Wobbler syndrome, correctly termed cervical stenotic myelopathy (CSM), is essentially a narrowing of the spinal canal in the neck that leads to compression of the spinal cord. This causes a range of neurological abnormalities in the limbs, resulting in a loss of balance called ataxia.
At its most extreme, this ataxia leaves the horse unable to stand or causes him to fall over frequently. Balance loss can be very mild, however, showing only subtle signs that can be misinterpreted as a mild lameness or even “exuberant” movement.
The cause of wobbler syndrome is unknown, but it is likely to be multifactorial and is considered a developmental disorder where genetic predisposition and nutrition appear to play important roles.
Thoroughbreds are most commonly affected, but the condition can occur in any breed, with male horses three times more likely to be affected than females. The age of onset is typically between six months and three years.
Physical and neurological examinations can establish a suspicion of the disease, yet an X-ray of the neck is required to make a definitive diagnosis.
Certain causes, such as osteoarthritis, abnormal bony development and subluxations (slight dislocation or misalignment of vertebrae), can sometimes be seen with basic X-rays. Specific measurements of the spinal canal width can be collected and compared with normal values to help determine if there is narrowing of the spinal canal.
These techniques can be unreliable, however, and more often than not a myelogram — an X-ray taken with a radiopaque dye injected around the spinal cord — is required to determine where the cord is being compressed.
The compression site is identified by a break in the dye column. Sometimes compression occurs only when the neck is flexed or extended and therefore a series of different views is needed to fully evaluate the condition.
The problem with myelography is that the horse must be under general anaesthesia, and this is only justified in certain cases. This is more risky than usual, because the horse is already suffering from neurological weaknesses and less steady on his feet — making anaesthesia recovery more difficult and injury more likely. In addition, the dye can be an irritant and so may exacerbate neurological signs.
What’s the prognosis?
Treatment will depend on the exact cause of the compression.
Anti-inflammatory treatment or injection of anti-inflammatory medications into the affected joint in the spine may be sufficient, but surgical stabilisation is typically required for the classic case of wobbler syndrome.
The aim of the procedure is to fuse the two vertebral bodies either side of the site of compression. This is achieved by inserting either a special surgical implant known as a kerf-cut cylinder, or attaching a bone plate on the underside of the vertebral body.
The operation is performed with the horse under general anaesthesia and lying on his back, while an incision is made on the underside of his neck. This complex procedure is carried out only at certain clinics and by a few surgeons, as it requires specialised equipment and knowledge of the anatomy.
The likelihood of treatment success is based on the site of compression (the lower down the neck, the harder it is to treat), the number of sites of compression (the more sites, the worse the prognosis) and the degree of ataxia.
Prognosis is poor for a severely affected horse, but these treatment options may well alleviate mild clinical signs. The difficulty with this disease is that riding a horse who may not be sure of where his feet are, is just too big a risk to take.
Ref: Horse & Hound; 9 July 2015