Diagnostic Accuracy of Uroflowmetry for Urethral Strictures in Pediatric Hypospadias: TIP versus Non-TIP Outcomes

Abstract

Introduction

This study aimed to evaluate the diagnostic accuracy of uroflowmetry in detecting pediatric urethral strictures following tubularized incised plate (TIP) and non-TIP urethroplasty.

Materials and Methods

A retrospective cohort study was conducted on children who underwent primary hypospadias repairs from June 2016 to June 2023 at our institution. Patients were categorized into the TIP and the non-TIP groups. Urethral calibration and uroflowmetry were used to evaluate urethral patency following urethroplasty. Data on demographic characteristics, perioperative information, uroflowmetry results, urethral calibration outcomes, and postoperative complications were collected.

The relationship between calibration and uroflowmetry and the diagnostic accuracy of uroflowmetry for urethral strictures were analyzed.

Results

A total of 62 cases were included, with 38 in the TIP group and 24 in the non-TIP group. Ten patients were diagnosed with urethral strictures. The maximum urinary flow rate (Q_max) exhibited a higher area under the curve (AUC) than the average urinary flow rate (Q_ave) in both the TIP and non-TIP groups. The Q_max in the non-TIP group demonstrated a higher AUC than in the TIP group (non-TIP: AUC = 0.94, cutoff = 6.65 ml/s, sensitivity = 100%, specificity = 81.0%; TIP: AUC = 0.80, cutoff = 5.75 ml/s, sensitivity = 100%, specificity = 58.1%). A significant quadratic correlation was found between Q_max and urethral calibration (non-TIP: C ² = 14.72 * Q_max, R ² = 0.96; TIP: C ² = 14.76 * Q_max, R ² = 0.88). The Q_max nomogram interval ≤ −3 standard deviation was a significant predictor for non-TIP urethral strictures (kappa = 0.70).

Conclusion

Uroflowmetry, particularly Q_max, shows promise as a noninvasive screening tool for detecting urethral strictures after hypospadias repair. It has high diagnostic accuracy in non-TIP cases but limited utility in TIP cases.

Keywords

urethral stricture - calibration - uroflowmetry - hypospadias

Introduction

Urethral stricture is a common complication following hypospadias repairs, posing significant management challenges and the risk of multiple reoperations.[1] Long-standing urethral strictures can lead to urinary retention, urinary tract infections, bladder dysfunction, and in severe cases, renal damage. In a minority of pediatric patients, urethral stricture may even result in erectile dysfunction.[2] Current diagnostic methods, such as retrograde urethrography and cystoscopy, are considered the gold standard for diagnosing urethral strictures.[3] [4] Urethral calibration is also a relatively straightforward method for diagnosing strictures and can be performed on an outpatient basis.[5] Sometimes, these invasive procedures need to be done under anesthesia, and may cause pain or other potential complications. Therefore, there is a need to explore noninvasive methods to reduce postexamination complications, minimize pain, and improve examination cooperation in pediatric patients.

Uroflowmetry records the volume of urine and the duration of micturition through electronic devices, providing the maximum urinary flow rate (Q_max) and the average urinary flow rate (Q_ave).[6] It is the best noninvasive urodynamic examination for detecting lower urinary tract obstruction.[7] However, the results of uroflowmetry do not always match the actual presence of urethral stricture in some patients,[8] [9] which limits its diagnostic utility in cases with suspicious urethral stricture. In clinical practice, further invasive examinations are still needed to confirm the presence of true urethral stricture in patients whose uroflowmetry suggests possible stricture. Moreover, current evidence is insufficient to recommend a specific cutoff value for Q_max or Q_ave to determine the appropriateness of treatment.

