Wobbler syndrome is associated with cervical stenotic myelopathy (CSM), a malformation of the vertebrae or a narrowing of the spinal canal that compresses the spinal cord, damaging nerves and affecting proprioception. Clinical signs include gait abnormalities signified by a loss of balance, loss of coordination (ataxia), weakness, and in some cases, paresis or paralysis.
Other scientific names for the disease include cervical vertebral malformation, cervical vertebral compressive myelopathy, cervical vertebral stenotic myelopathy, and cervical vertebral stenosis.
CSM is primarily classified as a developmental or orthopedic disease and typically occurs in young horses between the ages of 4 months and 4 years of age, with a mean age of 2 for diagnosis 8 and distinct peaks of diagnosis coinciding with periods of rapid skeletal growth around 6 months and 1.5 years of age. The prevalence of the disease in young horses is due to increased susceptibility to growth abnormalities and trauma until their skeletons fully mature and all of their growth plates close. The vertebral processes are the last part of the skeleton to mature, at between 3 and approximately 5 years of age. CSM also affects older horses as a secondary effect of arthritis.
Causes & Pathogenesis
The causes of CSM are multifactorial and may include one or a combination of the following, a high carbohydrate diet to promote faster than normal growth rates, nutritional imbalances leading to copper deficiencies and excessive dietary zinc, instability of the vertebral column, physical trauma, osteochondrosis and osteosclerosis due to the abnormal maturation of bone and cartilage, genetics, and arthritis.
There are two theories of pathogenesis, the developmental theory and the biomechanical theory. The developmental theory proposes that cervical vertebral malformations occur in response to an underlying bone disorder that affects cartilage formation and maturation. Whereas the biomechanical theory proposes that cervical static stenosis, narrowing of the cervical canal between C5 and C7, is the result of force and mechanical stress on the cervical column which causes structural changes in the vertebrae. Mechanical stresses include strain or physical trauma from injuries, such as when a young horse is restrained with a halter and tied up for the first time, or is cast in a stall, vibration, and oscillation of the neck.
Young horses diagnosed with CSM between the ages of 4 to 18 months typically have lesions between the C3 and C5 vertebrae which compress on the spinal cord when the neck is flexed, known as dynamic compression, or cervical vertebral instability caused by abnormal movement of the cervical vertebrae. Whereas, horses between 1 to 4 years of age are more likely have a narrowing of the cervical canal, which causes static compression to occur regardless of the position of the neck due to the abnormal formation and shape of the vertebrae. In some cases, narrowing of the spinal canal may be more extensive, extending from C3 to C7. In older horses static compression and narrowing of the cervical canal is most likely the result of chronic trauma to the neck or arthritis in the cervical vertebrae.
Breed & Genetic Predisposition
While it has been reported in a variety of breeds, there is a higher incidence of the disease in rapidly growing horses, Thoroughbreds, Warmbloods, Quarter Horses, and Tennessee Walking Horses, with males being affected more frequently than females. However, there is still considerable debate regarding genetic predisposition, see Prevention below. The reasons for the gender predisposition are thought to be related to the different effects of testosterone and estrogen on growth rates. Affected horses, generally, tend to be larger males with longer necks compared to age-matched horses in control groups.
Clinical Signs & Diagnostics
Clinical signs associated with CSM include mild to progressive ataxia (loss of coordination) most often observed in the hind limbs, weakness, and in some cases paresis or paralysis. Ataxia is generally more prevalent in the hind limbs than the forelimbs because the nerves that are connected to the hindlimbs are located along the outside of the cervical area of the spinal cord and are easily damaged by any compression or trauma of the cervical spine, whereas the nerves connected to the forelimbs are located deeper within the spinal cord and thus better protected.
There is a series of preliminary tests that can be performed, preferably with a veterinarian present for safety reasons, to confirm the presence of ataxia and impaired neurological function indicative of Wobbler syndrome. These include 1) turning the horse in a small circle – a horse with CSM will swing rear legs out while turning; 2) walking the horse backwards – wobblers will have trouble backing up, hopping with the rear legs rather than in the normal two-beat fashion; 3) while one person is walking the horse forward, another person will pull the horse to one side by the tail – a wobbler will have an impaired sense of proprioception and will easily be pulled over without being able to correct its position; 4) if the horse does not exhibit any resistance to having its tail lifted, this is a good indication of nerve damage along the spinal cord.
Following a complete neurological exam, advanced diagnostics will be used to determine the specific cause of the gait abnormality and confirm whether or not cervical damage has occurred. It is also necessary to rule out other diseases that can cause ataxia, including equine protozoal myeloencephalitis, West Nile, equine herpes virus, and rabies. A definitive diagnosis depends on radiographs of the cervical vertebrae to measure the diameter of the spinal canal and, if stenosis is indicated, a myelogram will also be performed to determine the presence and extent of spinal cord compression.
Treatment & Potential Complications
Initial treatment with anti-inflammatories and corticosteroids may help minimize acute inflammation but will not have any significant long-term benefit. Osmotic diuretics may be more effective at decreasing swelling of the nerve tissue and relieving intracranial pressure, but this too is a temporary measure. In young horses, if caught early enough, dietary adjustments and a reduced level of exercise for an extended period of time may be an adequate level of intervention and the horse may grow out of the problem. In some mature horses, depending on the degree of ataxia, the location of compression and number of vertebrae involved, surgical fusion may be performed. The most common surgical intervention involves placing a titanium basket around the affected vertebrae to promote fusion and relieve compression on the spinal cord.
In many cases, CSM is performance-limiting even after surgery. While Crabbe 4 indicates approximately 70% of horses treated surgically will experience some degree of improvement, only about 50% are likely to be able to perform in athletic disciplines. Even following improvements from vertebral fusion surgery, horses may still have residual neurological problems, including mild ataxia and should be considered unsafe to ride.
Orthopedic development of the disease may be preventable through balanced nutrition for both mare and foal to promote natural growth rates. Horse pairings that have previously produced foals with wobblers should not be bred again, as there may be a genetic predisposition. However, it has been reported that breeding trials between stallions and mares confirmed to have CSM did not produce any foals with Wobbler syndrome. More research is needed regarding the role of genetics in the development of the disease. Preventative measures for the mature horse were not indicated in the literature.
Horses with Wobbler syndrome pose a danger to themselves and others. Severe ataxia can increase chances of traumatic self-injury and casting, and the horses may also be difficult to safely handle and/or ride. These cases have significant financial impact related to diagnostic and treatment expenses, as well as a significant emotional impact, as owners must make a decision between treatment and euthanasia. For humane reasons, it is common for horses affected by CSM to be euthanized, particularly in more advanced or severe cases, for the safety of both horse and human.
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