There is a CSF intensity structure at the posterior fossa posterior to the cerebellum, mainly on the left side. The left cerebellar hemisphere seems to be smaller in size as compared to the right side. This could be either due to:
- loss of volume of this cerebellar hemisphere with subsequent passive dilatation of the related CSF spaces, as with ischemia, however, there is no supporting clinical data or underlying signal alteration seen.
- a compressing posterior fossa cystic lesion such as an arachnoid cyst, epidermoid, or a neoplastic process, however, there seems to be no mass effect exerted on the cerebellum. On the sagittal plane, the cerebellar surface opposing this CSF structure is convex, rather than concave which would've been the case if there was mass effect. Also, the lesion exerts no mass effect or midline shift exerted on the fourth ventricle as well.
- if this CSF space is not a compressing extra-axial structure nor an area of volume loss, then this small sized cerebellar hemisphere is likely due to hypoplasia. Considering the fact that this CSF structure seems to be in continuity with the cisterna magna, i.e no definite septum or separation is seen, and communicating with the fourth ventricle, Dandy Walker spectrum of abnormalities has to be considered.
Still, the findings we have here do not meet the classic triad of Dandy-Walker malformation, which are:
- enlarged posterior fossa with cystic dilatation of the fourth ventricle
- vermian hypoplasia (which is present in this case and best demonstrated at the coronal and axial planes)
- upward rotation of the torcular herophili and vermis
In this case, we rather have a normal sized posterior fossa with a cystic structure that seems to communicate with the fourth ventricle. Still there is vermian hypoplasia (best demonstrated in the coronal and axial planes). The torcular herophili and tentorium are of normal orientation. Therefore, we rather describe these findings as Dandy-Walker variant that represent a less severe form of Dandy-Walker malformation.
Another incidental interesting finding is the discrepancy in caliber between both transverse sinuses, as the right side seems to be of much prominent caliber than the left side, best appreciated on the sagittal plane appearing as a signal void structure on T2, just posterior to the tentorium. This is rather a normal variant but in this imaging study it can give a false impression on the axial images that there is asymmetry between the occipital bones on either side, which can lead to the conclusion that there is an expansive cyst on the left side causing scalloping of the skull. However by going back to the coronal and sagittal images we see that the occipital bones are of symmetrical contour and caliber.