Z-scores of fetal bladder distention for the antenatal differential diagnosis of posterior urethral valves and urethral atresia

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: A prospective cross-sectional study was carried out on singleton healthy pregnancies in order to construct nomograms of fetal urinary bladder distension between 15 and 35 weeks' gestation. Z-scores were calculated and validated in a retrospective cohort of fetuses with megacystis. Cases of fetuses presenting with megacystis with ascertained postnatal or postmortem diagnosis were collected from a retrospective multicenter study. Correlations between anatomo-pathological findings, based on medical examinations of the infant or postmortem examinations, and fetal megacystis were established. The accuracy of the Z-scores was evaluated by receiver-operating characteristic (ROC) analysis. RESULTS: Nomograms of fetal urinary bladder distension were calculated from 3D ultrasound volumes in 225 pregnant women between 15 and 35 weeks of gestation. A total of 1238 measurements of urinary bladder were obtained. Subsequently Z-scores, derived from fetal nomograms, were calculated in 106 cases with suspected LUTO (including 76 cases with PUV (72%), 22 cases with UA (21%), 4 with urethral stenosis (4%) cases and 4 with Megacystis-Microcolon-Intestinal Hypoperistalsis syndrome (MMIH, 4%). Fetuses with PUV showed a significantly lower LBD Z-score compared to those with UA (3.95 vs 8.83, p = 0.00). By a ROC curve analysis, we identified 2.4 as optimal cut-off to discriminate fetuses with PUV from the rest of the study population (area under the curve (AUC) of 0.84 (CI 0.748-0.936 p = 0.00, sensitivity 85%, specificity 98.5%). CONCLUSIONS: Z-scores of LBD can reliably distinguish fetuses with LUTO caused by PUV from those with other subtypes of LUTO. This normative data will be useful for prenatal counseling and management of LUTO. This article is protected by copyright. All rights reserved.