The International Consultation on Incontinence Research Society recently proposed the definition of coexisting overactive–underactive bladder (COUB) as a possible new syndrome. This is characterized by coexisting storage and emptying symptoms in the same patient without implying any specific urodynamic/functional findings. In COUB detrusor is abnormally activated during the filling phase without a complete rest, thus wasting energy required for the next voiding phase, which is then impaired due to muscle asthenia and exhaustion. Aim of this retrospective study was to verify this hypothesis.

We included male patients undergoing urodynamic study at our center between 2011-2022.All patients were affected by lower urinary tract symptoms (LUTS). Neurologic disease and patients with previous urinary tract surgery were excluded.
Patients were divided in two groups: normal detrusor function (NDF, Group A) and with detrusor overactivity (DO, Group B) during the filling phase. Number of patients with bladder outlet obstruction (BOO, defined for a bladder outlet obstruction index BOOI>40), number of patients with detrusor underactivity (DU, defined for a bladder contractility index, BCI<100), BCI, post-voiding residual (PVR) and voiding efficiency (bladder capacity-PVR/bladder capacity) were compared in the two groups. DO patients were divided according to the phasic or pre-voiding DU and mean BCI was calculated.

We analysed 499 male patients with LUTS (mean age 62,6 years). Fifty – nine (11,8%) patients had long-term urinary catheters and 4 (0,8%) performed clean intermittent catheterization (CIC). 225 patients (45,1%, Group A) showed NDF and 274 patients (54,9% Group B) DO, 194 of them (70,8%) have phasic detrusor contractions and 80 (29,2%) have pre-voiding detrusor overactivity.
In Group A and B respectively, bladder capacity was 420,3 mL and 298,4 mL (p<0,05), PVR was 90,0mL and 57,0mL (p<0,05) and voiding efficiency was 78,6% and 81,6% respectively. 
During the voiding phase,177 patients (40,7%) were obstructed (BOOI>40),100 (23,0%) were equivocal (20<BOOI>40) and 154 (35,4%) were not obstructed (BOOI<20).
Fifty-nine (26,2%) and 119 (43,3%) patients showed BOO in Group A and B, respectively (p<0,05).
Ninety-three (41,3%) and 94(34,3%) patients showed DU in Group A and B, respectively.
During the filling phase,179 (65,3%) patients of the group B showed a phasic DO, while 95 (34,7%) patients showed a pre – filling DO; their mean BCI was 116,8 and 117,0 respectively.

To our knowledge, this is the first study trying to assess possible differences in terms of detrusor contractility in patients with DO or NDF in the filling phase. Data coming from our study seem to contradict this hypothesis; the number of DU patients is similar in both groups and BCI is similar in patients with early and late appearance of DO. Limits of this study are the retrospective design and the absence of a control group.

Our study seems to contradict the hypothesis that in the COUB, DU could be caused by the wasting of energy used during bladder filling, determining impaired voiding due to muscle asthenia and exhaustion.

Continence 7S1 (2023) 100730
DOI: 10.1016/j.cont.2023.100730