Patients referred with chronic pelvic pain remain a challenge for treating physicians. Often, these patients are classified as having bladder pain syndrome (BPS). Given the highly heterogeneous (clinical) nature of BPS, with a range of subtypes, it can be challenging to implement the appropriate treatment for each patient. Additionally, the majority of patients diagnosed with BPS lack cystoscopic and pathological evaluation to differentiate interstitial cystitis (IC) with or without Hunner's lesions (HL) from other inflammatory and non-inflammatory subtypes of BPS. Previous research on chronic pelvic pain illustrated the difficulty in distinguishing patients with IC from patients with detrusor overactivity based on urodynamic studies.[1] Some studies on the other hand, did show significant differences in specific urodynamic parameters, such as reduced cystometric capacity in patients with BPS/IC compared to patients with overactive bladder.[2] Nevertheless, the exact relationship remains unclear. The aim of this study was to evaluate whether urodynamic findings differ between patients with self-reported different types of BPS, and between patients with and without macroscopic inflammatory and non-inflammatory types of BPS.

A retrospective patient file study was conducted on all patients who underwent invasive urodynamics between 2005 and 2023 for chronic pain related to the urinary tract (N=206). For this study, only female patients were considered (N=78). We compared the results of the urodynamic studies of patients who consulted with bladder-filling-related pain (reported bladder-filling-related pain) with those with non-filling-related pain. The following parameters of filling cystometry were analyzed and correlated with cystoscopic findings: sensation of pain elicited by bladder filling during urodynamics (cystometric bladder-filling-related pain), maximum cystometric capacity (MCC), impaired bladder compliance defined as values <25 mL/cm H20, normal is defined as values >25 mL/cm H20, and presence of detrusor overactivity. Descriptive and comparative statistics were performed using SPSS. Data are presented as N(umber)(%) or mean (±Standard Deviation). Chi square test was used to evaluate categorical variables (with Fisher’s exact where necessary), and independent student’s t-test and analysis of variance (ANOVA) with post-hoc Bonferroni for continuous variables. Statistical significance was set at p<0.05.

In total, 78 female patients were included, with a mean age of 59.4±16.1 years, and of which 31 reported bladder-filling-related pain and 47 non-filling-related pain. In 81% of patients with reported bladder-filling-related pain, pain was reproduced during filling cystometry (p=0.031). In 56% of female patients reported non-filling-related pain, cystometric bladder-filling pain was found. 
The micturition frequency was significantly increased in patients with non-filling-related pain, compared to patients with reported bladder-filling-related pain, with a daytime frequency of 14±7 (vs 10±5 –p=0.015) and a nighttime frequency of 4.4±4.0 (vs3.7±2.2- p=0.005). There was no difference in MCC (360±187mL vs. 341±233mL) or presence of impaired bladder compliance between patients with reported bladder-filling-related pain and non-filling-related pain. In patients with reported bladder-filling-related pain, we observed significantly less detrusor overactivity (15% versus 38%) compared to those with non-filling-related pain (p=0.037). Among 62 patients who underwent cystoscopy, signs of bladder wall inflammation were present in 47% (N=29); 13% (N=8) had HL, 34% (N=21) had erythema. Cystometric bladder-filling-related pain was associated with the cystoscopic findings suggestive of inflammation: 61% (N=22) with cystometric bladder-filling-related pain (N=36) had either HL or erythema, and 81% (N=13) of women without cystometric bladder-filling-related pain (N=16) had no cystoscopic signs of inflammation (p=0.013). Furthermore, patients with HL or bladder wall erythema had a significantly smaller MCC than those with normal cystoscopic findings (163±97mL and 253±166mL respectively vs. 410±175mL, both p<0.01). Patients with HL also had a significantly higher nocturnal micturition frequency compared to the other two groups (average 7.3±3.1 versus 3.7±2.7 and 2.6±2.1, p<0.001 and p=0.016 respectively).

In this analysis, we compared female patients with reported bladder-filling-related pain with patients with non-filling-related pain. 
Reported bladder-filling-related pain was associated with cystometric bladder-filling-related pain in 81%. Detrusor overactivity was rarely present in women with reported bladder-filling-related pain, potentially explaining why medication aimed at detrusor relaxation (antimuscarinics or beta3 agonists) is less effective in this group. Remarkably, urodynamic and cystoscopic findings were better associated than reported pain and cystoscopic findings. All patients with HL experienced cystometric bladder-filling-related pain. However, only 25% of them had reported bladder-filling-related pain in daily life. Similarly, among patients with bladder wall erythema on cystoscopy, 83% had cystometric bladder-filling-related pain. Only 43% of these patients also reported bladder-filling-related pain in daily life. This could be an important finding to allow urodynamic differentiation between BPS subtypes. 
A limitation of this study are the missing data due to its retrospective nature.

This study indicates a clear association between reported bladder-filling-related pain, cystoscopic and urodynamic findings in women. In the vast majority of women with reported bladder-filling-related pain, this could be reproduced during filling cystometry. Women with reported bladder-filling-related pain had a very low prevalence of detrusor overactivity, suggesting that bladder relaxing drugs may be less effective in this population. Finally, signs of bladder wall inflammation were often found in women with cystometric bladder-filling-related pain and these patients also had a significantly smaller MCC. Therefore, cystoscopic findings appear to relate better with urodynamic findings than with reported pain symptoms.

Chin-Chieh Hsu, Ching-Chung Liang, Shuenn-Dyh Chang, Chih-Wei Chien, Wu-Chiao Hsieh. Comparison of urodynamic results and quality of life between women with interstitial cystitis and overactive bladder, Taiwan J Obstet Gynecol . 2020;59(1):39-42
Sung Han Kim, Tae Beom Kim, Soo Woong Kim, Seung-June Oh, Urodynamic findings of the painful bladder syndrome/interstitial cystitis: a comparison with idiopathic overactive bladder,. J Urol . 2009; 181(6):2550-4.