According to the International Continence Society (ICS), bladder outlet obstruction (BOO) is a general term characterized by increased detrusor pressure, reduced urinary flow rate, and/or increased postvoid residual volume (PVR) [1]. It is typically diagnosed by evaluating simultaneous measurements of urinary flow rate and detrusor pressure, along with the presence of PVR. BOO may result from anatomical or functional obstruction at the bladder neck and/or urethra.

The reported prevalence of BOO in women ranges from 2.7% to 29% [2, 3]. This wide range is largely attributable to the lack of standardized diagnostic criteria. Voiding dynamics differ between women and men; most women void with low detrusor pressure or by relaxing the pelvic floor muscles. In some women, voiding may be augmented by habitual abdominal straining, which can increase voided volume or flow rate. Because normal detrusor pressure during voiding is substantially lower in women than in men, male nomograms used to define obstruction are not directly applicable to women [4].

Diagnosing BOO in women can be challenging with common urological tools such as history, physical examination, and uroflowmetry. Uroflowmetry alone cannot determine whether a reduced flow rate is due to impaired detrusor contractility or anatomical obstruction. To address this limitation, the Blaivas-Groutz [5] and Solomon-Greenwell [6] nomograms have been developed. Among these, the most widely accepted video-urodynamic criterion for diagnosing female BOO is free-flow maximum flow rate (Qmax) <12 mL/sec in combination with detrusor pressure at maximum flow (PdetQmax) >25 cm H₂O [7]. However, there is still no universally accepted nomogram or set of pressure-flow criteria for diagnosing BOO in women.

In this study, we aimed to evaluate pressure-flow study findings in women with confirmed anatomical BOO and to assess the diagnostic performance of various urodynamic parameters according to obstruction location.

Women who underwent urodynamic evaluation at the Urodynamics Laboratory of Kart Dr. Lütfi Kırdar City Hospital between January 2007 and October 2023 and were diagnosed with anatomical BOO were retrospectively analyzed. The study was approved by the Ethics Committee of Kart Dr. Lütfi Kırdar City Hospital.

Inclusion criteria were age ≥18 years, a clinical diagnosis of anatomical BOO, and completion of a pressure-flow study. Patients were excluded if they were unable to void during the pressure-flow study; had dysfunctional voiding; had detrusor underactivity (projected isovolumetric pressure 1 <30 cm H2O and Watts factor <7 W/m²) [8]; or had known neurogenic lower urinary tract dysfunction (including central and peripheral nervous system lesions, neurospinal dysraphism, degenerative systemic disorders, or iatrogenic causes).

Urodynamic assessments were performed using the Solar System (Medical Measurement Systems, The Netherlands). All patients underwent free uroflowmetry, PVR measurement, filling cystometry, and a pressure-flow study. Unless otherwise specified, the methods, definitions, and units used in this study adhered to standards recommended by the ICS [9]. Cystometry was conducted using air-charged 7F vesical and 7F rectal catheters, with the patient in the sitting position. Bladder filling was performed with room-temperature normal saline at a physiological filling rate. Abdominal leak point pressure was measured at a bladder volume of 150 mL and every 100 mL thereafter during the Valsalva maneuver. Detrusor overactivity was provoked using coughing (3 times in the standing position) and the sound of running water. After reaching maximum cystometric capacity, the pressure-flow study was performed while the patient voided in the sitting position. The patient was left alone in the urodynamics room and instructed to void; intravesical, abdominal, and detrusor pressures; flow rate; and external sphincter electromyography (EMG) (via surface electrodes) were recorded simultaneously. Final PVR was also measured.

All patients with suspected BOO initially underwent urethral calibration using a Hegar dilator (size 5), followed by cystourethroscopy to confirm the diagnosis. Cystometry and pressure-flow studies were performed in all patients. When bladder neck obstruction was suspected, voiding cystourethrography (VCUG) was additionally performed, and surface sphincter EMG activity was evaluated during pressure-flow studies to distinguish functional outlet obstruction.

