Effect of transcutaneous electrical stimulation on morphological characteristics of women with stress urinary incontinence

Background: Stress urinary incontinence is an involuntary leakage of urine due to a weak pelvic oor and weak sphincter when the intra-abdominal pressure increases. Its symptoms are known to improve upon electrical stimulation of the pelvic oor. This study aimed to determine the effects of transcutaneous electrical stimulation on ultrasonography variables, such as the bladder neck position (BNP), length of the urethra (LU), funneling index (FI), and posterior (PRT) and anterior rhabdosphincter thickness (ART), and the Incontinence Quality of Life scores. It also investigated the association between the relative changes in the two. Methods: Twenty-one patients with stress urinary continence were included and subjected to transcutaneous electrical stimulation for eight weeks. Ultrasonography was used to measure the BNP, LU, FI, ART, and PRT. Data were analyzed at rest and during the Valsalva maneuver and the difference ( △ ) between them was evaluated. Results: The BNP during the Valsalva maneuver and the △ BNP decreased signicantly. The LU during the Valsalva maneuver increased after eight weeks. The FI during the Valsalva maneuver and the △ FI changed signicantly. The ART and PRT and the total quality of life score signicantly increased after eight weeks (p<.05). The LU during the Valsalva maneuver positively correlated with the total quality of life (r=0.630; p=.002), psychosocial impact subscale (r=0.705; p=.000), and social embarrassment subscale (r=0.488; p=.025) scores. The correlations between the PRT and the avoidance and limiting behavior subscale score (r=0.624; p=.002) and between the △ FI and the social embarrassment subscale score (r=-0.515; p=.0.20) were signicant. Conclusions: Transcutaneous electrical stimulation can improve the BNP, ART, and PRT, along with the subjective indicators, in women with stress urinary continence. Improving stress urinary continence symptoms


Background
The International Continence Society de nes stress urinary incontinence (SUI) as an involuntary loss of urine when the intra-abdominal pressure increases due to sneezing, coughing, or physical exertion [1,2]. A possible cause of SUI is bladder neck hypermobility due to pelvic oor muscle (PFM) weakness [3]. SUI affects the patient's social, physical, and psychological well-being and his/her quality of life [4,5].
However, people with SUI often hesitate to disclose their symptoms. Various noninvasive or conservative interventions, such as medications, pelvic oor muscle training (PFMT), and pelvic oor electrical stimulation (PFES), are available for managing SUI. According to a previous study, noninvasive treatment results in a greater improvement of mild-to-moderate SUI [6].
The PFM, which supports the pelvic viscera and controls the pelvic outlets, is an active contributor to the continence mechanism [7]. During sneezing, coughing, or physical exertion, increases in the intraabdominal pressure are transmitted to the urethra to reinforce its closure only if the urethra remains in position between the anterior vaginal wall or the pubic symphysis and the PFM [8]. Assessment of bladder neck hypermobility has revealed that during functional tasks, patients with SUI show a larger excursion of the urethral angle than their continent counterparts [9,10]. Bladder neck hypermobility and reduced proximal urethral closure are associated with bladder neck funneling in SUI [11,12]. Several studies have evaluated bladder neck hypermobility parameters, such as the bladder neck position (BNP), which are associated with the PFM functions [13,14]. When the BNP is above the pelvic oor, the pressure transmitted to the bladder is equally transmitted to the urethra, which simultaneously increases the urethral closure pressure. If the BNP is lower than the pelvic oor, a greater pressure is transmitted to the bladder than to the urethra, which threatens the urethral closure and continence status. Due to bladder neck descent, the length of the urethra (LU) is shorter in incontinent women than in continent women [15]. Additionally, the volume of the external sphincter appears smaller in SUI women than in continent women on ultra-sonography [16].
PFES is frequently used in the conservative treatment of patients with SUI. It may lead to nerve regrowth and strengthening of the external sphincter, thereby increasing bladder outlet resistance over time [17,18]. It also assists women who face di culty in identifying and feeling the PFM and also allows an electrically-induced contraction of a weak PFM [19]. Transcutaneous electrical stimulation (TES) and intravaginal electrical stimulation are the two common PFES types for SUI and the effectiveness of each has been proven clinically [19,20]. In the intravaginal type, electrodes are inserted into the vagina; therefore, this type could be potentially limited by pain, intolerance to high stimulation intensity, discomfort associated with electrode insertion, di culty in hygiene management, risk of infections, and/or bleeding. In contrast, the TES is more convenient and acceptable to the user and safer with respect to infections. Recently, the Easy-K (Alphamedic Co., Ltd., Daegu, Korea) device was designed for contracting the PFM by electrical stimulation using cutaneous perivaginal electrodes in the sitting position. Previous studies suggest that TES is effective in treating urinary incontinence and can be performed using cutaneous electrodes in the perivaginal region [21,22].
Thus, the present study aimed to demonstrate the effect of TES on BNP, LU, bladder neck funneling, urethral external sphincter hypertrophy, and quality of life. It also aimed to determine the association between the relative changes in the total and subscale scores of the Incontinence Quality of Life (I-QOL) questionnaire and the relative change in the ultra-sonography variables in women with SUI. The current study hypothesized that the bladder neck hypermobility would improve and the I-QOL scores would be increase after eight weeks of TES.

