In this work; based on our experience of the prenatal detection of fetal posterior fossa abnormalities by prenatal US and fetal MRI, we established a diagnostic approach based on our own iconography including:I. Cystic lesions of the foetal posterior fossa:A- Dandy-Walker malformation:

Dandy -Walker malformation (DWM) is the most common posterior fossa malformation(2). It is characterized by the triad of:

All three features must be present to categorise the anomaly as DWM (1-3)..

Axial ultrasound and MRI shows communication between the fourth ventricle and cisterna magna. Sagittal images show an abnormal fastigium, vermian agenesis or hypoplasia with upward rotation and elevation of the tentorium (Figure 1).

Ventriculomegaly is often present in DWM. Supratentorial anomalies such as corpus callosal dysgenesis, heterotopias, and occipital encephalocele are commonly seen in as sociation with DWM (3).

Mega-cisterna magna is defined as enlargement of the cisterna magna (3).

On axial ultrasound and MRI no communication will be seen between the fourth ventricle and the cisterna magna. Transverse measurement of the cisterna magna will be ≥ 10 mm. Sagittal images will show that the tentorium is normally inserted and oriented. Cerebellar biometry, morphology and echogenicity are normal. Brainstem morphology is normal. The fourth ventricle shape is normal. The tegmento-vermian angle is normal (4) (Figure 2).

Blake pouch cyst is defined as failure of fenestration of the Blake’s pouch. The vermian biometry and morphology is normal. The fourth ventricle is enlarged and the tegmento-vermian angle is ranged between 19 and 26 degree. The expanded Blake’s pouch is more echogenic than the subarachnoid space of the surrounding cisterna magna. The inferior vermis may be compressed by the expanded Blake’s pouch (3, 6).

Arachnoid cyst is an extra-axial cyst. It correspond to a duplication of the arachnoid between the cerebellum and the occipital vault without communication with the 4th ventricle. The cerebellar anatomy is normal. The tegmento-vermian angle is normal.

A large cyst can lead to focal ascension of the tentorium and in some cases com press CSF pathways, leading to hydrocephalus (5).

The expanding porencephalic cyst results from an ischaemo-haemorrhagic parenchymal insult. This results in focal cerebellar liquefaction, leading to a cystic lesion, which can expand into the subarachnoid spaces and may therefore mimicanarachnoid cyst (4).

In global cerebellar hypoplasia, the transverse cerebellar diameter is smaller thanexpected for gestational age. Both the cerebellar hemispheres and vermis are small and sub arachnoid spaces are prominent (2, 5).

Posterior fossa is not enlarged and the tentorium cerebelli is in the correct position. The increase in size of the cisterna magna is relative and secondary to the absence or the decrease in size or volume (Figure 3) of the cerebellum.

Vermian hypoplasia is characterised by a small vermis that incompletely covers the 4th ventricle. The vermian height is smaller than expected for gestational age and the tegmento-vermian angle is usually between 24 and 40 degrees and must be mesured after 24 weeks’ gestation (Figure 4) (6).

Vermian agenesis is diagnosed by an abnormal cleft separating the cerebellar hemispheres on axial images with no vermis being visible (Figure 5).

In partial agenesis, a part of the vermis is present. The present part is anatomically of normal sharp and volume. Partial agenesis is always inferior (Figure 6). Abnormal communication between the fourth ventricle and the cisterna magna bordered on both sides by the cerebellar hemispheres (1, 3, 5).

Vermian agenesis/hypoplasia is distinguished from DWM by the absence of a posterior fossa cyst and a normal sized posterior fossa.

Joubert syndrome is a dysgenetic malformation of the midbrain. It is characterised by dysmorphology of the midbrain which on axial images resembles a molar tooth. This is due to abnormally deep interpeduncular fossa, parallel, thickened, and horizontally oriented superior cerebellar peduncles, associated with vermian hypoplasia (Figure 7)(1, 4, 5).

This is a rare malformation characterised by the absence of the cerebellar vermis with the cerebellar hemispheres fused across the midline. The cerebellum appears horseshoe shaped with horizontal orientation of the cerebellar folia (4).

It is seen in association with open spinal dysraphism. It is characterised by small posterior fossa, pear shaped cerebellum, descent of the brainstem and cerebellar vermis through the foramen magnum with tectal beaking. The 4th ventricle appears elongated and is low lying (Figure 8) (2, 3).

It is seen in association with open spinal dysraphism with herniation of meninges, cerebrospinal fluid (meningocele), and/or brain tissue (encephalocele) through a defect(6).