It has been known for some time that some dog breeds such as the Cavalier King Charles spaniels can have the Chiari-like malformation (CLM). Their small skulls crowd the brain and result in herniation of the cerebellum and brain stem through the foramen magnum. About 50% also have syringomyelia.
Among the clinical signs, as described by the Canine Inherited Disorders Database, is “phantom” scratching:
“The most important and consistent sign of CM/SM is pain, which your dog may show through crying out, withdrawn behavior, reluctance to exercise, intolerance of a neck collar or touch about the head and neck, and sleeping with the head raised. Affected dogs may have scoliosis (curvature of the spine), weakness and poor coordination and they may scratch on one side without touching the skin – called “phantom” scratching.”
Chiari-like malformation has also been found in other "toy" breeds such as Chihuahuas and the Brussels griffon. In order to determine the skull anatomy that results in CLM, a team of investigators from the United Kingdom, U.S. and Canada performed MRI measurements on skulls of “Griffon Bruxellois dogs with and without Chiari-like malformation and syringomyelia and identified several significant variables.” In the study, published recently in PLOS ONE, the authors
"... found that in the Griffon Bruxellois dog, Chiari-like malformation is characterized by an apparent shortening of the entire cranial base and possibly by increased proximity of the atlas to the occiput. As a compensatory change, there appears to be an increased height of the rostral cranial cavity with lengthening of the dorsal cranial vault and considerable reorganization of the brain parenchyma including ventral deviation of the olfactory bulbs and rostral invagination of the cerebellum under the occipital lobes."
"Our latest discoveries will be significant in driving this research forward. Our next steps will be to apply our technique to other breeds with Chiari malformation such as the Cavalier King Charles spaniel and Chihuahua."
"We also want to investigate more sensitive ways of screening so that risk of disease can be detected easier, at an earlier age and with a single MRI scan."