Low urinary flow rates are common after hypospadias repair, especially following tubularized incised plate (TIP) urethroplasty. Previous studies and clinical experience suggest that the diagnostic accuracy of uroflowmetry may vary between TIP and non-TIP patients.[10] [11] TIP urethroplasty utilizes only the urethral plate, while non-TIP procedures may involve the foreskin or other tissues. These differences and biological variations between tissues may affect postoperative urination, urethral compliance, and scar formation. Therefore, this study compares the diagnostic performance of uroflowmetry for urethral stricture in TIP and non-TIP patients using urethral calibration as the reference standard and explores the potential quantitative relationship between uroflowmetry and urethral calibration.

Materials and Methods

Study Design

This study is a retrospective cohort study. Data were collected retrospectively from consecutive patients who underwent primary hypospadias repairs at our center from June 2016 to June 2023. Patients were divided into TIP and non-TIP groups based on their primary surgical techniques. This study complied with all principles and regulations of the Helsinki Declaration and was approved by the Ethics Committee of our hospital, with the ethical number 2024-IRB-0262-P-01.

The inclusion criteria were as follows: (1) Primary hypospadias; (2) age <18 years; (3) the non-TIP group included patients who underwent one-stage and staged Duckett, Duckett + Duplay procedure, Onlay, and Mathieu procedures, and the TIP group consisted of patients who only underwent TIP procedure; (4) the availability of both urethral calibration data and uroflowmetry data within 1 month before calibration postoperatively; (5) follow-up >12 months. Exclusion criteria were (1) uroflowmetry data with a voided volume of less than 50 mL; (2) urethral dilation, cystoscopy, or catheterization were performed within 1 month prior to uroflowmetry; and (3) patients with concomitant posterior urethral valves or neurogenic bladder.

Surgical Techniques Nuances

The surgeries were all performed by doctors with more than 10 years of rich experience in hypospadias surgery. The materials for urethroplasty vary considerably among different hypospadias repair techniques. In the TIP or Duplay procedures, the native urethral plate is used as the urethroplasty material. By contrast, the Duckett procedure utilizes a vascular pedicled prepuce, and the Onlay or Mathieu procedures incorporate a combination of the native urethral plate and the prepuce. The severity of hypospadias cases suitable for different surgical techniques differs significantly. The one-stage or staged Duckett, or Duckett + Duplay procedure, is generally indicated for proximal hypospadias or those accompanied by severe chordee. Conversely, the TIP, Onlay, and Mathieu procedures are commonly used for mid or distal hypospadias. Moreover, the development state of the urethral plate plays a vital role in determining the appropriate surgical approach. The TIP procedure relies on a well-developed urethral plate. In contrast, the Onlay and Mathieu procedures show greater adaptability to cases with suboptimal urethral plate development.

Calibration and Uroflowmetry

Calibration could be conducted on an outpatient basis without anesthesia when there was a parental request or suspicion of urethral stricture. It could also be conducted under anesthesia during surgeries for associated conditions not requiring urethral reconstruction, such as prepuce epidermoid cysts, hydrocele, and undescended testes. Urethral calibration was performed by surgeons experienced in hypospadias repairs. The calibration value (C) was defined as the largest size of the urethral sound that could be inserted without significant resistance, breakthrough feeling, or causing noticeable pain for outpatients. The urethral stricture was defined as a calibration value less than 8 Fr (<1 year), less than 10 Fr (1 year to prepuberty), and less than 12 Fr (postpuberty).[12] [13] Uroflowmetry was performed by trained professionals using a uroflowmeter (Medical Measurement Systems, the Netherlands) in patients without untreated postoperative urethral fistulas or dehiscence. The assessment included Q_max, Q_ave, and Siroky nomogram intervals. Data were collected on demographic information, surgical techniques, uroflowmetry results, urethral calibration outcomes, and postoperative complications.

