Overactive bladder (OAB) is a multi-factorial progressive syndrome [1] in many individuals, and the severity is quantified using voiding diaries and validated, subjective questionnaires. However, there is no currently available objective method to identify the degree of OAB. Recent studies have used signal analysis techniques such as Fast Fourier Transform (FFT) analysis to objectively quantify the amplitude and frequency of spontaneously rhythmic contractions identified as Detrusor Overactivity (DO) [2]. Furthermore, comparative-fill urodynamics has been used to identify and quantify acute dynamic elasticity that is present in individuals without DO and diminished or absent in individuals with DO [3]. Dynamic elasticity is characterized by a reduction in vesical pressure (Pves) (loss of elasticity) during a bladder fill due to strain-induced stress softening (strain softening) caused by previous filling and passive emptying of the bladder, and a return of Pves (restoration of elasticity) during a fill caused by active voiding during the previous fill [3]. The purpose of the present study was to develop a method to pinpoint an individual on an OAB spectrum based on both their level of dynamic elasticity and the magnitude of any rhythmic contractile activity.

Participants with and without OAB were recruited into a prospective comparative-fill urodynamics study. OAB was determined based on response to the urgency question (Question 5A) of International Consultation on Incontinence Questionnaire on OAB (ICIq-OAB). The urgency question asks “How often do you have to rush to the toilet to urinate?”  Individuals were considered to have OAB if 5a≥3 and considered to have no OAB if 5a=0. In the study two biomechanical properties of the bladder were calculated for each participant: the dynamic elasticity index and the maximum rhythmic amplitude.  The dynamic elasticity index was calculated using previously published methods [3] by measuring pre-strain-softening pressure, post-strain softening pressure, and reversal pressure in comparative urodynamic fills. The dynamic elasticity index was defined as the sum of the loss in Pves due to strain softening (pre - post) and the gain in Pves due to restoration of elasticity (reversal - post) divided by the change in percent capacity used to cause strain softening in the protocol (40% capacity). The maximum rhythmic amplitude of Pves was determined in the frequency range 1.75 and 8.0 cycles/minute for each participant using a previously developed FFT algorithm [2]. For each participant, maximum rhythmic amplitude was plotted on the Y-Axis and the dynamic elasticity index was plotted on the X-Axis.

Based on responses to the ICIq-OAB 5a question, there were 13 participants with OAB and 15 without OAB. The average dynamic elasticity index was 0.03±0.05 cm-H2O/%capacity (range -0.36 to 0.32 cm-H2O/%capacity) for participants with OAB and 0.08±0.04 cm-H2O/%capacity (range -0.23 to 0.39 cm-H2O/%capacity) for individuals without OAB.  Average rhythmic amplitude participants with OAB was 2.3±0.4 cm-H2O (range 0.8 to 5.5 cm-H2O) and 1.6±0.5 cm-H2O (range 0.2 to 7.4 cm-H2O) for participants without OAB.  The combined plot generates a widely distributed spectrum (Fig1A and Fig1C) for participants with OAB which appears more narrowly distributed in participants without OAB (Fig1B and Fig1C). When both datasets were analyzed together, those with maximum rhythmic amplitude >1 cm-H2O were more likely to have OAB (Fig1C, above horizontal blue line, p=0.006).

When the dynamic elasticity index is plotted against the maximal Pves rhythmic amplitude obtained from comparative fill urodynamics, individuals with OAB appear to be more widely distributed, creating an OAB spectrum. This spectrum allows an individual’s position on the spectrum to be pinpointed objectively. In contrast, individuals without OAB appear to have a narrower distribution, highlighting the potential benefit of the OAB spectrum.

The generation of an OAB spectrum using bladder biomechanical properties of the dynamic elasticity index and the maximum Pves rhythmic amplitude obtained from comparative fill urodynamics may enable improved phenotyping of individuals with OAB. However, further research will be required to determine the clinical utility of this novel OAB spectrum.

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Cullingsworth ZE, Kelly BB, Deebel NA, Colhoun AF, Nagle AS, Klausner AP, Speich JE. Automated quantification of low amplitude rhythmic contractions (LARC) during real-world urodynamics identifies a potential detrusor overactivity subgroup. PLoS One. 2018;13(8):e0201594.
Cullingsworth ZE, Klausner AP, Li R, Nagle AS, Carroll AW, Roseman JT, 2nd, Speich JE. Comparative-fill urodynamics in individuals with and without detrusor overactivity supports a conceptual model for dynamic elasticity regulation. Neurourol Urodyn. 2019, DOI:10.1002/nau.24255.