Outcome of apparently isolated fetal posterior fossa anomalies: systematic review and meta-analysis.

Objective: To report the outcomes of apparently isolated fetal posterior fossa anomalies diagnosed prenatally.

Methods: MEDLINE, EMBASE and Cochrane databases were searched from inception to 1 September 2024. The inclusion criteria were studies reporting the outcomes of fetuses with a prenatal diagnosis of apparently isolated posterior fossa anomaly, defined as an anomaly with no associated cerebral or extracerebral malformations at the time of the primary diagnosis, including mega cisterna magna (MCM), Blake's pouch cyst (BPC), Dandy-Walker malformation (DWM) and vermian hypoplasia (VH). The outcomes observed were: chromosomal or genetic anomalies; associated structural anomalies detected exclusively on follow-up ultrasound, fetal magnetic resonance imaging (MRI) or postnatal imaging; and adverse neurodevelopmental outcome. Random-effects meta-analyses of proportions were used to combine data.

Results: Thirty-one studies (including 676 fetuses with an apparently isolated posterior fossa anomaly and a known outcome) were included. In fetuses with a prenatal diagnosis of isolated MCM, there were no cases of chromosomal anomaly detected on G-banded karyotype analysis, copy number variants (CNVs) detected on chromosomal microarray analysis (CMA), anomalies detected on next generation sequencing (NGS), or associated structural anomalies detected on follow-up ultrasound or fetal MRI, while additional anomalies not detected on prenatal imaging were identified exclusively postnatally in 3.5% (95% CI, 0.7-8.3%) of cases. An abnormal neurodevelopmental outcome was reported in 4.9% (95% CI, 1.5-10.0%) of cases. In fetuses with a prenatal diagnosis of isolated BPC, chromosomal anomaly was reported in 2.7% (95% CI, 0.8-5.4%) of cases and CNVs detected on CMA were reported in 2.9% (95% CI, 0.5-7.0%), while there were no cases of genetic anomaly detected on NGS. There were no cases of additional anomalies detected exclusively on fetal MRI, while on postnatal imaging, 3.2% (95% CI, 0.8-7.1%) of fetuses with a prenatal diagnosis of BPC had an anomaly that was not detected prenatally. In-utero resolution of the cyst was reported in 42.7% (95% CI, 35.7-49.8%) of cases. An abnormal neurodevelopmental outcome was reported in 2.0% (95% CI, 0.5-4.5%) of cases. In fetuses with a prenatal diagnosis of isolated DWM, a chromosomal anomaly was detected in 10.7% (95% CI, 5.4-17.5%) of cases, CNVs were detected on CMA in 15.4% (95% CI, 6.2-27.8%) of cases and genetic anomalies were detected on NGS in 16.5% (95% CI, 4.6-33.8%) of cases. Associated structural anomalies detected exclusively on follow-up ultrasound and fetal MRI were reported in 8.2% (95% CI, 0.2-25.6%) and 17.3% (95% CI, 1.3-45.6%) of cases, respectively, while 11.3% (95% CI, 2.4-25.7%) of fetuses had associated structural anomalies detected only on postnatal imaging. Abnormal neurodevelopmental outcome affected 45.5% (95% CI, 17.3-75.4%) of cases. In fetuses with a prenatal diagnosis of isolated VH, a chromosomal anomaly was reported in 6.0% (95% CI, 0.7-15.9%) of cases, CNVs were detected on CMA in 15.4% (95% CI, 1.0-42.1%) of cases and genetic anomalies were detected on NGS in 17.2% (95% CI, 3.3-38.9%) of cases. There were no cases presenting with associated structural anomalies detected exclusively on follow-up ultrasound or fetal MRI, while 18.1% (95% CI, 5.4-36.0%) of cases had associated structural anomalies detected only on postnatal imaging. An abnormal neurodevelopmental outcome was reported in 23.6% (95% CI, 5.0-50.4%) of cases.

Conclusions: There are still significant weaknesses in the current literature on fetal posterior fossa anomalies, mainly characterized by the lack of standardized tools for neurodevelopmental assessment and the heterogeneity in timing of follow-up for most included studies. Fetuses with a prenatal diagnosis of isolated MCM or isolated BPC have a generally favorable outcome. Conversely, DWM is associated with a significant risk of genetic anomaly and adverse neurodevelopmental outcome, while the small number of cases with VH still precludes an objective evaluation of the actual burden of neurodevelopmental delay in these children. © 2025 International Society of Ultrasound in Obstetrics and Gynecology.