Sample Size

In this retrospective cohort study, participants were divided into two groups: the urethral calibration group and the uroflowmetry group, with urethral stricture serving as the primary outcome measure. The diagnostic difference between the two groups for urethral stricture is 10%, with a standard deviation (SD) of uroflowmetry being 4. Assuming a two-sided α of 0.05 (one-sided 0.025), a power of 0.9 (1 − β), and a noninferiority margin of 4, and referring to the method by Hueber et al,[10] the calculated sample size for the urethral calibration group is 23 cases, and for the uroflowmetry group is 23 cases. Considering a 10% loss to follow-up and refusal rate, a minimum of 26 cases per group is required.

Statistical Analysis

Data analysis was conducted using SPSS 26.0 and R 4.4.0. Non-normally distributed continuous variables were expressed as median with interquartile range (IQR), and comparisons between groups were made using the Mann–Whitney test. Categorical variables were expressed as frequencies and percentages, and comparisons between groups were made using chi-square tests, Kappa tests, or Fisher's exact tests. Missing values were replaced with the median. The receiver operating characteristic (ROC) curve was used to evaluate the diagnostic efficacy of uroflowmetry for urethral stricture, with the Youden index used to set the optimal cutoff value. Curve regression was used to investigate the correlation between uroflowmetry and urethral calibration. A p-value of less than 0.05 was considered statistically significant.

Results

Baseline Characteristics

A total of 62 children were included in the study ([Fig. 1]), with 38 in the TIP group and 24 in the non-TIP group, followed up for a median of 4.00 years (IQR 3.00–6.75). The median Q_max was 5.50 (IQR 3.50–7.50) ml/s in the TIP groups and 8.70 (IQR 6.60–10.62) ml/s in the non-TIP groups, showing a statistically significant difference between the two groups (p = 0.003). The median urethral calibration was 10.00 (IQR 9.25–11.00) Fr in the TIP group and 11.00 (IQR 10.00–13.00) Fr in the non-TIP group, also showing a statistically significant difference (p = 0.021). Ten children were diagnosed with urethral strictures. Baseline characteristics are presented in [Table 1].

Table 1 Baseline characteristics

Variables	Total (n = 62)	TIP (n = 38)	Non-TIP (n = 24)	Statistic	p-value
Surgical information
Age at surgery (months), M (Q1, Q3)	15.00 (9.00, 31.50)	14.50 (8.25, 36.25)	15.00 (10.00, 23.00)	Z = − 0.06	0.949
Glans diameter (mm), M (Q1, Q3)	14.00 (13.00, 15.00)	14.00 (13.00, 16.50)	13.25 (13.00, 14.00)	Z = − 1.33	0.182
Degloved curvature, M (Q1, Q3)	20.00 (5.00, 45.00)	15.00 (2.50, 20.00)	60.00 (45.00, 90.00)	Z = − 4.97	<0.001
Urethral plate width (mm), M (Q1, Q3)	4.00 (2.50, 5.00)	4.00 (2.75, 5.00)	2.50 (2.00, 4.50)	Z = − 0.95	0.345
Meatus location[a], n (%)				−	0.006
Proximal	18 (32.73)	7 (18.92)	11 (61.11)
Midshaft	30 (54.55)	25 (67.57)	5 (27.78)
Distal	7 (12.73)	5 (13.51)	2 (11.11)
Surgical techniques, n (%)				−	−
TIP	38 (61.29)	−	−
Non-TIP	24 (38.71)	−	−
One-stage Duckett	11 (45.83)	−	−
Staged Duckett	6 (25.00)	−	−
Duckett and Duplay	5 (20.83)	−	−
Onlay	1 (4.17)	−	−
Mathieu	1 (4.17)	−	−
Reconstruction length (mm), M (Q1, Q3)	25.00 (18.00, 40.00)	20.00 (15.00, 25.00)	40.00 (36.50, 46.25)	Z = − 5.42	<0.001
Uroflowmetry
Age at uroflowmetry (years), M (Q1, Q3)	4.00 (3.00, 6.75)	4.50 (3.00, 6.75)	4.00 (3.00, 5.50)	Z = − 0.30	0.764
Qmax, M (Q1, Q3)	6.60 (4.78, 9.50)	5.50 (3.50, 7.50)	8.70 (6.60, 10.62)	Z = − 2.92	0.003
Qave, M (Q1, Q3)	4.50 (3.10, 5.80)	4.05 (2.50, 4.68)	5.55 (4.45, 6.45)	Z = − 3.22	0.001
Calibration
Calibration, M (Q1, Q3)	10.00 (10.00, 12.00)	10.00 (9.25, 11.75)	11.00 (10.00, 13.00)	Z = − 2.06	0.040
Age at calibration (years), M (Q1, Q3)	4.00 (2.00, 6.00)	5.00 (3.00, 6.75)	3.00 (2.00, 5.00)	Z = − 1.34	0.180
Postoperative complications
Urethral fistula, n (%)				χ2 = 0.08	0.773
No	35 (56.45)	22 (57.89)	13 (54.17)
Yes	27 (43.55)	16 (42.11)	11 (45.83)
Urethral diverticulum, n (%)				χ2 = 3.69	0.055
No	56 (90.32)	37 (97.37)	19 (79.17)
Yes	6 (9.68)	1 (2.63)	5 (20.83)
Residual curve, n (%)				−	1.000
No	60 (96.77)	37 (97.37)	23 (95.83)
Yes	2 (3.23)	1 (2.63)	1 (4.17)

