Comparison of Canine and Human Intervertebral Disk Disease
Intervertebral disk disease is a major neurologic problem affecting both canines and humans. Both species can be affected with cervical (neck) disk disease with similarities in the symptoms and outcome. However, in the thoracolumbar (mid-back) area, unique species differences alter the symptoms and outcome of canine versus human disk disease. The major neuroanatomic difference is in which vertebra the end of the spinal cord lies. In humans, the end of the spinal cord lies approximately inside the second lumbar (L2) vertebra (mid-back). Nerves exiting the spinal cord then descend inside the remaining lumbar and sacral (pelvis) vertebral segments. In comparison, the spinal cord in dogs ends at approximately the sixth lumbar (L6) vertebra (low-back area) and nerves descend through the last lumbar, sacral and coccygeal (tail) vertebral segments.
Why is this knowledge so important in determining the differences we see in canine versus human disk disease? To answer this we must first consider the postural differences between humans and dogs and the directions from which forces are applied to the vertebrae and intervertebral disks. Humans walk upright with most jarring forces being transferred straight up the spinal column from the legs. The lumbar vertebra are “first in line” to absorb and dissipate those forces. They also are not “fixed” by an attached rib cage and therefore absorb major twisting forces (axial rotation) as well. Consequently the lumbar intervertebral disks are at highest risk for injury and possible herniation in humans. When disk herniation occurs, the herniated material presses on nerve roots and not the spinal cord itself resulting in a great deal of pain but seldom profound leg paralysis. In comparison, dogs walk on all four limbs with jarring forces normally applied at a right angle to the spinal column. However, when dogs jump down onto their front limbs, the major line of force redirects down the length of the spinal column causing greater end on compression of individual disks and an increased likelihood of herniation. Additional twisting forces are greatest where the immobile thoracic spine meets the mobile lumbar spine. As a result, the most common site of disk herniation in the back of dogs is this thoracolumbar junction where the spinal cord is present and is secondarily compressed by herniated disk material. Thus the clinical presentation of thoracolumbar disk herniation in dogs can be far worse than just shooting pains down the legs. It is common for dogs to show profound paralysis of their hind limbs from the resulting spinal cord damage.
Patricia J. Luttgen, DVM, MS