Stress urinary incontinence (SUI) is a highly prevalent condition in women and considerably impacts the patients’ quality of life. SUI is characterized by involuntary urine loss during episodes of high abdominal pressure such as coughing, sneezing, or physical exertion. In healthy individuals, continence is maintained by synergic coordination of low intravesical pressure, achieved by relaxation of the detrusor smooth muscle, and simultaneous sufficiently high resting pressure in the urethra, compared to the intravesical pressure, to prevent leakage of urine from the bladder. During episodes of high abdominal pressure, the urethral pressure has to rise to match the increase in intravesical pressure resulting from the force the abdomen exerts on the bladder.
	A novel therapeutic option for SUI patients utilizes the insertion of a small air-filled balloon into the bladder transurethrally. The air-filled balloon is proposed to serve as an efficient and minimally invasive treatment option for patients. Since gasses compress more easily than liquids, the balloon has been proposed to act as an absorber for increases in intravesical pressure in response to an increase in intra-abdominal pressure. It is fabricated as a free-floating biocompatible medical-grade polyurethane balloon and is implanted through the urethra without the need for anesthesia. After the balloon is inserted into the bladder and filled with 30 mL of air and 0.5 mL of a perfluorocarbon liquid that prevents the balloon from deflating, buoyant forces cause the balloon to float to the top of the bladder dome. The device successfully prevents the loss of urine during episodes of high intra-abdominal pressure. Previous clinical studies have investigated the use of this intravesical balloon to alleviate symptoms of SUI and have confirmed its therapeutic effectiveness.
	We hypothesize that during coughing episodes the size and volume of the balloon, measured during video urodynamic investigation, will decrease as a result of successful transfer of abdominal pressure to the air-filled balloon. Absorbance of the increase in intra-abdominal pressure by the balloon is expected to cause a reduction in incontinence episodes or volume.

The current study was approved by the local ethical committee, and informed consent was obtained from each of our participants. We recruited 10 female patients with SUI according to ICS criteria (mean age: 55.6, SD 7.8). The video urodynamic studies of 6 patients could be evaluated for this analysis after balloon implantation (4 patients had missing data due to technical difficulties).
	After implantation of the balloon, participants were invited for a follow-up visit approximately 7-10 days after balloon placement and underwent a video urodynamic investigation at a sampling rate of 20 frames per second while participants were exposed to episodes of high intra-abdominal pressure using Valsalva maneuvers and coughing episodes during bladder filling with 100ml of saline with a contrast medium.
	We designed a custom MATLAB (MathWorks, R2018b) script which enabled us to subtract information regarding the size of the balloon in an automated fashion. Due to the air present in the balloon it can be distinguished from the surrounding structures and tissues based on its bright appearance in the grayscale images. After optimizing the contrast of each frame of the video urodynamic assessment, we manually defined a cut-off gray value that would allow for the most optimal differentiation between the balloon and the rest of the image. Images were then converted to binary images where each pixel above our threshold was assigned the value 1 (white) and each other pixel 0 (black). The largest continuous object consisting of white pixels was considered to correspond to the balloon and was selected for further processing. The coordinates of the edge extremities (Fig. 1.A) of this object were computed and plotted over the original image grayscale image. Correspondence of the computed dimensions of the balloon with the original image were then visually verified by the researchers. The diameters of the balloon along the mediolateral axis (horizontal, or x direction) and cranio-caudal axis (vertical, or y direction) were then used for frame-by-frame computing of the size of an ellipse corresponding to the size of the balloon. The difference between the maximal (rest) and minimal (high intra-abdominal pressure) size of the computed ellipse during the video urodynamic assessment was statistically assessed using an SPSS 26 (IBM, 2019) implementation of the Wilcoxon Signed-Rank non-parametric test. Since the abdominal pressure forces emerging from the coughing episodes travel in a cranio-caudal fashion we expect the dimensions of the balloon to mainly be affected in the vertical (y) direction. We tested this by statistically assessing the difference between the maximal and minimal diameters of the balloon along the y direction using a Wilcoxon Signed-Rank test.
	In addition, using micturition diaries, we compared participants’ daily number of episodes of involuntary urine loss and daily pad use before and after balloon placement using a Wilcoxon Signed-Rank non-parametric test.

The surface area of an ellipse based on the dimensions of the balloon in the frontal plane was found to significantly decrease in size during coughing episodes (p = 0.028, mean decrease = 9.26%, SD: 2.44) (Fig. 1.B). Because abdominal pressure forces due to coughing mainly travel in cranio-caudal (vertical) direction we, additionally, assessed changes in the diameter of the balloon along the vertical axis and found a significant decrease of the diameter of the balloon during coughing (p = 0.028, mean decrease = 11.5%, SD: 3.61, n = 6) (Fig. 1.B).
	The micturition diaries showed a significant decrease in participants’ SUI symptoms. The daily number of episodes of involuntary urine loss decreased from an average of 4.4 episodes before intervention to 1.8 episodes after balloon placement (p = 0.020). Average daily pad use decreased from 3 pads per day to 1.3 after placement of the intravesical balloon (p = 0.017).

The dimensions of the balloon in the frontal view show a significant decrease during episodes of high abdominal pressure, mainly along the cranio-caudal axis. We argue that the decrease in size of the balloon is a direct result of compression of the air inside the balloon. This indicates that the balloon is successful in absorbing at least part of the intra-abdominal pressure which is transferred to the bladder. Thereby decreasing the extent to which the intravesical pressure increases in response to episodes of high intra-abdominal pressure.
	The results of the current study confirm the therapeutic effectivity of the intravesical placement of an air-filled balloon 7 days post-placement on SUI by their significantly positive effect on patients’ daily involuntary urine loss and pad use. We propose that the therapeutic effect is related to absorption of the increase in intravesical pressure by the balloon.

In the present study, we show that implantation of an intravesical balloon in patients with SUI complaints significantly reduces patients’ daily episodes of involuntary urine loss and pads used. The balloon significantly decreased in size indicating that it successfully absorbed intra-abdominal pressure increases. The observations made in this study can be utilized to further maximize the potential benefits of this therapy and help us understand which patients can be expected to obtain the largest advantage of the implantation of an intravesical balloon.