Abbreviations: −, Fisher's exact; M, median; Q₁, first quartile; Q₃, third quartile; Q_ave, average urinary flow rate; Q_max, maximum urinary flow rate; TIP, tubularized incised plate; Z, Mann − Whitney test; χ², chi-square test.

^a Only 55 cases were reported in the meatus location.

Fig. 1 Flow chart. TIP, tubularized incised plate.

Diagnostic Accuracy of Q_max and Q_ave for Urethral Stricture

ROC analysis was used to assess the diagnostic accuracy of Q_max and Q_ave for urethral strictures ([Supplementary Fig. S1] [available in the online version only]). In the non-TIP group, the area under the curve (AUC) for Q_max was 0.94 (cutoff = 6.65 ml/s, sensitivity = 100%, specificity = 81.0%), higher than that of Q_ave with 0.91 (cutoff = 4.95 ml/s, sensitivity = 100%, specificity = 71.4%). In the TIP group, the AUC for Q_max was 0.80 (cutoff = 5.75 ml/s, sensitivity = 100%, specificity = 58.1%), higher than that of Q_ave with 0.70 (cutoff = 3.20 ml/s, sensitivity = 71.4%, specificity = 67.7%).

Diagnostic Accuracy of Uroflowmetry Nomogram Intervals for Urethral Stricture

Uroflowmetry Siroky nomogram intervals Q_max ≤ −3SD, Q_max < − 2SD, Q_ave ≤ −3SD, and Q_ave < − 2SD were used to perform Kappa consistency tests for the diagnosis of urethral stricture. In the non-TIP group, the Q_max nomogram interval ≤ −3 SD showed good consistency for the diagnosis of urethral stricture (kappa = 0.70), with the ROC curve showing an AUC of 0.95 (cutoff = 0.5, sensitivity = 100%, specificity = 91.0%; [Supplementary Fig. S1] [available in the online version only]). In contrast, the intervals Q_max < − 2SD, Q_ave ≤ −3SD, and Q_ave < − 2SD showed poor consistency for the diagnosis of urethral stricture (kappa = 0.29, 0.00, and 0.50, respectively). In the TIP group, the intervals Q_max ≤ −3SD, Q_max < − 2SD, Q_ave ≤ −3SD, and Qave < − 2SD also showed poor consistency for the diagnosis of urethral stricture (kappa = 0.37, 0.13, −0.18, and 0.08, respectively).

