SYSTEMATIC REVIEW AND META-ANALYSIS OF ISOLATED POSTERIOR FOSSA MALFORMATIONS ON PRENATAL ULTRASOUND: NOMENCLATURE, DIAGNOSTIC ACCURACY AND ASSOCIATED ANOMALIES

OBJECTIVE: The aim of this systematic review and meta-analysis was to explore the pregnancy outcome in fetuses with prenatal diagnosis of isolated posterior fossa anomalies. METHODS: Medline and Embase were searched electronically utilizing combinations of the relevant medical subject heading for "posterior fossa", and "outcome. The posterior fossa anomalies analysed were: Dandy Walker malformation (DWM), mega cisterna magna (MCM), Blake's pouch cyst (BPC) and vermian hypoplasia (VH). The outcomes observed were: the rate of chromosomal abnormalities, additional anomalies detected only at pre-natal magnetic resonance imaging (MRI), additional anomalies detected only at post-natal imaging and concordance between pre-natal and post- diagnosis. Only isolated cases of posterior fossa anomalies were included in the analysis, defined as having no cerebral or extra-cerebral additional anomalies detected at the scan. Quality assessment of the included studies was performed using the Newcastle-Ottawa Scale (NOS) for cohort studies. We used meta-analyses of proportions to combine data and fixed or random effects models according to the heterogeneity of the results. RESULTS: A total of 22 studies including 531 fetuses with posterior fossa anomalies were included in the systematic review. The prevalence of chromosomal abnormalities in fetuses with isolated DWM was 16.3% (95% CI 8.7-25.7). The prevalence of additional CNS abnormalities detected only at prenatal MRI but missed at ultrasound was 13.7 (95% CI 0.2-42,6), while those of additional CNS and extra-CNS anomalies missed at prenatal imaging and detected only after birth were 18.2% (95% CI 6.2-34.6) and 18,9 (6,3-36,2) respectively. Prenatal diagnosis was not confirmed in 28.2% (95% CI 8.5-53.9) of the cases (Table 4). None of the fetuses with isolated MCM tested prenatally were found to have a chromosomal abnormality (0%, 95% CI 0-4,7). There were no significant associations with associated anomalies detected at pre-natal MRI, nor with associated CNS and extra-CNS anomalies detected after birth and missed pre-natally. Prenatal diagnosis was not confirmed in 7.1% (95% CI 2.3-14.5) of the cases. The rate of chromosomal anomalies in fetuses with isolated BPC was 5.2% (95% CI 0.9- 12.7) and there was no associated CNS anomaly detected only at prenatal MRI and missed at the scan (0%, 95% CI 0-6,4). Likewise, no associated CNS or extra-CNS anomalies were detected only after birth. Prenatal diagnosis of BPC was not confirmed after birth in 9.8% (95% CI 2.9-20.1) of the cases. The rate of chromosomal anomalies in fetuses with isolated VH was 6.5% (95% CI 0.8-17.1) and no additional anomalies were detected at prenatal MRI and missed at the scan (PP: 0%, 95% CI 0-45,9). Finally, the proportions of cerebral and extra-cerebral anomalies detected only after birth were 14.2% (95% CI 2.9-31.9) and 0% (95% CI 0-18,5), respectively. Prenatal diagnosis was not confirmed in 32.4% (95% CI 18.3-48.4) of the cases. CONCLUSIONS: Isolated DWM is a condition at high risk of chromosomal and associated structural anomalies. Isolated MCM and BPC have a low risk of aneuploidy or associated structural anomalies. The small number of cases with isolated VH precludes drawing any robust evidence regarding their management.