Low amplitude rhythmic contractions (LARC) are observed in the intravesical pressure (Pves) tracings of many patients suffering from detrusor overactivity (DO). These pressure changes are thought to be the result of synchronization of contractions that cause micromotion (MM) in different regions of the bladder wall. The goal of this study was to develop a non-invasive method to measure bladder wall micromotion in vivo. The first aim was to measure bladder wall MM with ultrasound and validate the method by correlating the frequency characteristics of the MM with Pves in a porcine model. The second aim was to apply the validated method in humans with and without DO to determine its effectiveness in identifying a MM mediated subtype of DO.

For the first aim, female pigs were anesthetized with urethane and, when necessary, isoflurane and underwent an ultrasound urodynamic study (Laborie model Aquarius TT). Anatomical Motion Mode (AMM) ultrasound cine loops were obtained at pauses in filling using a GE Voluson E8 ultrasound system and used to optimize a texture tracking algorithm to measure bladder wall width over time. The frequency characteristics between wall width and Pves were compared using Fast Fourier Transform analysis to validate the algorithm.

For the second aim, 13 people with urinary urgency and six with no urgency symptoms based on validated ICIq-OAB survey scores underwent an ultrasound urodynamic study. AMM cine loops were obtained at 40% cystometric capacity and were analysed using the texture tracking algorithm to calculate wall width as a function of time (Fig. 1). A urodynamicist blinded to the results of the MM study diagnosed DO based on UDS tracings following standardized ICS protocols.

In the pig study, FFT characteristics of wall width and Pves showed the same peak frequencies with proportional amplitudes demonstrating that the texture tracking algorithm was effective. In the human study, significant MM was considered to be wall width changes with peak frequencies in the range of 1.5-7 cycles/min and amplitudes greater than 0.14 mm and was found in five of 19 individuals (26%). DO was independently diagnosed in ten of the 19 individuals (53%). A significant association was found between MM and DO (p = 0.03). All of those with significant MM had DO, which yielded a specificity of 100%. Half of those with DO were found to have significant MM, corresponding to a sensitivity of 50% and implying that there may be a MM-associated subtype of DO.

The results of the pig study show that the texture tracking algorithm can measure changes in bladder wall width and that these changes share important frequency characteristics to Pves. Future studies are needed to determine effects of other phasic physiological processes including breathing, ureteric jetting, bowel motion, and bladder volume on MM in the porcine model. The results of the human study demonstrate that the non-invasive ultrasound-based method to monitor bladder wall width changes can be effective in identifying a potential subtype of DO. In addition to validating the method in a much larger population, future studies are needed to determine the relationship between changes in Pves and changes in wall width and to analyse wall width MM in different bladder regions as well as associations with bladder sensation.

This study demonstrates the feasibility of a non−invasive method to measure bladder wall MM using transabdominal AMM ultrasound. Identification of a MM-associated subgroup could enable better targeting of certain DO treatments to this group without the need for an invasive urodynamic study.