Relationship between Uroflowmetry and Urethral Calibration

Curve regression analysis was used to examine the relationship between uroflowmetry and urethral calibration ([Supplementary S2] [available in the online version only]). In the non-TIP group, a strong quadratic correlation was found between urethral calibration and Q_max (C ²= 14.72 * Q_max, R ²= 0.96, p < 0.001), which was superior to the correlation in the TIP group (C ²= 14.76 * Q_max, R ²= 0.88, p < 0.001). Similarly, in the non-TIP group, a strong quadratic correlation was found between urethral calibration and Q_ave (C ²= 23.92 * Q_ave, R ²= 0.96, p < 0.001), which was superior to the correlation in the TIP group (C ²= 23.21 * Q_ave, R ²= 0.87, p < 0.001).

Discussion

This study evaluates the diagnostic accuracy of uroflowmetry in detecting urethral strictures following hypospadias repair in children. By comparing uroflowmetry results with urethral calibration, we found that uroflowmetry, particularly Q_max, can effectively indicate the presence of urethral strictures in non-TIP patients. For non-TIP patients with a urine flow rate lower than 6.65 ml/s, the presence of urethral stricture should be considered. The diagnostic accuracy of urethral stricture is superior when the nomogram interval is less than −3SD compared with −2SD. The use of the formula C ²= 14.72 * Q_max allows for the noninvasive estimation of urethral calibration. The diagnostic significance of uroflowmetry for urethral strictures in TIP patients is limited, yet it still holds considerable value for screening purposes. In summary, uroflowmetry can serve as a routine follow-up procedure after hypospadias surgery to aid in the detection of urethral strictures.

In the diagnosis of urethral strictures, methods like retrograde urethrography, cystoscopy, urethral calibration, and uroflowmetry are commonly used.[11] Visual methods such as retrograde urethrography and cystoscopy show stricture details but have low patient compliance (54.4%).[14] Urethral calibration, using a sound to measure urethral circumference, is a simple diagnostic method for urethral strictures that can be performed in an outpatient setting. Wein et al[7] and Snodgrass and Bush[15] suggest that calibration <8 Fr with urination symptoms indicates stricture. However, some researchers find it hard to differentiate calibration from dilation due to potential stricture dilation during the procedure.[16] Therefore, the current study defines calibration as the maximum size of sound entering without significant resistance, breakthrough sensation, or severe pain for outpatients. Considering the convenience of clinical application, urethral calibration was chosen as the main diagnostic method, with only a few patients undergoing retrograde urethrography and cystoscopy (not included in the data).

Uroflowmetry is being investigated as an independent diagnostic tool for urethral stricture. Erickson et al[17] demonstrated that reduced uroflowmetry accompanied by urinary symptoms has 99% sensitivity and 98% specificity for diagnosing urethral strictures. Yanagi et al[18] showed the effectiveness of uroflowmetry in predicting the anatomical success of urethroplasty, with positive predictive values of 86% for Q_max and 87% for ΔQ (Q_max − Q_ave). Considering that ΔQ may reflect urinary flow obstruction,[19] the current study included ΔQ in analyses, finding it not more accurate than Q_max, as detailed in [Supplements S3] and [S4] (available in the online version only). Lambert et al[20] established a predictive model for adult urethral strictures, using the equation [1.675Ln(Q_max) − 4.360Sqrt(ΔQ) + 4.146] to score patients, distinguishing healthy individuals from those with urethral strictures. These studies highlight uroflowmetry's value, but most were on adults, and pediatric data are lacking. Unlike adults, where posterior strictures are common, children mostly have anterior strictures posthypospadias surgery.[21] This study proposed a noninvasive formula using Q_max for pediatric strictures, which holds practical significance for clinical application.

