X-rays and Chiari malformation

Lateral skull radiographs of 2 Griffon Bruxellois (GB) and 2 Cavalier King Charles Spaniels (CKCS)

The dogs on the left are normal with no Chiari-like Malformation (CM) or Syringomyelia (SM). The dogs on the right have both Chiari-like Malformation (CM) or Syringomyelia .

Red – approximates outline of parietal and interparietal bone.
Blue approximates outline of supraoccipital bone.
Green approximates the basicranium.

In the dog with CM there is shortening of the basicranium, the supraoccipital bone is shorter and straighter and there is an apparent compensatory lengthening of the parietal bone.

Chiari-like malformation (CLM) and syringomyelia (SM) have recently been identified in the Griffon Bruxellois. This condition is characterised by mismatch in size between the brain (too big) and the skull (too small). There is insufficient room for the hindbrain and consequently the cerebellum and medulla are pushed out the back of the skull through a hole called the foramen magnum. This obstructs cerebrospinal fluid flow (CSF) and leads to pockets of fluid within the spinal cord substance. These fluid pockets are referred to as syringes (singular syrinx) or syringomyelia. Wide syringes can cause pain and other neurological signs. The purpose of this study was to describe CLM and SM in the GB and to determine if CLM could be identified by the radiographic dimensions of the skull.
Method
Head and upper spinal cord MRI from 56 GB dogs over 2.5 years of age were assessed for CLM and CSF pathway abnormalities. Lateral skull radiographs (i.e. X-rays of the head when the dog is laying on its side) were obtained in 33 of 56 dogs and measurements were taken from landmarks that could be identified on all the skulls in a consistent manner.  Four lines (2 horizontal and two vertical) were measured and the ratios of one length to another were then calculated. Dogs with and without CLM were compared statistically. Other factors such as age, sex, SM and dilatation of the lateral ventricles were also evaluated.
Results
In this selected group CM was present in 60.7 % of dogs and SM occurred with and without CLM (37.5% and 8.9% study population respectively). There was a significant association between having CLM and SM i.e. the dog was more likely to have SM if CLM was present. The radiographic study demonstrated that one measurement ratio could be used to predict CLM (sensitivity 87%, specificity of 78% i.e. low false positive rate and moderate false negative rate). This measurement ratio reflected the height the back of the skull (shorter in CLM dogs) to the length of the back of the skull in particular the parietal bone (longer in CLM dogs). Syringomyelia could not be predicted by the measurement ratios however female dogs with syringomyelia had more exaggerated skull changes.  Age was not significant for the presence of CLM. Pedigree analysis suggested a hereditary predisposition to both CLM and SM and that there may be a “carrier” state for CLM as normal dogs may have affected offspring.
Conclusions
The study suggested that CLM in the GB is characterised by a shortening of the base of the skull and also the supraoccipital bone which forms the back of the skull. To compensate for the reduction in these bones, the parietal skull bone, forming the roof of the skull, lengthens so that the forebrain is adequately accommodated. However there appears to be insufficient space at the back of the skull for the brain stem and cerebellum. The cause of this shortening is not known however it may be due to premature fusing of the sutures (joints) between some of the bones of the skull.
Another feature of SM in the GB was that it could occur without CLM suggesting other underlying causes of SM. Dogs with SM but without CLM tended to have dilatation of the entire ventricular system suggesting that there may be other causes of  obstruction of CSF flow though the foramen magnum.

Future studies are concentrating on the MRI characteristics, pathogenesis and genetics of this condition.

Chiari-C Rusbridge¹, SP Knowler¹, L Pieterse² AK McFadyen³

1 Stone Lion Veterinary Centre, Wimbledon, UK.
2 Statuesque Griffon Bruxellois, Sydney, Australia.
3. Glasgow Caledonian University, UK.