Canine Degenerative Disk Disease pt 1

What is a disk, and what is its purpose?

The spinal cord is one of the most important and most sensitive organs in the body. If it is traumatized, its cells will not regenerate; injuries usually result in permanent damage. Therefore, the spinal cord is protected in a very special fashion. It goes through a bony canal within the spine; it is surrounded by protective bone everywhere except over the disks. This extreme protection reflects its importance and its fragility.

Disks are rubber-like cushions between the vertebrae. They allow the back to move up and down and sideways without allowing contact between the bones of the spinal column.

What does it mean for a disk to rupture, and how does it happen?

The disk is composed of two parts. The outer covering is much like a thick shell. It is comprised of tough fibers that protect and contain the central part. It is thinnest at the top; this thin area is located just below the spinal cord. The central part of the disk has the consistency of thick tooth paste; it is much softer than the outer part.

When the outer shell degenerates, it allows the central part of the disk to escape. This is called a disk rupture or a ruptured disk. Since the shell is thinnest near the spinal cord, disk material that escapes almost always goes upward, putting pressure on the cord. Because the spinal cord is encased within its bony canal, it cannot move away from the pressure and it becomes pinched.

Degenerative disk disease causes spontaneous degeneration of the outer part of the disk, resulting in escape of the central part. It is not related to injury, although trauma can cause disks to rupture. It is also not related to age. Most dogs with degenerative disk disease are 3-7 years old. It is just a spontaneous event that is most likely controlled by genetic factors. Certain breeds, notably the Dachshund, Poodle, Pekinese, Lhaso Apso, and Cocker Spaniel have a high incidence of disk disease. Other breeds, such as the German Shepherd and Doberman Pinscher, also have disk disease but with a lower incidence. Many breeds never have degenerative disk disease.

Most owners report that a disk rupture occurred following some traumatic event, such as a relatively small jump or fall. Although this act is frequently blamed for the disk rupture, if the disk had not already been degenerating, the rupture would not have occurred.

How does a ruptured disk affect the spinal cord?

The spinal cord is much like a telephone cable that is carrying thousands of tiny wires. When it is crushed, transmission of information through the wires is stopped. When the disk degenerates and ruptures, a similar event occurs. The central part is forced upward, putting pressure on the spinal cord and/or the nerves that leave the spinal cord over the disks (i.e., spinal nerves). Pressure on the spinal nerves results in pain; pressure on the spinal cord results in pain and/or loss of information transmission. This results in paralysis or partial paralysis.

Most disk ruptures occur in the middle to lower part of the back. However, they may also occur in the neck. The former often causes paralysis without severe pain; the latter often causes severe pain without paralysis. If paralysis affects all four legs, the disk rupture must be in the neck. Because of the way the nerve tracts are arranged in the spinal cord, disk ruptures in the neck may affect the rear legs first or even exclusively.
How fast do disks degenerate and rupture?

Disk degeneration usually occurs relatively slowly, i.e., over several days or weeks. The dog usually experiences pain and becomes reluctant to move. It may lie around for a few days allowing the body to resolve the problem, often without the owner being aware that a problem existed. However, disks may also rupture very acutely. Some dogs will go from normal walking to total paralysis in less than one hour.

How is a disk rupture diagnosed?

A presumptive diagnosis of disk disease is made based on the dog's history of neck or back pain, incoordination when walking, or paralysis when there is no history of trauma. The physical examination will indicate that the problem originates from the spinal cord, giving further evidence to disk disease. Another important factor is the breed. If the dog is one of the high incidence breeds, the diagnosis is even more likely.

In some cases, plain radiographs (x-rays) may assist the diagnosis, but they may also be normal since neither the disk nor the spinal cord are visible. If the diagnosis is in doubt or if surgery is to be performed, a myelogram may be done. This procedures involves injecting a special dye around the spinal cord while the dog is under anesthetic. When radiographs are taken, the dye will be seen outlining the spinal cord. A break in the dye column means that there is pressure on the spinal cord.

How do you know if the pressure on the spinal cord is due to a disk or something else?

It is possible that the pressure is due to a blood clot or a tumor. Both are possible but not very common, especially when compared to disk ruptures. If the breed of dog is correct for disk disease, there has been a sudden onset, and there has been no trauma, there is about a 95% chance that a disk rupture is causing the pressure. However, the diagnosis is not definite until the time of surgery.