What causes Chiari-like malformation (CM)?
CM is not yet fully understood. Somehow the miniaturisation process in the Cavalier went awry and unlike many other toy breeds the brain did not decrease in size in proportion with the skull. The Cavalier appears to have a brain more appropriate for a bigger dog. Studies in the Griffon Bruxellois (Rusbridge et al 2009) have suggested that CM in this breed is characterised by a short skull base. This shortening results in an compensatory increase in the size of some of the other skull bones meaning that the forebrain is adequately accommodated however there is no compensatory increase in size of the back of the skull meaning that there is not enough room for the cerebellum and brain stem.