Kate, actually dilated ventricles are not the same as hydrocephalus (but hydrocephalus involves dilated ventricles). All my scanned dogs except one had dilated ventricles but none, according to Geoff Skerritt and Clare Rusbridge, has hydrocephalus. Hydrocephalus is a very extreme version of dilated ventricles... (see below). It also isn't the vetricles pushing down on the cerebellum (alone?) but the Chiari-like malformation and the fact that the skull typically slopes in and presses inwards at that point in this breed, causing hind brain compression (compression on the cerebellum). When this is extreme it actually forces the cerebellum down into the opening into the spinal canal (the foramen magnum).
According to Clare Rusbridge, no one yet knows what role dilated ventricles play, if any. In some breeds, huge dilated ventricles are actually normal. The neurologists note them on scans on the possibility that this may be significant and it may be useful to have those readings in future.
From Clare's website:
You can see an image there as well: http://www.veterinary-neurologist.co.uk/part2.htm#8What is ventricular dilatation / ventriculomegaly?
The ventricles are four communicating cavities within the brain that are continuous with the central canal of the spinal cord. The four ventricles consist of the two lateral ventricles, the third ventricle and the fourth ventricle: They are filled with CSF, which is formed by structures called choroid plexuses located in the walls and roofs of the ventricles. When there is an obstruction at the FM the CSF can “back up” in the ventricular system, dilating the cavities. When ventricular dilatation is considered an incidental finding it is often described in MRI reports as ventriculomegaly. Severe ventricular dilatation resulting in brain compromise is described as hydrocephalous. Hydrocephalous as a consequence of CM is rare and most cases with ventricular dilatation have no associated clinical signs. Ventriculomegaly is also seen in other situations unrelated to CM and is particularly common in brachycephalic (dome headed) breeds.
I know a couple of the researchers do consider dilated ventricles to be significant in some way as yet unknown, and they often dilate further as SM progresses.