The coexistence of detrusor overactivity (DO) and detrusor underactivity (DU) (originally called “detrusor hyperactivity with impaired contractile function”, DHIC) was first described in 1987, when it was suggested that it may represent a more advanced stage in the natural history of DO [1]. 
It has been theorized that: a) during the filling phase the overactive detrusor is abnormally activated, wasting energy required for the next voiding phase, and b) that chronic muscular overactivity may lead to increased muscle mass, progressive ischemia and fibrosis that can drive DU [2]. 
A recent study that explored for the first time the association between DO and DU could not find differences in detrusor voiding function between men with and without DO [3]. 
The aims of this study of men with DO were: i) to compare DO parameters in patients with DU, normal voiding and bladder outflow obstruction (BOO), and ii) to correlate these parameters with the detrusor contraction index (DCI).

This is a retrospective cross-sectional study of an urodynamics database prospectively acquired in a single tertiary-level hospital between January 2012 and December 2021, following the recommendations of the International Continence Society. 
Consecutive men aged ≥ 18 years old subjected to standard urodynamic studies with demonstrated DO were included. Normal voiding function was defined by a Bladder Outflow Obstruction Index (BOOI = pdetQmax – 2 Qmax) less than 20 and a DCI (DCI = pdetQmax + 5 Qmax) equal or over 100, pure DU by a BOOI less than 20 and a DCI less than 100 and pure BOO by a BOOI over 40 and a DCI equal or over 100. 
Wilcoxon rank-sum test was used to compare DO parameters among the different patient groups. Spearman’s rank correlation coefficient was applied to correlate DO parameters and DCI. The data was analyzed with Jamovi and statistical significance was defined as two-sided p value < 0.05.

We identified 657 patients with DO in their urodynamic studies; 404 of them voided between 150 and 750 ml. Among these, 82 had pure DU, 111 had normal voiding and 181 had pure BOO. Thirty patients with DU and BOO were not analyzed.
The comparison of pressure-flow study and DO parameters in men with different voiding diagnoses are shown in Table 1 and Table 2, respectively. There were no significant differences in pre-voiding bladder volume between the groups and some of the DO parameters were significantly higher in patients with normal voiding and/or with BOO than in patients with DU.
In the entire group of men there were significant direct correlations between DCI and some DO parameters, although these were very weak: total height of DO contractions (rho = 0.216, p < 0.001), mean DO contraction height (rho = 0.233, p < 0.001) and DO index (rho = 0.119, p = 0.021).

In men with DO and different voiding diagnoses, some DO parameters were significantly higher in patients with normal voiding and/or with BOO than in patients with DU, and there were very weak significant direct correlations between some of these DO parameters and DCI.
This study shows a trend that DO is associated with greater detrusor contraction strength, contradicting the hypothesis that DU results from muscle asthenia due to DO during the previous bladder filling. Future research should focus on the long term follow up of patients with DO, evaluating the detrusor contraction strength.

This study shows a trend that DO is associated with greater detrusor contraction strength and does not support the hypothesis that DO is associated with DU in the same bladder filling and emptying cycle.

Resnick NM, Yalla SV. Detrusor hyperactivity with impaired contractile function. An unrecognized but common cause of incontinence in elderly patients. JAMA 1987; 257: 3076-81.
Mancini V, Tarcan T, Serati M, Wyndaele M, Carrieri G, Abrams P.  Is coexistent overactive-underactive bladder (with or without detrusor overactivity and underactivity) a real clinical syndrome? ICI-RS 2019. Neurourol Urodyn. 2020; 39 Suppl 3: S50-S59.
Rosato E, Mollo L, Laurendi V, Orecchia L, Bachetti P, Finazzi Agrò E. May coexisting overactive-underactive bladder (COUB) be due to muscle fatigue? Data from a single-centre urodynamic database. Continence 9 (2024) 101061.