The Wobbler Syndrome (“Wobbles”)
Wobbler Syndrome is a neurological condition of young horses that results in abnormal neurological control of the limbs. Signs of brain disease such as changes in behavior, seizures, or severe depression, are not evident. Horses affected with Wobbler syndrome are typically brought to the veterinarian with incoordination and weakness between the ages of 6 months and 4 years.
Signs of incoordination and weakness are most prominent in the hind limbs but do also affect the forelimbs. Both sides of the body are affected to a similar degree, unlike the asymmetry observed with Equine Protozoal Myelitis (EPM). Wobbles results from physical impingement (compression) of the spinal cord as it courses down the neck in areas where the vertebrae are malformed or unstable. The malformed vertebrae are the result of developmental abnormalities which are poorly understood. The condition is also known as cervical vertebral malformation and/or cervical stenotic myelopathy.
The major effect of compression of the cervical spinal cord is a reduction in the horse’s sense of where his legs are (proprioception). The affected horse is at risk for falling during training, exercise, or even walking out of the stall. Often, the trainer notes that the horse has fallen during training and, following the fall, it was clearly evident that incoordination and weakness (spinal ataxia) were apparent. Typically, the prominent neurological symptoms following a fall have been attributed to the affect of trauma to the neck during the fall.
In actual fact, the reason was a pre-existing (mild) neurological dysfunction. Certainly, aggravation of the neurological symptoms could have been brought about by concomitant neck injury (the spinal cord is poorly protected in this condition). Other characteristic signs of Wobbler syndrome include inflexibility of the neck and, in severe cases, visible misalignment of the neck. The veterinarian is also able to recognize specific abnormalities in the manner in which the fore limbs and the hind limbs are being used by the horse during walking and trotting. In very severe cases, the affected horse is unable to stand up without considerable assistance.
The underlying problem is abnormal bony development in the neck, often due to such common bony diseases of young horses, such as physitis and osteochondrosis dessicans (OCD). Although the limbs are much more common places for serious bony disease due to OCD, the neck is another highly mobile sight, with rapid development in growing horses, that can be affected.
The reasons that developmental orthopedic diseases sometimes occur in young horses are not completely understood; it is likely a multifactorial disease. Several important predisposing factors have been identified and include
- Genetics, an inherited risk
- Rate of growth, which is affected by diet and genetics
- Dietary factors, which affect the rate of growth and are important for maturation of bone
- Hormones – male horses are at higher risk than female horses
- Work or excessive exercise on immature bones
Young male thoroughbred horses are at particular risk for this condition, especially those that have gained weight rapidly during their first two years of life.
The cervical vertebrae are supposed to protect the spinal cord while allowing normal movement of the neck. In Wobbler syndrome, these bones have grown in such a manner as to compress the spinal cord and cause both degeneration of the cord and signs of neurological impairment in the function of the limbs. The spinal cord seems to “grow out” of the small space left inside the deformed vertebrae.
Wobbler syndrome should be high on the list of likely diseases that cause signs of spinal ataxia in young horses (mostly males) between 6 months and 4 years of age. Other important possible causes of similar symptoms include neck trauma, EPM, viral (herpes) myeloencephalopathy, and equine degenerative myeloencephalopathy. The positive diagnosis of Wobbler syndrome relies on radiography of the affected horse’s neck. Radiographic images of the cervical vertebrae may reveal specific characteristic changes that occur in this disease. Sometimes it is necessary to resort to a myelographic (dye) study in order to isolate the exact location or locations of spinal cord compression. This is always necessary if surgical treatment is being considered.
A myelographic study entails the injection of a radiographic contrast into the spinal canal in order to highlight the width of the spinal cord on X-ray. Clearly, myelography can only be performed with the horse under anesthesia. Using myelography, the radiographic images are used to pin-point areas of compression of the spinal cord when the neck is moved into different positions, typically flexed and extended positions.
It is important to consider the risks associated with general anesthesia and myelography prior to undertaking this diagnostic procedure. There is risk of injury during recovery from general anesthesia for horses affected with neurological disease. The performance of myelography itself may cause further compressive damage to the spinal cord and result in worsening of the neurological signs. Furthermore, it has been suggested that myelography may increase the risk of exacerbation for EPM. Before undertaking a myelographic study, the horse owner should have a grasp of the risks as well as a clear understanding of the implications of the result.
The primary short-term treatment for Wobbler syndrome is directed against the spinal cord inflammation that is caused by cord compression. Along with rest, various different anti-inflammatory strategies are used. In the long term, some veterinarians have advocated surgical treatment of affected horses. This treatment involves the surgical fusion (arthrodesis) of affected and adjacent vertebrae.
Clearly, it is imperative that any and all sites of spinal cord compression have been identified by myelography, which is not always possible. Under general anesthesia, the two adjacent cervical vertebrae are surgically fixed together to prevent any movement at sites of spinal compression. Those veterinarians who perform this surgery claim a high rate of success, meaning that horses generally improve, but do not always become normal.
At present, there are no controlled studies comparing surgery with conservative medical treatment in sufficient numbers of horses to make a conclusion. Many veterinarians believe that this type of surgery should not be done because the horse may be sold to someone else who would not be aware that the horse is neurologically incompetent. The surgery may give a false sense of security for owners who are anxious to ride their horse, whereas most horses with Wobbles, surgery or not, should never be ridden again.
Another treatment strategy for Wobbler syndrome involves a strict reduction in the affected horse’s ration. This will probably only be effective in foals. It is well established that rapid growth increases the risk for the development of Wobbler syndrome. By reducing the ration to that just satisfactory for growth, the risks associated with Wobbler syndrome may be substantially reduced. Dietary reduction is especially important for the purposes of reducing the risk of this condition in young growing horses.