is stopped through surgery [8]. In addition, Rezapour et al suggested that surgical treatment can be applied even to MUI, while reporting that the success rate for MUI is up to 85% after the TVT operation [9]. Compared to SUI, however, the success rate of surgical treatment for MUI is not high enough to be satisfactory. There may be some difference in interpreting the result of operations, but almost all studies show that unlike the high success rate of operations for SUI, the success rate of operations for MUI and patient satisfaction are rather low and UUI continues after surgical treatment [10].

Because any kind of incontinence after surgery reduces patient satisfaction, persistent UUI after the sling procedure in MUI is the main cause for reducing the success rate of surgery for MUI and patient satisfaction in general [11]. Therefore, to identify risk factors for surgical failure in patients with MUI and to increase patient satisfaction after the operation, it may be clinically useful to find risk factors related to persistent postoperative UUI. Thus, we evaluated postoperative variables as well as the clinical characteristics of MUI patients who underwent the TOT procedure in an attempt to determine whether clinical factors had an effect on persistent UUI after the TOT procedure.

This study included 175 women with MUI who underwent TOT procedures. The preoperative parameters of the 175 women are summarized in Table 1. The women's mean age was 55.4±9.8 years; 139 patients (79.4%) were under 65 years old, and 36 patients (20.6%) were over 65 years old. The mean follow-up time was 15.9±0.4 months. The mean parity was 2.7±1.3, and the mean symptom duration was 4.86±4.15 years. Postoperative maximal flow rate (Qmax) and PVR were 27.1±2.0 ml/s and 30.4±15.8 ml, respectively. Those variables had not significantly changed in relation to the preoperative Qmax (28.2±1.9 ml/s) and PVR (26.5±13.4 ml). Postoperative urine retention occurred in 9 patients (5.1%), but all patients recovered within 3 days. Persistent SUI after the TOT procedure occurred in 16 patients (9.1%), and persistent UUI after the TOT procedure occurred in 51 patients (29.1%). In addition, absence of persistent UUI after the TOT procedure occurred in 124 patients (70.9%), and among the 124 patients, persistent urgency after the TOT procedure occurred in 25 patients (14.3%).

To determine the possible factors influencing the persistence of UUI after the TOT procedure, logistic regression analysis was performed. In the univariate analysis, old age and previous anticholinergics use were associated with persistence of UUI after the TOT procedure. Those two significant variables were analyzed in the multivariate model to determine the independent risk factors for the persistence of UUI. In the multivariate model, only old age was a significant risk factor for the persistence of UUI, and elderly age (over 65 years old) was associated with a 3.3-fold risk for persistence of UUI after the TOT procedure compared with middle age (under 65 years old) (Table 2).

As shown in Table 2, there was no association between the presence of uninhibited detrusor contractions and persistent UUI after the TOT procedure. We conducted statistical analysis to verify the factors that affected persistent UUI after the TOT procedure in 49 patients who showed uninhibited detrusor contraction in the UDS, but found no significant factors.

To date, data on factors predicting the persistence of urge incontinence or failure after the sling procedure in patients with MUI have been limited, and there are no consistent predictors of persistent UUI after the sling procedure in MUI. Segal et al reported that they found no factor to be related to surgical failure after the TVT operation [14]. Paick et al suggested that a low MUCP may be associated with persistent UUI after the TVT procedure in patients with MUI [15]. Jeong et al suggested that maximal cystometric capacity and involuntary detrusor contraction were important predictors of surgical failure in MUI [16]. Another study reported that women with stress-predominant MUI and without detrusor overactivity (DO) or with low pressure DO were more likely to be cured of their UUI after a midurethral sling procedure [5].

In comparison with studies related to the TVT procedure, detailed outcomes and clinical factors predicting the persistence of UUI after the TOT procedure have yet to be sufficiently reported. Furthermore, there is no significant difference in the surgical results between TVT and TOT, but because there is some difference in postoperative voiding symptoms and complications between the two procedures [17], there remains a possibility that factors related to UUI lasting continuously after both TOT and TVT procedures do not correspond to each other. As a result, in this study we attempted to determine whether the clinical characteristics and preoperative examination results of MUI patients can predict the risk of persistence of UUI after the TOT procedure. Unlike previous studies, we found that old age is a risk factor for persistent postoperative UUI.

In other studies, factors related to the UDS were mainly suggested as risk factors for persistent UUI after surgery. Because some cases have reported that symptoms of MUI patients are not really reproduced as they are in UDS, or that DO appears in the UDS but UUI symptoms do not appear [18], there is a possibility that the accuracy of finding a risk group through factors related to the urodynamic study may be low. On the other hand, if risk factors are assumed on the basis of age as suggested by this study, it is possible to explain the chance of surgical failure by selecting patients who are at risk of UUI easily and accurately without an invasive test like UDS, which will be clinically easier and more useful.

The following explains why the persistence of UUI after operation appears high for elderly women with MUI. First, older women often experience an outlet obstruction effect [19]. As a result, voiding problems, like incomplete emptying and overactive bladder symptoms, lead to DO, further leading to a possibility that UUI occurs more often [20,21]. Second, elderly women are more likely to suffer from severe intrinsic sphincter deficiency [22,23]. Such patients may not experience urgency because of severe stress incontinence. However, after a TOT operation, their urethral resistance can be reestablished, and their masked UUI is more likely to reoccur. Third, it is said that out of patients with MUI, those with urge-predominant MUI are more likely to have a surgical failure than are those with stress-predominant MUI [24,25]. This study does not clarify which factor is more serious, stress or urge, but the younger the patients are, the more likely they are to have SUI, and the older the patients are, the higher the prevalence rate of MUI or UUI [2,26,27]. The failure rate of the operation will appear higher for patients of old ages, on the premise that there are lots of older patients with urge-predominant MUI. Fourth, because old women tend to have shorter functional urethras, smaller urethral mobility, and larger urethral rigidity, such characteristics can influence the effect of the tape after the operation and are more likely to worsen the surgical result, although it is not clear how the mechanism works [28].

We suggest that old age can function as a factor useful enough to predict the result of treatment for MUI patients who want the TOT procedure. Furthermore, explaining the possibility of persistent UUI to elderly women ahead of surgery will lower their expectation for a successful operation, which can increase the patients' overall satisfaction. In addition, the possibility of postoperative complications with anesthesia is found to be higher in older patients [29,30]. It is essential to carefully choose the surgical treatment by comprehensively considering all the possibilities of surgical failures and nonsurgical complications if the patient is an elderly woman with MUI who has severe or multiple medical problems.

A limitation of this study is that the follow-up period was as short as only 6 months, and the study was not really prospective. To confirm our findings, we need to do additional research, including a large population and long-term follow-up. In addition, more studies on the effect mechanism of old age should be conducted.

Our results suggest that elderly women (over 65 years old) have a higher possibility of persistence of UUI after the TOT procedure than do middle-aged women (under 65 years old). Therefore, elderly women with MUI should be sufficiently informed of the possibility of persistent UUI after the TOT procedure, which will lead to an increase in patient satisfaction after surgery.