Co-existent overactive-underactive bladder (COUB) has been proposed as a term for the combination of overactive bladder (OAB) and underactive bladder (UAB) [1]. There are few studies on the epidemiology of COUB because there is no established definition of UAB. On the other hand, an EPIC study showed that OAB with additional voiding as well as postvoiding symptoms has a greater impact on patients’ lives than OAB without additional symptoms [2]. In this study, using the International Continence Society Working Group (ICS-WG) definition of UAB [3], we investigated the prevalence of OAB with UAB (COUB), OAB alone, UAB alone, and non-OAB/UAB by gender and age, and unveiled their impact on quality of life (QOL) as well as health care-seeking behavior. We hypothesized the increased prevalence of COUB with age, the poorest quality of life, and the highest physician visit rate in the COUB population among the four groups.

We conducted a large-scale nationwide epidemiological online survey of lower urinary tract symptoms in 6210 participants (3088 females and 3122 males) aged 20–99 years old who were selected by probability sampling based on the composition of the Japanese population. To determine the prevalence of each category, OAB and UAB were diagnosed by the OAB symptom score (urgency score ≥2 and total score ≥3) and the definition by the ICS-WG (participants with a slow urinary stream, hesitancy, and straining to void) [3], respectively. The prevalence of COUB in the OAB and UAB population was also examined. QOL was evaluated by a modified King’s Health Questionnaire. In addition, health care-seeking behavior was assessed. Chi-square and Cochran-Armitage tests were used for statistical analyses, and p <0.05 was considered statistically significant.

The overall prevalences of COUB, OAB, UAB, and non-OAB/UAB were 4.3% (n = 135), 9.6% (n = 299), 5.0% (n = 155), and 81.1% (n = 2533) in men, and 1.6% (n = 50), 8.2% (n = 254), 2.4% (n = 73), and 87.8% (n = 2711) in women, respectively. The prevalence of all categories significantly increased with age (Figure 1A and B). The overall prevalence of COUB in the OAB population was 31.1% (135/434) and 16.4% (50/304) in men and women, respectively, and significantly increased with age only in women (Figure 1C). The overall prevalence of COUB in the UAB population was 46.6% (135/290) and 40.7% (50/123) in men and women, respectively, which significantly increased with age only in women (Figure 1D). In the COUB population, the impact on QOL rated as slight or more and the proportion of physician visits was the highest among the groups (Figure 2A to 2D).

The prevalence of COUB increased with age. However, the prevalence of COUB with age in the OAB and UAB population differed between men and women. This suggests that gender differences should be kept in mind when considering the pathophysiology of COUB. Consistent with the results of EPIC study [2], COUB adversely affects participants’ QOL to a substantial degree, which should be associated with the higher physician visit rate than in other groups. Major limitations of this study are that the incidence as well as remission rates of OAB and UAB were not taken into account because it was a cross-sectional study in nature, and a transition rate from OAB to UAB, if the phenomenon exists, has not been established.

In this large-scale cross-sectional study, the prevalence of COUB increased with age, but certain gender differences may exist. Further longitudinal studies focusing not only on OAB but also on UAB are required to elucidate the precise epidemiological aspects of COUB. This study highlighted that COUB deteriorated QOL more profoundly than OAB or UAB alone, promoting the health-care seeking behavior of the participants.

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