The following parameters were recorded and calculated: freeflow Qmax (mL/sec); the Blaivas-Groutz nomogram [5]; OCO (obstruction coefficient=PdetQmax/(40+2×Qmax)) [10]; BOOIf (Bladder Outlet Obstruction Index–Female=Pdet Qmax–2.2×Qmax) [6]; the Dybowski index (PdetQmax= 1.5×Qmax+10) [11]; URA (urethral resistance factor=[(1+4d Q² PdetQmax)^1/2–1] / [2d Q²]) [12]; and WOI (Woman Obstruction Index) [13]. Threshold values for obstruction were defined as BOOIf >18 [6], URA >20 [12], OCO >0.35 [10], and Dybowski index >10 [11]. The Barco-Castillo nomogram [14] was also used to evaluate obstruction.

Patients were categorized into 3 groups based on the anatomical location of the obstruction: external urethral meatus stenosis, urethral stricture, and bladder neck obstruction.

Continuous variables were presented as mean±standard error of the mean. Descriptive statistics were used to summarize patient characteristics. Data normality was assessed using the Shapiro-Wilk test. Comparisons between groups were performed using the chi-square test for categorical variables and the Kruskal-Wallis test for continuous variables. Post hoc comparisons were conducted using Dunn multiple-comparison test. A P-value <0.05 was considered statistically significant.

Seventy-two female patients who met the inclusion criteria were included in the analysis, with a mean age of 50.42±1.44 years. Of these, 18 (25.0%) had external urethral meatus stenosis, 49 (68.1%) had urethral strictures, and 5 (6.9%) had bladder neck obstruction. Table 1 summarizes the clinical characteristics and storage-phase urodynamic findings by obstruction location. Statistically significant differences were observed among groups in urethral caliber and free-flow Qmax (P<0.05). Post hoc analysis demonstrated that the significant difference in Qmax was between the urethral stricture and bladder neck obstruction groups (P=0.024), whereas the difference in urethral caliber was between the urethral stricture and meatal stenosis groups (P=0.015) (Table 1).

Detrusor overactivity was observed in 39 patients (54.1%), stress urinary incontinence in 4 (5.5%), and mixed urinary incontinence in 5 (6.9%). However, the distribution of incontinence types did not differ significantly by obstruction location (P=0.887).

Table 2 presents the pressure-flow study parameters. Statistically significant differences were observed in BOOIf, Dybowski index, URA, OCO, and WOI values among the groups (P<0.05). These differences were attributed to comparisons between the urethral stricture and meatal stenosis groups (P=0.007, P=0.008, P=0.009, P=0.010, and P=0.010, respectively). Similar to the other indices, the WOI value was higher in the urethral stricture group; however, since no defined cutoff value exists for WOI, no further analysis was performed (Table 2).

Obstruction detection rates according to anatomical localization are presented in Table 3. The Blaivas-Groutz nomogram identified BOO in all patients regardless of obstruction localization: 23.6% (n=17) were classified as mild, 40.2% (n=29) as moderate, and 36.2% (n=26) as severe obstruction. No statistically significant difference in obstruction severity was observed across anatomical groups (P=0.114) (Table 3).

BOO in women is a general term encompassing anatomical or functional narrowing at various levels, including the bladder neck, urethra, and external urethral meatus. In this study, urodynamic findings varied by the level of anatomical narrowing, suggesting that pressure-flow results are not uniform across BOO subtypes in women.