Subjects and design
The G*power software version 3.1.2 (Franz Faul, University of Kiel, Kiel, Germany) was used for power analysis based on the results of a pilot study on ve subjects. Sample size calculation was performed with a power of 0.80, alpha level of 0.05, and effect size of 0.95; the necessary sample size was found to be 10 subjects. A total of 21 women who were diagnosed by gynecologist with grade 1 or grade 2 SUI according to the Stamey's grading system (Table 1) were recruited in this study through a social networking site. Their mean age was 51.3 years and their mean body mass index was 24.12k kg/m 2 . Other demographic characteristic of the subjects is summarized in the Table 2. The exclusion criteria included pregnancy and a history of urogenital surgery such as urinary incontinence surgery, colporrhaphy, and hysterectomy. All subjects read an explanation about the experimental procedures and then signed an informed consent form. This study was approved by the Yonsei University Wonju Institutional Review Board (approval number: 1041849-201808-BM-077-02).
This study was conducted on a single group using a pre-and post-design over an eight-week period. All subjects were assessed twice at baseline and eight weeks after the initiation of the intervention. During the treatment, they were asked to write a daily report on the usage of TES; a researcher checked the subjects' compliance daily through this report and text messages.

Transcutaneous electrical stimulation
The Easy-K is a TES device that uses cutaneous electrodes in the perivaginal region for stimulating the PFMs and the surrounding structures ( Figure 1); the shape and position of the electrodes are such that the electrodes come in contact with the entire vulva while the user is sitting on the stimulator ( Figure 2). A layer of wet tissue is used as a conducting material between the skin and the electrodes. Before using the device, we cleaned the vulva and laid wet tissues on the electrode. This generated an electrical stimulus that directly contacted the perivaginal regions and stimulated the PFM extensively while the subject was sitting on the Easy-K. The device was placed on the toilet seat and the subjects were instructed to sit on the device as the cutaneous electrodes came in contact with the perivaginal regions. Thereafter, the physical therapist set the stimulation amplitude to evoke perceivable and comfortable stimulation levels.
The Easy-K delivered biphasic, asymmetric impulses of 21-27 Hz and provided controlling sessions for pulse (5 seconds) and resting (4 seconds) durations. The subjects were instructed to use the device once a day in a 20-minute session for ve-to-six days a week over a period of eight weeks. Furthermore, they underwent an Easy-K session with possible increases in the stimulation amplitude (as tolerated by the patients).

