Mid-sagittal visualisation of the vermis and, therefore, accurate classification of posterior fossa abnormalities as part of the sonographic second-trimester screening can be challenging, making it a frequent indication for fetal magnetic resonance imaging (MRI) [1,2,3]. While Dandy-Walker malformation and confirmed hypoplasia of the cerebellar vermis correlate with adverse neurologic development in a large proportion of cases, about 90% of fetuses with either isolated Blake’s pouch cyst or mega cisterna magna are reported to have normal developmental outcome [4]. Delayed rotation of the cerebellar vermis may be an imaging pitfall and a potential risk of unnecessary pregnancy interruption [5].
Blake’s pouch cyst is believed to be caused by failed fenestration of the Blake’s pouch, an outpouching of the posterior membraneous area continuous with the fourth ventricle. Embryologically, its neck becomes the foramen of Magendie usually around 18 weeks’ gestation. In cases of persistent Blake’s pouch, the vermis is elevated but the fastigial recess, the primary fissure, and the lobulation appear normal [6]. In contrast to Dandy-Walker malformation, the tegmento-vermian angle will be < 45° in case of a Blake’s pouch cyst or vermian hypoplasia, with an angle of approximately > 25° suggesting the latter rather than the former [7]. A detailed description is to be preferred over unspecific and sometimes confusing terms like “Dandy-Walker variant” or “Dandy-Walker spectrum” [8, 9]. Unfortunately, the term “inferior vermian hypoplasia” is increasingly used despite known limitations in our ability to distinguish it from persistent Blake’s pouch [2, 5, 6]. In his much-noticed editorial on the pre- and misconception of inferior vermian hypoplasia, Robinson pointed out that, since the vermis develops more in a ventral-to-dorsal direction, it may not necessarily be the inferior vermis that is abnormal, or the vermis may not be abnormal at all. [6]. Albeit difficult to detect, the position of the choroid plexus is a potential discriminator between true vermian hypoplasia and Blake’s pouch cyst [6, 10].
In fetal MRI, switching to higher field strength requires adapted imaging sequences, handling of artefacts, and a limitation of the specific absorption rate to 4 W/kg for maternal whole-body exposure [11]. Superior image resolution at 3 Tesla as compared to 1.5 Tesla has recently been proven for a number of brain structures including the cerebellum between 20 and 24 weeks' gestation [12]. Follow-up imaging at a later gestational age most probably further increases diagnostic accuracy. However, spontaneous resolution of Blake’s pouch in the third trimester will not necessarily answer the question if the vermis is normal or not.
Here, we aim to illustrate the potential of second- and third-trimester MRI at 3 Tesla and assess its added value in persistent Blake’s pouch with suspected vermian hypoplasia.