Chiari malformation is a condition that occurs when the lower parts of the brain have been pushed down toward the spinal cord. The condition is estimated to occur in 1 in 1,280 people. Now, researchers have uncovered the debilitating effects of the condition by studying the brains of "toy dog" breeds.
This is according to a study recently published in the journal PLOS One.
The research team, led by Dr. Clare Rusbridge of the University of Surrey in the UK, says they hope the findings will improve the treatment of Chiari malformation (CM) for both humans and dogs.
According to the National Institute of Neurological Disorders and Stroke (NINDS), CMs are structural abnormalities in the brain's cerebellum - the region that controls balance.
These defects can occur when the skull is smaller than normal. This causes the cerebellum and brain stem (the bottom part of the brain that connects with the spinal cord) to be pushed downward into the foramen magnum (a large opening in the base of the skull) and the upper spinal canal.
The added pressure on the cerebellum and brain stem can stop the functions that are controlled by these areas, as well as block the flow of cerebrospinal fluid (CSF) to and from the brain. CSF is clear liquid that protects the brain and spinal cord.
Although some people have no symptoms of CM, many others can experience neck pain, problems with balance, muscle weakness, numbness in the arms and legs, dizziness, vomiting and hearing loss.
Individuals with CM often have other related conditions, including spinal curvature and spina bifida - a condition that can cause complete paralysis.
Toys dogs with CM 'have taller foreheads'
Selective breeding of "toy dogs" - such as griffon Bruxellois (also known as the Brussels griffon), Cavalier King Charles spaniels, Chihuahuas and their crosses - to make them look more "doll-like" has caused Chiari malformation (CM) to become much more common in dogs.
With this in mind, the investigators decided to take 14 brain, skull and vertebrae measurements of 155 griffon Bruxellois dogs.
Researchers found that griffon Bruxellois dogs with Chiari malformation had taller foreheads, which caused the shape of their brain to change.
These were compared with the measurements of three dogs with a mesaticephalic skull shape - a beagle and two Australian terriers. The measurements were also compared with three affenpinschers. These are toy dog breeds that are genetically close to the griffon Bruxellois.
On comparing the brain scans of griffons that had been affected by CM with the brain scans of normal dogs, the researchers found that dogs with CM had taller foreheads, which caused the shape of their brain to change.
Furthermore, severe forms of CM had caused the affected dogs' cerebellum to be pushed underneath the main part of their brain.
The investigators say their findings support the view that CM is a "multifactorial" condition.
They say it may be caused by shortening of the bones that form the base of the skull, "loss of convexity" in the supraoccipital bone (a bone on the dorsal side of the skull's foramen), the folding of the cerebellum under the occipital lobes (the main region for visual processing), and possibly the increased proximity of the brain to the back of the skull.
Commenting on the findings, Dr. Rusbridge says:
"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."
The researchers say they also want their findings to show the general public that breeding dogs in a certain way may not be in the animal's best interest.
"There are responsible breeders out there, who have invested in screening and who are breeding for health as well as producing attractive puppies, and it is vital that people only look to buy from them," adds Dr. Rusbridge.