Outcome measures
The subjects were asked not to urinate for three to four hours prior to the examination. The objective variables including the BNP, LU, funneling index (FI), and rhabdosphincter thickness (RT) were measured using an ultrasound scanner (ALOKA, SSD-α10, JAPAN) with a 5.0 Mhz vaginal-type transducer. The subjects were examined in the lithotomy position by a skilled gynecologist. Their morphological characteristics were evaluated at rest and during the Valsalva maneuver. These characteristics were analyzed on frozen images. The differences in the values of these variables between resting and during the Valsalva maneuver were calculated and statistically analyzed; these differences were denoted by delta (△). The BNP (°) was quanti ed by calculating the pubourethral angle between two axes: The x-axis was de ned by a line drawn from the lower margin of the pubic symphysis to the bladder neck, while the y-axis was de ned by the midline of the pubic symphysis ( Figure 3) [23]. The LU (mm) is the distance from the bladder neck to the lower margin of the pubic symphysis; the longer this distance, the higher is the position of the bladder. The FI is measured by multiplying the width (mm) of the bladder neck by its height (mm) [24]. The rhabdosphincters, a part of the external urethral sphincter, are located in the middle of the urethra [16]. The RT (mm) was measured at a distance of 5 mm from the bladder neck point towards the bladder in the sagittal plane [25]; the anterior RT (ART) and the posterior RT (PRT) are measured from the anterior and posterior walls of the urethra, respectively.
The subjective variables were self-reported using the Korean version of the I-QOL questionnaire. The I-QOL is used to assess the incontinence-speci c quality of life. It is in the form of a Likert-type response scale and contains 22 items. The subjects assessed themselves using the 5-point scale questions on daily life related to SUI and scored themselves as 1 (extreme), 2 (quite a bit), 3 (moderate), 4 (a little), or 5 (not at all). The total sums of these scores were documented for analysis. There are three subscales: the avoidance and limiting behavioral (AL), psychosocial impacts (PI), and social embarrassment (SE) subscales. A higher total score means a higher self-satisfaction and better SUI-related quality of life.

Statistical analysis
The data were analyzed using SPSS for Windows version 24.0 (SPSS Inc., Chicago, IL, USA). The Kolmogorov-Smirnov test was performed to con rm the normal distribution of the data. The paired t-test was used to analyze the BNP, LU, FI, ART, and PRT in each condition and the I-QOL scores before and after the intervention. Pearson's correlation was used to determine the association between the relative changes in the total and subscale scores of the I-QOL and the ultrasonographic variables in the women with SUI. The signi cance level was set at a p-value of .05. Table 3 presents the ultrasonographic ndings. The BNP during the Valsalva maneuver and the difference values representing bladder neck hypermobility signi cantly decreased after eight weeks of intervention (p<.05). The LU signi cantly increased after eight weeks only during the Valsalva maneuver (p<.05). While the FI during the Valsalva maneuver and the △FI decreased signi cantly (p<.05), the FI at rest did not improve signi cantly. The ART and PRT, which are the indicators of external sphincter hypertrophy and were measured both at rest and during the Valsalva maneuver, signi cantly increased post intervention (p<.05); the △ART also increased signi cantly (p<.05).

Results
The total I-QOL score increased from 64.81 to 71.86 after intervention (p<.05). Among the three subscale scores, the AL and PI sub-scores signi cantly improved (p<.05). Although the SE sub-score did not improve signi cantly (p>.05), it steadily increased from 14.33 to 14.9 over the eight-week treatment period (Table 4). Table 5 indicates the Pearson's correlation between the I-QOL sub-scores and the BNP, LU, FI, ART, and PRT. Signi cant positive correlations were observed between the total I-QOL score and the LU during the Valsalva maneuver, between the AL sub-score and the PRT at rest and during the Valsalva maneuver, between the PI sub-score and the LU during the Valsalva maneuver, and between the SE sub-score and the △FI and LU during the Valsalva maneuver.

