Degenerative Disc Disease in Dogs

What is a disc, and what is its purpose?

The spinal cord is one of the most important and sensitive organ systems in the body. If it is damaged, the nerve cells do not regenerate but are replaced with fibrous or scar tissue. Spinal cord injuries usually result in permanent, irreversible damage. To protect it from damage, the spinal cord runs through a bony canal within the spine and is surrounded by protective bone everywhere except at the junctions of the vertebrae. These junctions are filled by rubber-like cushions called intervertebral discs. The individual vertebrae and intervertebral discs allow the back to move up and down and sideways without allowing contact between the bones of the spinal column. This extreme protection of the spinal cord reflects its importance and fragility.

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

The disc is composed of two parts, and has a structure similar to a jelly donut. The outer covering (fibrous ring) is much like a thick shell. It is made of tough fibers that protect and contain the central part (pulpy nucleus). The central part of the disc is softer and has the consistency of thick toothpaste.

Degenerative disc disease causes spontaneous degeneration of the outer part of the disc, resulting in sudden disc rupture or herniation. Since the disc is thinnest near the spinal cord, the disc material that escapes through the tear usually goes upward, putting pressure on the spinal cord. Because the spinal cord is encased within its bony canal, it cannot move away from the pressure and it becomes "pinched."

"Because the spinal cord is encased within its bony canal, it cannot move away from the pressure and it becomes 'pinched'."

A disc rupture may or may not be related to injury. In many cases, the disc rupture occurs after a fall or a relatively small jump. Although this act is frequently blamed for the disc rupture, the injury actually occurred due to chronic disc degeneration.

Most dogs with degenerative disc disease are middle-aged, from three to seven years old. Certain breeds, especially the Dachshund, Poodle, Pekinese, Lhasa Apso, German Shepherd Dog, Doberman, and Cocker Spaniel have a high incidence of intervertebral disc disease.

How does a slipped disc affect the spinal cord?

The spinal cord is like a telephone cable carrying thousands of tiny wires. When it is crushed, transmission of information through the wires is slowed or stopped. When the disc degenerates and ruptures, it crushes the spinal cord and the nerves that leave the spinal cord over the discs (i.e., spinal nerves). Pressure on the spinal nerves result in pain. Pressure on the spinal cord itself results in pain and/or loss of information transmission. This, in turn, causes partial or complete paralysis, along with other neurological problems.

Most disc ruptures occur in the middle to lower part of the back. However, they may also occur in the neck. Back injuries often cause paralysis without severe pain, while neck injuries usually cause severe pain without paralysis. Due to the way nerve tracts are arranged in the spinal cord, if paralysis affects all four legs, the disc rupture must be in the neck. Sometimes, however, a lesion in the neck area may affect the rear legs first and may not involve the front limbs.

How quickly do intervertebral discs degenerate and rupture?

Disc degeneration occurs relatively slowly. A rupture may occur slowly and gradually, with degenerated disc material slowly escaping from the disc over several days or weeks. In these cases, the dog often experiences pain and is reluctant to move. It may lie around for a few days allowing the body to try to heal the injury, often without the owner being aware that a problem exists. However, discs may also rupture very suddenly. Some dogs will go from normal walking to total paralysis in less than one hour.

How is this problem diagnosed?

A probable diagnosis of disc disease will be based on the dog's history of neck or back pain, uncoordinated walking, or paralysis, with or without a history of recent trauma. If the dog is one of the high-incidence breeds, this disease will be high on the list of potential causes of the patient's symptoms.

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

It is possible that the pressure on the spinal cord is due to a blood clot or tumor. Both are possible but rare, especially when compared to disc ruptures. If the patient is a high-risk breed and there has been a sudden onset of paralysis without severe trauma, there is an approximately 95% chance that a disc rupture is causing the clinical signs. However, the diagnosis is not definite without surgery or a CT scan.