The results of this study indicate that in the non-TIP group, both Q_max and Q_ave have high sensitivity and specificity in diagnosing urethral strictures, with Q_max showing superior performance. Boroda et al[22] proposed that the repaired hypospadias urethra, especially those with longer tube lengths, may not behave like a standard rigid tube, which could affect Q_max more significantly than Q_ave. Furthermore, our study indicates that the urinary flow rate nomogram interval of less than −2SD does not effectively reflect the presence of urethral stricture. This is likely because the standards for obstructive urination established from adult studies are not applicable to children. A threshold of ≤ −3SD can effectively reflect urethral stricture in children who have undergone non-TIP surgical techniques. This is important because children have smaller bladders and urethras, leading to lower flow rates compared with adults. Additionally, structural abnormalities are inherent in hypospadias cases in children. Therefore, using a threshold of ≤ −3SD offers greater diagnostic accuracy and sensitivity for urethral stricture compared with the < − 2SD.

The correlation between uroflowmetry and urethral calibration in TIP patients is not as good as in non-TIP patients, which may be related to the “gourd-shaped” configuration of the urethra[23] or decreased urethral compliance[10] following TIP surgery. Due to possible structural and/or functional abnormalities of the urethral corpus spongiosum in children with hypospadias, the spongiosum is limited in expansion during urination, leading to low uroflow rates. Nonetheless, such abnormalities do not impact the outcomes of urethral calibration assessments. While uroflowmetry's diagnostic value is limited, its 100% sensitivity and noninvasive nature offer significant screening value. long-term observational studies have demonstrated a substantial increase in uroflowmetry values during puberty for TIP patients. Consequently, the uroflowmetry and urethral calibration of TIP patients may become more aligned after puberty, which requires further follow-up studies to validate.[11] [24]

This study has several limitations. First, as a retrospective study, the long follow-up period and the fact that some patients were followed up locally led to missing data. The use of median values to replace missing data may introduce information bias. Second, uroflowmetry results can be affected by the volume of urine voided, making it difficult to standardize the voided volume, which may impact the results. However, this study analyzed the uroflowmetry nomogram intervals to minimize the influence of voided volume on uroflowmetry results. Third, urethral calibration is not a routine examination for patients with normal urine flow and urine flow rate, leading to a lack of a large amount of normal urethral data and causing selection bias. Lastly, this study is based on a single center with a small sample size, and it did not grade the severity of urethral strictures, which limits the generalizability of the findings. Therefore, future multicenter, large-sample, prospective cohort studies are needed to further validate our findings and refine the diagnostic thresholds of uroflowmetry for urethral strictures of varying severities.

Conclusion

Uroflowmetry, particularly Q_max, shows promise as a noninvasive screening tool for detecting urethral strictures after hypospadias repair, with high diagnostic accuracy in non-TIP cases but limited utility in TIP cases. Further multicenter prospective studies are needed to validate these findings, refine diagnostic thresholds, and integrate uroflowmetry into standardized pediatric follow-up protocols.

Conflict of Interest

None declared.

Authors' Contributions

W.R.: Concept and design, methodology, data verification, statistical analysis, revision of the manuscript, and funding acquisition. H.L.: Concept and design, methodology, data collection, statistical analysis, and manuscript drafting. J.Z.: Concept and design, methodology, data collection, revision of the manuscript, and critical appraisal of the manuscript. Q.H.: Methodology, statistical analysis, revision, and critical appraisal of the manuscript. W.Y.: Reviewed proposal, statistical analysis, revision, and critical appraisal of the manuscript. L.H.: Reviewed proposal, revision, and critical appraisal of the manuscript. G.C.: Concept and design, methodology, project administration, revision, and critical appraisal of the manuscript. X.Y.: Concept and design, methodology, project administration, revision, final approval of the version to be published.

Ethical Approval

This study was approved by the Ethics Committee of Children's Hospital, Zhejiang University (2024-IRB-0262-P-01).

Data Availability

Data are available upon request.

Consent for Publication

This is a retrospective cohort study using an anonymized dataset.

^* These authors contributed equally.

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Publication History

Received: 26 September 2024

Accepted: 09 February 2025

Accepted Manuscript online:
11 February 2025

Article published online:
27 March 2025