When an intervertebral disk bulges and pushes on the spinal cord, it affects the relay and transmission of signals in the nervous system. The severity of the compression of the spinal cord directly can relate to the severity of the clinical signs seen in our patients. The grade of compression may be listed on a scale of 1-5.
A mild compression may result in only a pain sensation in a dog. The bulging disk presses on the spinal cord, transmitting a pain signal to the brain. Additionally, the muscles of the back around that area tend to be tense and rigid, eventually resulting in a muscle spasm. This rigidity is a natural reaction to holding as still as possible to create as little additional pain as a patient can. When touched, these muscles tend to start having fasiculations. Fasiculations are a muscle twitch, a small, local, involuntary muscle contraction and relaxation which may be visible under the skin. This is a grade 1 compression.
A moderate compression may lead to pain and a loss of “conscious proprioception.” This is the conscious awareness of body position and where body parts are in space. Typically, we see this in the hindlimbs when there is a mid-back spinal compression. On physical examination, we will flip a back foot so it is resting on the top of the toes instead of on the pawpads. A dog that has lost its conscious proprioception will not put its foot back in its appropriate position; instead, it will continue standing on its foot inappropriately. A dog that still retains the ability will flip its foot back into its rightful place. This is a grade 2 compression.
A more severe compression will lead to an animal having changes in its ability to appropriately use its legs. Frequently, this is seen as a condition we call “ataxia.” Ataxia is defined as a lack of voluntary coordination of muscle movements. These dogs are able to move their back legs of their own free will. However, they typically have uncoordinated movements, almost appearing to have a drunken stumble when they try to walk normally. This is a grade 3 compression.
A very severe compression of the spinal cord will lead to a loss of voluntary movement in an animal’s legs. This can happen in either the front or back legs, but it more frequently is seen in the hindlimbs. An animal will abruptly be unable to stand, walk, or use its affected legs. When its toes are pinched, an animal may retain its automatic reflexes and pull its leg away from you or whimper in pain. It is able to do this because it is an automatic response that requires no conscious thought. This type of paralysis indicates a grade 4 compression.
Finally, the most severe type of compression results in a loss of sensation in the affected limb. In these animals, when its toes are pinched, it will no longer pull its leg away or whimper in pain. It has lost the ability to feel its leg, indicating a grade 5 compression.
The clinical signs an animal is displaying typically are indicative of the degree of compression. The gold standard diagnostic for intervertebral disk disease is an MRI. With an MRI, we are able to see definitively which disks are affected, and the degree to which the spinal cord is compressed. These findings may be indicative of whether or not a surgery to relieve the compression is a good option for a patient. Other diagnostic options include a CT scan or a myelogram. A myelogram uses X-rays and a special dye called contrast material to make pictures of the bones and the fluid-filled space between the vertebrae and spinal cord. These more advanced diagnostic techniques are performed at a veterinary neurologist.