Wobbler syndrome (Cervical Vertebral Malformation, CVM) is a common condition of Thoroughbred horses. Affected horses are not really sure where their limbs are. They also show signs of weakness - stumbling, toe dragging and knuckling - resulting in the characteristic “wobbly” gait.” These signs are caused by compression of the spinal cord.
In some cases, the signs have been linked to arthritic changes in the cervical articular process joints. These are the joints between adjacent vertebrae in the neck.
As with other joints, they are susceptible to inflammation, which can result in soft tissue swelling, thickening of the joint capsule and new bone formation. Any increase in the size of those joints is a cause for concern, as they lie so close to the spinal cord.
Some “wobblers” have marked bony and soft tissue swelling around the joints. Could distension of these joints, as might occur if they were inflamed, result in compression of the spinal cord, even if no other bony of soft tissue changes were present? If so, detection and treatment of the inflammation at an early stage might prevent the disease progressing.
Recent research at the Royal Veterinary College used Computed Tomography (CT) to investigate the structure of these joints. The plan was to record the 3-dimensional anatomy of the articular process joints of the cervical vertebrae, to find out exactly where they extend to, and to assess their relationship with the spinal cord.
Holly Claridge performed the study as an undergraduate project. She examined six necks from clinically normal horses that had been euthanased at the abattoir. Shaving foam was injected into the articular process joints to make them stand out in the CT image. “Shaving foam is essentially air” she explains. “So the joints show up nicely. It also distends the joint so mimicking them when they are inflamed.”
The CT scanner produced a series of 2 dimensional slices through the tissues. Further computer processing created a 3-dimensional image of the neck.
She found that these joints do indeed extend towards, but do not actually touch, the spinal cord.
“With this method we can really see where the joints go. From the 2D images we can see that the joints don’t come in contact with the spinal cord at all. That was consistent throughout the study in all specimens.”
“We can also see the direction in which the joints project towards the spinal cord. They protrude towards the dorso-lateral aspect of the spinal cord. If you think about the spinal cord as a clock face they point towards the spinal cord at the 10 and 2 o’clock position.”
So, in the absence of any soft tissue or bony changes, it seems unlikely that inflammation or swelling of these joints causes spinal cord compression.
However, these joints are capable of marked bony change. “If you do have massive bony and soft tissue change around these joints then spinal cord compression might result.” Further work is now being carried out at the RVC looking at the structure of the joints in diseased horses.
The findings have implications for the diagnosis of Wobbler syndrome. Current practice is to take side to side views when conducting myelography (x-ray examination with contrast material injected around the spinal cord to show narrowing of the spinal canal). In the light of these findings Ms Claridge suggests taking additional oblique views to ensure that no lesions are missed.
Holly Claridge graduated from the Royal Veterinary College in London earlier this year, and was awarded a medal by the College for her research. She has presented the results at the Congress of the British Equine Veterinary Association in Birmingham, and was sponsored by Intervet Schering Plough Animal Health to present them to the Thoroughbred Racing and Breeding Seminar in Cheltenham.
For more details see:
The 3D anatomy of the cervical articular process joints in the horses and their relationship to the spinal cord.
H Claridge, R Piercy, A Parry, R Weller.
Thoroughbred Racing and Breeding Seminar Handbook 2009, p 32-33