Fetal posterior fossa fluid collections

The term “posterior fossa fluid collections” refers to different conditions characterized by the presence of “cystic” areas in the posterior fossa ruled out during the second trimester anomaly scan. They include:
• Dandy Walker malformation (DWM)
• Cerebellar vermis hypoplasia (CVH)
• Blake’s pouch cyst (BPC)
• Megacisterna magna (MCM)
• Arachnoid cyst (AC)
The prognosis of these conditions is quite different: usually good in isolated BPC and MCM, frequently poor in DWM and CVH, depending on the cyst size in case of AC. For this reason the differential prenatal diagnosis is useful for a correct counseling.
The routine axial scan is can frequently be doubtful, particularly in differentiating DWM, CVH and BPC, which are all characterized by the presence of a median “cystic cleft” between the cerebellar hemispheres. In these case is extremely useful the midsagittal scan on the posterior fossa showing the brainstem and cerebellar vermis. This section allows evaluating the fourth ventricle, the shape and size of the vermis and its rotation in relation to the brainstem.
In DWM the vermis is severely hypoplasic and upward rotated; the posterior fossa is enlarged with high insertion of the tentorium.
In CVH the vermis is partially hypoplasic in its inferior area; it is slightly upward rotated; the size of the posterior fossa in normal as well as the insertion of the tentorium.
In BPC the vermis is normal with a slight upward rotation secondary to the posterior protrusion of a cystic dilatation of the forth ventricle, which is still not fenestred (Blake’pouch). The insertion of the tentorium is normal.
For the differential diagnosis the measurement of the angle between the vermis and the brainstem may be useful.

Anahtar Kelimeler