Posterior fossa abnormalities involving upward rotation of the cerebellar vermis (URCV) and posterior fossa expansion exist on a continuum that confuses fetal radiologists and clinicians. Variable terms with imprecise definitions for identical radiopathological entities limit evidence – based prognostic counselling and result in ill – informed pregnancy termination. The continuum prototype, the Dandy – Walker malformation (DWM), is defined by “hypoplastic” and “URCV” with “enlarged” fourth ventricle, “enlarged” posterior fossa, and “elevated” torcula without any of these terms being precisely defined. A recent study of 10 children with DWM found delayed global developmental in 78%, language in 100% and intellectual disability in 40% (Pinchevsky et al) broadly aligned with outcomes in other cohort studies (Bolduc et al, Klein et al). Early shunting of obstructed ventricles may mitigate morbidity (Klein et al). The persistent Blake’s pouch cyst (PBPC) or “isolated” URCV can have mild posterior fossa enlargement, reduction in vermis height (VH) and AP diameter (Pinto et al, Conte et al). Resolution of URCV by late fenestration by 26/40 occurs in 1/3 (Salsi et al). Contrasting DWM, isolated PBPC is associated with typical neurodevelopment in > 90% regardless of de-rotation (Gandolfi, Salsi et al). “Isolated inferior vermis hypoplasia (IIVH)” has likely been used, possibly erroneously (Goergen et al), to describe cases of URCV / PBPC on fetal MRI. The largest series of IIVH to date (n= 20, Tarui et al) demonstrated postnatal resolution of “IIVH” in 30% and typical neurodevelopment in 90%, including when “IIVH” persisted on postnatal imaging,similar to PBPC. Mega cisterna magna is typically associated with no URCV but likely reflects a variation of PBPC. Distinguishing delayed fenestration of PBPC and IIVH from DWM in the fetus is problematic due to imprecise terms artificially segregating entities on a continuum. Unfortunately, labelling drives counselling. Moreover, PBPC may evolve into an appearance indistinguishable from DWM.The fetal radiologist’s focus in PBPC/ DWM / IIVH image analysis should shift from labelling to understanding the strengths and limitations of imaging and non - imaging findings with purported prognostic significance for fetuses on the PBPC / IIVH / DWM continuum. A systematic “risk stratification” approach as outlined here can obviate the need for labelling.
1.Measurement of the cerebellar vermis to determine "hypoplasia". The vermis is reduced in size (in the range of 1 – 2 standard deviations below the gestational age mean for AP and CC diameter) in many cases of PBPC, is unassociated with persistence of URCV, is seen in cases where URCV resolves (Pinto et a, Conte et all) and failed to distinguish arbitrarily – labelled fetal MRI cases of PBPC and DWM in fetuses <23.1/40 (Nagaraj). Futhermore, compression of the vermis by the PBPC can mimic vermis hypoplasia. Using the term “vermis hypoplasia” or IIVH to describe normal mild reduction of vermis biometry in PBPC / URCV can create confusion with other prognostically adverse causes true vermis hypoplasia.
2.The “cerebellar tail sign (CTS)” as an indicator of Dandy – Walker malformation and by association, a more adverse prognosis. The cerebellar tail is is a specific dysplasia of the nodular lobule of the posterior cerebellum. The CTS on fetal MRI and ultrasound is a thick linear, posteriorly projecting extension of the midline posterior vermis. This now pathologically-proven (Haldipur et al) posterior vermian dysplasia with an imaging correlate has been demonstrated in human cases labelled DWM and a mouse model of DWM. Using the CTS to distinguish PBPC and DWM is problematic. Its clinical significance in the absence of classical DWM remains unclear due to lack of outcome data for fetuses with CTS and inclusion of cases in Haldipur et al's series whose imaging meets criteria for PBPC / URCV . Many fetal MRI case reports show examples of CTS followed derotation of the URCV to a normal appearance later (Chapman et al, Pinto et al, Conte et al) or persistent CTS with normal neurodevelopment (Guibaud et al).
3.Measurement of the tegmentovermian angle and cisterna magna depth to predict derotation and neurodevelopmental outcome. Conte et al found the combination of 2 or more of: tegmentovermian angle >23 degrees, cisterna magna AP diameter >9mm and tail sign predicted failure of vermis derotation with sensitivity of 84.2% specificity of 80.8% and NPV of 87.5% (95% CI, 70.9–95.2%) in 45 fetuses with mean gestation of 21.5 weeks. However, the neurodevelopmental consequence, if any, of failure of derotation is unclear and not examined in this study.
4.Coexistent brainstem and supratentorial malformations OR microarray, chromosomal + /- sonographic or fetal MRI evidence of a syndrome, present in up to 25% and 50% of fetuses with PBPC, respectively (D’Antonio et al. Behram et al) are prognostically important.
6. Subependymal nodular heterotopia and the “tilted telephone receiver sign” in association with mega cisterna magna and DWM, respectively, may point to FLNA pathogenic variant and PHACES and should be specifically sought in every case.

Acknowledgements

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