Schmidt MJ, Kramer M, Ondreka N.
Our aim was to determine the relative volume of the occipital bone of Cavalier King Charles spaniels with and without syringohydromyelia and normal French bulldogs to reappraise the role of a possible insufficiency of the paraxial mesoderm in the pathogenesis of the caudal occipital malformation syndrome. Analysis of the occipital bone volume of 43 dogs based on computed tomography datasets was performed. Volume was determined by means of three-dimensional models. Using manual segmentation of the occipital bone in sagittal, transverse, and dorsal images, the volume of the occipital bone and the rest of the skull was calculated. The absolute occipital bone volume was put in relation to the total skull volume, the occipital bone index. For the Cavalier King Charles spaniels without syringomyelia, the median occipital bone index was 0.0681; for Cavalier King Charles spaniels with syringomyelia, it was 0.0646 and for French bulldogs, it was 0.0676. There was no global difference of the occipital bone index between examined groups (P = 0.4331). A reduced volume of the occipital bone was not found in Cavalier King Charles spaniels in general in comparison to French bulldogs, or in Cavalier King Charles spaniels with syringomyelia compared to Cavalier King Charles spaniels without syringomyelia. These results do not support occipital hypoplasia as a cause for syringomyelia development, challenging the paraxial mesoderm insufficiency theory. This also suggests that the term Chiari-like malformation, a term derived from human studies, is not appropriate in the Cavalier King Charles spaniel.