Both free-flow Qmax and urethral caliber differed significantly by obstruction location. Urethral calibration is a simple and effective method for assessing clinically meaningful urethral narrowing in women. Studies in healthy women have reported mean urethral diameters of 22F, 23.7±1.9F, and 26F [15]. Inability to pass a 14F dilator may support the diagnosis of female urethral stricture [16]. However, there is no consensus regarding a specific caliber threshold for this diagnosis. Some studies consider urethral diameters <12F to 20F to be indicative of pathological narrowing. Because calibration alone cannot determine the precise location or length of a stricture, additional diagnostic modalities such as cystourethroscopy or VCUG are recommended [17]. In this study, women with urethral strictures had the most marked reductions in Qmax and caliber. To our knowledge, no prior studies have examined the association between narrowing location and Qmax or caliber values. Although low Qmax may indicate urethral narrowing, it may also occur with nonanatomical causes or detrusor underactivity; therefore, Qmax should be interpreted alongside other diagnostic tools.

BOO was detected in >90% of cases using the Barco-Castillo, OCO, and Dybowski indices. Detection rates varied by anatomical location, with the lowest detection observed in external urethral meatus stenosis. Only the Barco-Castillo nomogram demonstrated a detection rate >90% in this subgroup. Because of the small number of women with bladder neck obstruction in our cohort, definitive comparisons for this subgroup were not possible. In contrast, urethral strictures were detected in >95% of women using the Barco-Castillo, OCO, and Dybowski parameters.

The Blaivas-Groutz nomogram is one of the most widely used tools for assessing BOO in women [5], but it has been criticized for potentially overdiagnosing obstruction [18]. Consistent with this concern, all women in our study were classified as obstructed by this nomogram, regardless of anatomical location. One major limitation of this nomogram is its reliance on 2 separate voiding events—1 invasive and 1 noninvasive—based on the assumption that their pressure characteristics are equivalent. In addition, the nomogram may classify patients as obstructed even in the absence of measurable flow if PdetQmax is high. Akikwala et al. [18] reported that some patients were diagnosed with BOO despite no clinical suspicion, based on discrepancies between invasive and noninvasive flow rates. According to this nomogram, a PdetQmax >60 cm H₂O indicates at least moderate obstruction, irrespective of flow rate. Therefore, a high-pressure detrusor contraction alone may lead to an overestimated diagnosis of BOO. Massolt et al. [19] applied the Blaivas-Groutz nomogram to incontinent women and reported an obstruction prevalence of 70%; however, no anatomical or clinical findings were present to support this, calling the nomogram’s clinical validity into question.

In this study, all urodynamic indices used to evaluate BOO were highest in the urethral stricture group. WOI, developed by Valentini, followed a similar trend. However, because no defined threshold exists for WOI, further interpretation was not possible.

According to the BOOIf nomogram by Solomon and Greenwell [6], a score >18 corresponds to a 90% probability of BOO. In this study, 79.17% of women with anatomical BOO had BOOIf values above this threshold. Stratified by location, 85.71% of women with urethral strictures and 55.56% of those with meatal stenosis had BOOIf >18. All bladder neck obstruction cases also met this threshold, but the small sample size may limit generalizability. Notably, among all parameters, BOOIf had the lowest diagnostic rate in the urethral stricture group. To date, no studies have compared the diagnostic accuracy of these indices (BOOIf, Dybowski index, OCO, and URA) by stricture location or overall BOO detection rates.

This study included 72 women, with adequate numbers in the urethral stricture and meatal stenosis groups. All patients underwent standard urodynamic testing (cystometry and pressure-flow studies) in accordance with ICS guidelines.

The retrospective study design, small sample size in the bladder neck obstruction subgroup, lack of a control group, and absence of video-urodynamic evaluation are the primary limitations of this study.

Unlike in men, there is no standardized method for evaluating BOO or detrusor contractility in women during pressure-flow studies. Diagnostic variability exists among the currently used nomograms and parameters, and it remains unclear which of these provides the most accurate assessment. Based on our findings, pressure-flow studies can effectively detect anatomical urethral and bladder neck obstruction. However, in women with external urethral meatus stenosis, relying solely on pressure-flow assessment may lead to underdiagnosis. The diagnostic variability by obstruction location highlights a major challenge in standardizing BOO diagnosis in women. Further comprehensive studies are required to fully elucidate this topic.