Dynamic elasticity was previously identified during comparative-fill urodynamic studies (UDS) in individuals with overactive bladder (OAB) [1], and is the clinical correlate of adjustable preload tension in human detrusor strips, a mechanism for acutely regulating detrusor wall tension [2]. Dynamic elasticity is due to reversible strain induced stress softening ("strain softening"). Dynamic elasticity is lost via repeat passive filling and emptying and is regained after active voiding. The aim of this study was to test the hypothesis that individuals with detrusor overactivity (DO) exhibit less dynamic elasticity than those without DO.

Individuals with and without urinary urgency were enrolled in this prospective study based on their answers to the International Consultation on Incontinence Questionnaire (ICIq-OAB) question 5a, "Do you have to rush to the toilet to urinate?" Participants who answered "never" (score = 0) were categorized as being without urgency, and participants who answered "most of the time" or "all of the time" (score = 3 or 4, respectively) were categorized as having urgency.  

Vesical pressure (Pves) data were collected during repeat fill-and-empty UDS. An initial fill-active void cycle was performed to determine cystometric capacity (CCap) and identify any DO based on a blinded review of the UDS pressure tracings by a neurourologist/urodynamicist. Dynamic elasticity was quantified by comparing three fills (Fig 1, Fills 1-3). Fill 1 was defined as “before strain softening” and used a baseline after an active void. Fill 2 was defined as “after strain softening” (i.e. after passive emptying via syringe aspiration) and used to show the degree of dynamic elasticity. Fill 3 was defined as “after active voiding” and used to show how much dynamic elasticity was recovered after active voiding. For each fill, an average Pves value was calculated for the filling range from 0% CCap to 40% CCap (Figure 1, red lines) and these values were normalized to the average Pves for Fill 1 (Fig 2).

UDS data from 21 participants were analyzed. Eleven participants were identified with DO (10 women and 1 man), and DO was not identified in the remaining 10 participants (8 women and 2 men). Nine of the 10 participants without DO exhibited dynamic elasticity characterized by a loss in elasticity (decrease in Pves) during a fill subsequent to passive emptying (Figure 2, Fill 2, *) and a gain in elasticity (Pves return to baseline) during a fill subsequent to active voiding (Figure 2, Fill 3). Contrastingly, only three of the 11 participants with DO exhibited dynamic elasticity (Figure 2). The absence of dynamic elasticity was significantly associated with the presence of DO (Fischer’s exact test, p<0.01). Dynamic elasticity was shown in all five healthy participants without OAB or DO.

The present data support the hypothesis that contractile activity during active voiding or during filling phase DO reverses some or all of any loss in dynamic elasticity due to a fill-passive empty cycle. In patients without DO, the decrease in average Pves (loss dynamic elasticity) observed in Fill 2 (Figure 2) due to the Fill 1-passive empty cycle was restored by the active voiding contraction following Fill 2 and observed as the increase in Pves in Fill 3 (Figure 2).  However, in patients with DO a decrease in average Pves due to the Fill-1 passive empty cycle was not observed in Fill 2 (Figure 2) because either 1) DO during Fill 1 prevented the decrease in Pves or 2) DO during Fill 2 restored the decrease Pves during that particular fill.

Healthy bladders exhibit dynamic elasticity. DO may alter the bladder’s mechanism for acutely regulating dynamic elasticity, possibly contributing to OAB. Quantification of dynamic elasticity could lead to patient sub-typing and targeted  treatments for OAB.

Colhoun AF, Klausner AP, Nagle AS, Carroll AW, Barbee RW, Ratz PH, et al. A pilot study to measure dynamic elasticity of the bladder during urodynamics. Neurourol Urodyn. 2017;36(4):1086-90.
Colhoun AF, Speich JE, Dolat MT, Habibi JR, Guruli G, Ratz PH, et al. Acute length adaptation and adjustable preload in the human detrusor. Neurourol Urodyn. 2016;35(7):792-7.