Chiari malformation is the term used in human medicine that equates in veterinary medicine to Caudal Occipital Malformation Syndrome, COMS. The most commonly affected breed with COMS is the Cavalier King Charles Spaniel. With the advent of MRI we are finding this condition in several other breeds as well such as the Chihuahua, Yorkshire Terrier, and other small breeds of dogs.
COMS is a condition where the base of the skull in the dog is abnormally shaped. This leads to a more 'rounded skull' and, in turn, causes the part of the brain at the base of the skull (the cerebellum) to be 'crowded'. Because this skull defect is present at birth and through the growth phase of the puppy, once the dog reaches adulthood, the skull changes are static and don't change. But the after-effects are progressive.
This is a sagittal MRI of a dog with COMS. You can see the cerebellum at the base of the skull has slipped out of the base of the skull into the 1st vertebrae of the neck in what is termed a cerebellar herniation. The SM secondary to the COMS is present as the white area in the spinal cord just behind the cerebellum.
You see, COMS is associated with another condition that takes place in the spinal cord 'downstream' from the skull in the dog's neck. This condition is called syringohydromyelia or SM. SM is a progressive degenerative condition in the spinal cord that can happen anytime the flow of spinal fluid is disrupted.
Scoliosis, or a left-right bending malformation f the spinal cord can also happen in cases of COMS and SM
Most people know that the spinal cord has a left and right side that correspond directly to the left and right sides of the body. If you damage the left side of the spinal cord, the signs associated with that damage will be seen on the left side of the body. However, did you know that the spinal cord also has a 'front' and 'back' that carry specific types of information?
The part of the spinal cord that is towards the chest of a person or animal carries information from the brain to the body, instructions on how to move. The part of the spinal cord towards the back of the person or animal carries information from the body to the brain, sensory information if you will.
Interestingly enough, SM almost always forms in the sensory part of the spinal cord. So it leads to abnormal sensations rather than weakness or abnormal movments. That means that dogs with SM manifest some very odd and quite varied behaviors secondary to the abnormal sensations that their spinal cord is causing them to feel. Imagine, that every time you got excited your toes began to itch and burn, your ear began to feel hot, or your arm became numb and tingly...
The most striking manifestation of this abnormal sensation is called 'phantom scratching'. In this situation, dogs will begin to scratch 'at' their ear, but just a little bit off to the side. Almost as if they were scratching the air!
Diagnosis of COMS and SM requires an MRI. No other tests are able to definitively diagnose this condition. There are other tests that have been used to support the diagnosis though, including BAER hearing tests, CT scans, ultrasound, and even infrared imaging.
Treatment for COMS is sometimes medical, and sometimes surgical. There are several drugs that can be used to help control the condition, and surgery can be very successful (depending on how one defines 'success').