Changes in bladder geometry during filling may play a role in the development of bladder urgency and overactive bladder, because tension sensitive afferent nerves relay filling sensation. This study’s objective was to characterize changes in bladder geometry throughout filling, and to compare any patterns of geometry changes in filling between OAB patients and volunteers with normal bladder function. The hypothesis was that ultrasound urodynamics could be used to identify a subset of OAB patients with bladder shapes significantly different than those of normal volunteers.

This prospective study included 24 participants consisting of 5 men and 5 women without OAB and 14 women with OAB. All participants underwent ultrasound urodynamic testing with concurrent 3D ultrasound images acquired once every minute during bladder filling at an infusion rate of 10% cystometric capacity per minute. Ultrasound was performed using a GE Voluson E8 system with a 4-8 MHz transabdominal probe. Bladder diameters in the depth, height, and width directions were measured using 4D View software by GE at every 10% increase in capacity and were plotted as a function of normalized capacity (bladder volume divided by the voided volume). The height to width ratio at the end of filling (100% capacity) of the normal participants was used to determine if any bladders were outliers by defining a 95% confidence interval (Fig. 2A).

The mean and standard deviation of age and BMI were 27.5 ± 7.04 years and 26.0 ± 3.54 kg/m2 for those with normal bladder function and 53.7 ± 11.1 years and 33.5 ± 9.7 kg/m2 for those with OAB, respectively. As expected, age and BMI were significantly different between the two groups (p<0.001) Mean bladder diameter increased in the depth (Fig. 1A), width (Fig. 1B), and height (Fig. 1C) directions in both groups over the course of filling. There were significant differences between normals and OABs in the depth and height at several capacities above 50% (Fig.1 asterisks). However, there were no significant changes in the width direction. The greatest degree of change in diameter was seen in the height direction which grew by about two fold (100% strain) in both the normals and the OABs. The mean and standard deviation of the height-to-width ratio of the normals was 1.06 ± 0.12 yielding a 95% confidence interval of 0.82 to 1.30. No normal participant had a height-to-width ratio outside of this interval, but one OAB had a height-to-width ratio above it and three had height-to-width ratio below it (Fig. 2), identifying these individuals as possibly having shape mediated OAB.

In the depth and height directions, there were normalized capacities at which the normals had significantly larger bladder dimensions than the OABs. This is to be expected because OABs tend to have reduced bladder total capacities overall. Two different types of abnormal shape were identified. One OAB participant had an abnormally large height-to-width ratio (a tall, elongated bladder shape, Fig. 2C) while three had abnormally small height-to-width ratio (a short, flattened bladder shape, Fig. 2D).

This study shows that non-invasive ultrasound can potentially be used to identify differences in bladder shape and dimensions between individuals with OAB and without OAB. This method was used to identify a subset of OAB patients with abnormal HWR at the end of filling who may benefit from treatment targeted towards bladder shape such as weight loss, bowel care, and changes in posture