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Degenerative Disc Disease in Dogs
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 where it is surrounded by protective bone everywhere except over the discs. This extreme protection reflects its importance and its fragility.
Discs 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.
Most owners report that a disc rupture occurred following a traumatic event, such as a relatively small jump or fall. Although this act is frequently blamed for the disc rupture, if the disc had not already been degenerating, the rupture would not have occurred.
Most dogs with degenerative disc disease are 3-7 years old. It is most often considered to be a spontaneous event that is most likely controlled by genetic factors. Certain breeds, notably the Dachshund, Poodle, Pekingese, Lhaso Apso, and Cocker Spaniel, have a high incidence of disc disease. Large breeds, such as the German Shepherd, Labrador Retriever, and Doberman Pinscher, also have disc disease.
The disc 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 disc has the consistency of thick toothpaste and is much softer than the outer part.
When the outer shell degenerates, it allows the central part of the disc to escape. This is called a ruptured disc. Since the shell is thinnest near the spinal cord, disc 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 or compressed.
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 disc 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 discs (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 disc 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 disc rupture must be in the neck. Because of the way the nerve tracts are arranged in the spinal cord, disc ruptures in the neck may affect the rear legs first or even exclusively.
Disc degeneration usually occurs relatively slowly, i.e., over several weeks or months. 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, discs may also rupture very acutely. Some dogs will go from normal walking to total paralysis in less than one hour.
A presumptive diagnosis of disc 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 disc 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 disc nor the spinal cord are visible. If the diagnosis is in doubt or if surgery is to be performed, a myelogram or CT scan may be done. A myelogram is a procedure that involves injecting a special dye around the spinal cord. When radiographs are taken, the dye will be seen outlining the spinal cord. A break or disruption in the continuity of the dye column means that there is pressure on the spinal cord. A myelogram is performed with the dog under general anesthesia. There is some risk of complications associated with the dye used for myelograms and therefore, CT scans are done more commonly. CT scans can also help determine which side of the spinal canal the disc is located on so that surgery can be planned more accurately.
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 the frequency of disc ruptures. If the breed of dog is correct for disc disease, there has been a sudden onset, and there has been no trauma, there is about a 95% chance that a disc rupture is causing the pressure. However, the diagnosis is not definite until the time of surgery.
Treatment is based on the stage of the disease. Stage I disc disease produces mild pain and is usually self-correcting in a few days. Stage II disc disease causes moderate to severe pain in the neck or lumbar (lower back) area. Stage III disc disease causes partial paralysis (paresis) and results in the dog walking in staggering or uncoordinated movements. Stage IV disc disease causes paralysis but the ability to feel is present. Stage V disc disease causes paralysis and loss of feeling. These stages tend to overlap in some dogs, and dogs may move from one stage to another over a period of hours to days.
Dogs with Stage II and III disease are usually treated with anti-inflammatory drugs, pain relievers, and restriction from exercise. Surgery may be considered if the pain or incoordination persists after 4-7 days of treatment or if the neurological status declines from one day to the next. It is important that the dog not receive pain medication unless total confinement to a crate or cage is enforced. If the pain sensation is taken away, the dog is more likely to progress to total rupture of the disc. The sensation of pain is important for limiting motion. The length of confinement will vary among different dogs.
Dogs with Stage IV disease should have surgery, although a small percentage will recover without it. Dogs with Stage V disease should have surgery, and the sooner that surgery is performed the better the prognosis. If at all possible, these dogs should be operated within the first 24 hours of the onset of paralysis.
The goal of surgery is to remove pressure from the spinal cord. If the disc rupture occurs in the lower back, a window is made in the side of the vertebral bone to expose the spinal cord. This window allows removal of disc material and relieves pressure from the cord. If the disc rupture occurs in the neck, a window is made in the bone exposing the spinal cord. This may be done either from the top or the bottom, depending on the situation and the training of the surgeon.
Following surgery, your dog will be hospitalized for 3-7 days. Bladder and bowel control are often lost when the dog is paralyzed, so it is best for control of these functions to return before going home. However, it is generally best not to extend hospitalization beyond 7 days because regaining the ability to walk partly depends on exercise and motivation. Since motivation is such an important part of the recovery process, visitation is encouraged beginning the day after surgery. Please ask about scheduling your visits.
If paralysis was present before surgery, your dog may not be able to walk when it is discharged from the hospital. You will be given detailed instructions on the procedures that should be performed. Recovery is dependent on four factors: whether or not permanent damage was done before surgery, if the surgery was performed promptly, physical therapy performed at home, and the motivation of your dog. You will be instructed on ways to achieve the last two.
More than 85% of dogs that have motor function (use of their legs) at the time of diagnosis will recover without surgery. Most of the studies have shown that 85-95% of dogs that need surgery recover completely provided they still have sensation at the time of surgery. There is a small chance that your dog could have a second episode requiring surgery. Approximately 1 in 5 dogs will herniate a second disc at some time in their lives; however, the chances are just as good that they will not require surgery.
Other Potential Considerations
1. Spinal Shock. This is a temporary loss of spinal function that is generally associated with trauma. It occurs suddenly and is somewhat like a concussion of the brain. It may leave permanent damage, or full recovery may occur. Recovery from spinal shock generally occurs within a few hours to a few days.
2. Fibrocartilaginous Infarct or Embolism (FCE). In this condition, a small amount of disc material ruptures and gets into one of the blood vessels leading to the spinal cord. As the vessel narrows, the disc material obstructs it, depriving a certain segment of the spinal cord of its blood supply. Without proper blood supply, that segment of the spinal cord quits working, resulting in paralysis. Surgery will not help these dogs because there is no pressure on the spinal cord. Often, paralysis involves only one rear leg, or one rear leg is more severely affected than the other. Complete recovery may occur in a few days to weeks, or there may be permanent damage to a portion of the spinal cord.
Diagnosis of fibrocartilaginous infarct/embolism is based on the correct clinical signs and a normal myelogram or CT scan.
3. Degenerative Myelopathy. This condition means that the spinal cord is slowly dying. It results in progressive paralysis that begins with the dog dragging its rear feet as it walks. This is called “knuckling over” and results in the toenails of the rear feet being worn because they drag the ground with each step. It progresses to weakness of the rear legs, then paralysis. It generally takes several weeks before paralysis occurs, and it generally occurs in large breeds of dogs, especially German Shepherds. Because there is no successful treatment and paralysis includes loss of urine and bowel control, euthanasia is generally recommended.
Diagnosis of degenerative myelopathy is based on the correct clinical signs, especially in a large breed of dog, and a normal myelogram. Previously confirmation required a biopsy of the spinal cord so the diagnosis was confirmed only with an autopsy; however, there is now a genetic test available for degenerative myelopathy.
A normal myelogram in a dog with slowly progressive paralysis is very frustrating because the two most likely diseases (numbers 2 and 3) cannot be confirmed without an autopsy.