Overactive bladder (OAB) has diverse etiologies, of which pelvic organ prolapse (POP) is recognized as an important one in female OAB.  In patients with POP, the prevalence of OAB is up to six times higher than in patients without it.  Bladder outlet obstruction (BOO) as well as posterior fornix syndrome (PFS) can cause OAB in patients with POP [1,2].  However, the association between the compartment / degree of POP and OAB remains to be determined.
Recently, dynamic magnetic resonance imaging (dMRI) is thought to be useful for delineating the descent of each pelvic organ during abdominal straining without radiation exposure.  Alt et al. proposed that pelvic organ mobility (POM) measured by dMRI be used as one of the indicators of treatment outcomes after pelvic floor reconstructive surgery [3].  
In this study, we hypothesized that the mobility of the bladder primarily correlates with the occurrence of OAB as well as its progress, and tried to demonstrate these associations by the use of an OAB symptom score (OABSS) and parameters derived from dMRI findings.

We included 118 patients (mean age: 60.2 years old) who had POP of stage II or less at rest and stage III or more when straining during dMRI.  
The presence or absence and severity of OAB were evaluated by OABSS, which has been widely utilized as a diagnostic test as well as an outcome measure for management of OAB in Japan.  Diagnosing OAB requires three or more points on OABSS.
Using the sagittal images of dMRI, we measured representative points of the pelvic organs, that is, coordinate positions of the bladder neck (BN), the most dependent position of the bladder after straining (B), uterine cervix (C), and anorectal angle (AR) before (at rest) and during intense straining.  To measure them, we used a sacrococcygeal inferior pubic point line (SCIPP) line as an x-axis with the inferior margin of the pubic symphysis as an origin, and a perpendicular line to the SCIPP line at the origin as a y-axis.  The x and y coordinates of each representative point (e.g., BNx, and BNy, respectively) were measured before and during straining.  POM was evaluated by the distance between coordinate positions of the representative points before and during straining in x (e.g., BNxx = BNx during straining – BNx at rest, median, -12.4 mm; interquartile range (IQR), -19.1 to -5.7 mm) and y directions (eg., BNyy = BNy during straining – BNy at rest, median, -20.9 mm; IQR, -26.8 to -15.9 mm) and the distance derived from the Pythagorean theorem (e.g., BNp = [(BNxx)2 + (BNyy)2]1/2.
Student’s t-test and Spearman’s rank correlation were used to evaluate the association between the presence or absence of OAB and POM as well as coordinate positions, and between the sum of OABSS and them, respectively.  P <0.05 was considered to be statistically significant.

Thirty-five (29.7%) of the patients had OAB based on OABSS.  POM and coordinate positions of pelvic organs in patients with or without OAB (Figure 1) demonstrated that POM and coordinate points of BN as well as B were not associated with the occurrence of OAB.  On the other hand, correlations between POM as well as coordinate positions of pelvic organs and OABSS (Figure 2) showed that POM and coordinate points of BN or B correlated weakly with the severity of OAB.

The presence or absence of OAB is associated with POM and coordinate positions not of BN and B, but of C.  According to the PFS theory, defective support for C undermines the stability of the bladder trigone, which results in increased susceptibility to distension of the bladder.  In addition, cyclic strain on the peripheral nerves would cause sustained nerve damage, leading to partial denervation of the bladder.  These could contribute to the occurrence of OAB.

Defective support for the uterine cervix is associated with the occurrence of OAB, while bladder descent correlates with the severity of OAB.

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