The Wobbler Syndrome (“Wobbles”)
Dr. Philip Johnson
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. Genetics, an inherited risk
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
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.