Discussion
The present study aimed to demonstrate the effects of TES on bladder neck hypermobility, morphological characteristics, and psychological satisfaction in women with SUI. To our knowledge, this study is the rst to evaluate the BNP, LU, FI, ART, and PRT using ultra-sonography after eight weeks of TES in women with SUI. Although previous studies have suggested that PFMT may decrease bladder neck hypermobility in SUI [3,26], no studies have investigated the improvement of bladder neck hypermobility after TES of the PFM. The main ndings of the present study indicate that TES could improve not only the bladder neck hypermobility, but also the subjective symptoms of SUI. Therefore, TES is bene cial for improving the subjective symptoms in patients with SUI by improving the bladder neck hypermobility, which is the fundamental mechanism of SUI [3].
The stimulus is transmitted by the afferent pudendal nerve to the efferent pudendal nerve and the hypogastric nerve induces the contraction of the periurethral muscle and the PFM [27]. In patients with SUI, this pudendal nerve conduction is disturbed, which along with sphincter and PFM weakness, delays the re exive response of the PFM [17]. The electrodes of the TES device are positioned to t the dermatome of the pudendal nerve and the base of the pubococcygeus and iliococcygeus muscles, which belong to the PFM. Because TES directly stimulates the muscle, it may be effective in not only strengthening the muscles, but also in imparting the ability of identifying and recognizing the location and contraction of the PFM to women who cannot contract the muscle voluntarily [19]. These effects can restore the normal neuromuscular activities [19]. Restoring neuromuscular control and PFM function increases the bladder neck stability and urethral closure [17,18]. The TES-induced muscle contraction provides an involuntary exercise for improving the urethral closure mechanism [19]. We found moderate to high correlations between the I-QOL scores and the variables of ultra-sonography.
There was a positive correlation between LU under Valsalva and the total score (r=0.630; p=.002), PI (r=0.705; p=.000), and SE (r=0.488; p=.025). This indicates that the total score and the PI and SE subscores of I-QOL increase as the LU during the Valsalva maneuver increases. Incontinent women have a shorter LU as compared to continent women, especially during the Valsalva maneuver, because the bladder neck funneling increases and the LU decreases [25]. Furthermore, Kim et al reported that the LU of SUI women increased, with an improvement in the SUI symptoms, after eight weeks of TES [25]. In this study, we identi ed that an increase in the LU is highly correlated with the SUI women's subjective symptoms. The increase in LU after TES may be related to the reduction of subjective symptoms because a functional LU was obtained. We also found a positive correlation between the PRT and AL (r=0.624; p=.002). The rhabdosphincters are directly connected to the PFM and play an important role as urine ow regulators. Therefore, TES could indirectly affect the PRT by improving the AL score. Finally, we observed a correlation between △FI and SE (r=-0.515; p=.0.20). The higher a woman's FI, the higher the pressure of the urine from the bladder to the urethra. This nding could indicate an improvement in the urethral pressure against the load after eight weeks of TES. In conclusion, ultra-sonography variables could serve as important clinical indicators for determining the most reasonable mechanism behind the improvement of SUI symptoms after TES The ndings of our study can provide evidence that TES improves the ability of maintaining urinary continence through the development of a strong and rapid voluntary control of the PFM and sphincters in women with SUI when their intra-abdominal pressure increases during sneezing, coughing, or physical exertion. TES is bene cial in improving the stability of the PFM and sphincter support systems by providing an adequate resistive force against increased intra-abdominal pressure [30]. We believe that in this experiment, the anticipatory and reactive abilities of individuals with SUI may have been greatly affected by the improvement of the AL and PI sub-scores of I-QOL. However, this study has certain limitations. Firstly, it is di cult to generalize our result because this experiment included a relatively small sample size and did not include a control group. Secondly, we did not measure parameters such as electromyography data, strength, power, and endurance of the PFM. Further research with a larger sample size and a control group is necessary. It is essential to identify improvements in the neuromuscular function of the PFM through electromyography in women with SUI.

Conclusions
The present study identi ed that an eight-week course of TES affected the BNP, LU, FI, ART, PRT, and I-QOL scores of SUI women. TES can be bene cial as a conservative method for improving bladder neck hypermobility, urethral sphincter thickness, and subjective symptoms in women with SUI. Improving the SUI symptoms can ameliorate the women's self-esteem in social and psychological aspects. The study ndings may be useful for developing guidelines for SUI treatment.

Consent for publication
The present study obtained publishing consent from participants.

Availability of data and materials
The datasets used and/or analyzed during the present study are available from the corresponding author on reasonable request.

Competing interests
The authors declare that they have no competing interests.  The transcutaneous electrical stimulation device