Z-scores of fetal bladder size for antenatal differential diagnosis between posterior urethral valves and urethral atresia

Abstract

Objective: To construct reference values for fetal urinary bladder distension in pregnancy and use Z-scores as a diagnostic tool to differentiate posterior urethral valves (PUV) from urethral atresia (UA).

Methods: This was a prospective cross-sectional study in healthy singleton pregnancies aimed at constructing nomograms of fetal urinary bladder diameter and volume between 15 and 35 weeks' gestation. Z-scores of longitudinal bladder diameter (LBD) were calculated and validated in a cohort of fetuses with megacystis with ascertained postnatal or postmortem diagnosis, collected from a retrospective, multicenter study. Correlations between anatomopathological findings, based on medical examination of the infant or postmortem examination, and fetal megacystis were established. The accuracy of the Z-scores was evaluated by receiver-operating-characteristics (ROC)-curve analysis.

Results: Nomograms of fetal urinary bladder diameter and volume were produced from three-dimensional ultrasound volumes in 225 pregnant women between 15 and 35 weeks of gestation. A total of 1238 urinary bladder measurements were obtained. Z-scores, derived from the fetal nomograms, were calculated in 106 cases with suspected lower urinary tract obstruction (LUTO), including 76 (72%) cases with PUV, 22 (21%) cases with UA, four (4%) cases with urethral stenosis and four (4%) cases with megacystis-microcolon-intestinal hypoperistalsis syndrome. Fetuses with PUV showed a significantly lower LBD Z-score compared to those with UA (3.95 vs 8.83, P < 0.01). On ROC-curve analysis, we identified 5.2 as the optimal Z-score cut-off to differentiate fetuses with PUV from the rest of the study population (area under the curve, 0.84 (95% CI, 0.748-0.936); P < 0.01; sensitivity, 74%; specificity, 86%).

Conclusions: Z-scores of LBD can distinguish reliably fetuses with LUTO caused by PUV from those with other subtypes of LUTO, with an optimal cut-off of 5.2. This information should be useful for prenatal counseling and management of LUTO. © 2021 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

Keywords: congenital LUTO; fetal cystoscopy; fetal megacystis; fetal therapy; lower urinary tract obstruction; posterior urethral valves; urethral atresia; vesicoamniotic shunt.

Figure 1

Postmortem examination (a,b) and ultrasound images (c,d) in a 20 + 6‐week fetus with posterior urethral valves, tortuous ureters and multicystic renal dysplasia. RK, right kidney.

Figure 2

Postmortem examination (a–c) and ultrasound images (d,e) in a 23 + 2‐week fetus with urethral atresia. BLA, bladder; L, left; PU, posterior urethra; URE, ureter.

Figure 3

Receiver‐operating‐characteristics curve for Z‐score of fetal longitudinal bladder diameter in prediction of posterior urethral valves.