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.
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 with rubber-like cushions called intervertebral discs.
The individual vertebrae and 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.
The disc is composed of two parts and has a structure like 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.
What is degenerative disc disease?
Degenerative disc disease causes spontaneous degeneration of the outer part of the intervertebral disc, resulting in sudden disc rupture (also known as a slipped disc or herniated disc). 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."
A disc rupture may or may not be related to injury. In many cases, disc rupture occurs after a minor fall or a relatively small jump. Although this act is frequently blamed for the disc rupture, the injury occurred due to chronic disc degeneration.
Most dogs with degenerative disc disease are middle-aged, from three to seven years old. Certain breeds have a high incidence of intervertebral disc disease, especially the Dachshund, poodle, Pekinese, Lhasa Apso, German Shepherd, Doberman, beagle, French bulldog and cocker spaniel.
How does a ruptured 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 results in pain. Pressure on the spinal cord itself results in pain and/or loss of information transmission. This loss of transmission, 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, but 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. They 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 suddenly. Some dogs go from walking to total paralysis in less than one hour.
How is degenerative disc disease diagnosed?
A probable diagnosis of degenerative 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 your veterinarian considers.
It is possible for pressure on the spinal cord to be 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. Advanced imaging, such as an MRI or myelogram, is required for a definitive diagnosis. These tests would be done prior to surgery to identify which disc is involved.
Are all disc ruptures treated with surgery?
Not all disc ruptures are necessarily treated with surgery. The recommended treatment is based on the stage of the disease:
- Stage 1 disc disease produces mild pain and is usually self-correcting in a few days.
- Stage 2 disc disease causes moderate to severe pain in the neck or lumbar (lower back) area.
- Stage 3 disc disease causes partial paralysis (paresis) and results in the dog walking in staggering or uncoordinated movements.
- Stage 4 disc disease causes paralysis but the ability to feel is present.
- Stage 5 disc disease causes paralysis and loss of feeling.
These stages overlap in some dogs, and dogs may move from one stage to another over a period of hours to days. Note that these stages may be referred to in Roman numerals, as well (e.g., stage I, stage II, stage III).
Dogs with stage 2 and 3 disease are usually treated with anti-inflammatory drugs, pain relievers, muscle relaxers, and exercise restriction. If the pain or lack of coordinated movements persists after four to seven days of treatment or if the neurological status declines from one day to the next, surgery may be required. The time of confinement will vary based on each patient's condition.
Dogs with stage 4 disease should have surgery, although a small percentage will recover without it. Dogs with stage 5 disease should have surgery immediately. The sooner the surgery is done, the better the prognosis. Ideally, these dogs should be operated on within the first 24 hours of the onset of paralysis. Rapid identification and transfer to a specialist for advanced imaging and surgery is vital for these dogs.
What is the goal of surgery?
The goal of surgery is to remove pressure from the spinal cord. If the disc rupture occurs in the lower back, a "window" (called a fenestration) is made in the side of the vertebral bone to expose the spinal cord. If a disc rupture occurs in the neck, a window is made in the top or bottom of the bone (depending on the situation and the veterinarian's preference), exposing the spinal cord. This window allows removal of disc material and relieves pressure from the cord.
Unfortunately, success cannot be determined immediately. The return of neurological function, walking ability, and relief from pain may not occur for several days to weeks after surgery.
When can my dog go home?
Following surgery, your dog will be hospitalized for several days. Dogs often lose bladder and bowel control when they are paralyzed. It is better for your dog to remain hospitalized until control of these functions has returned. It is generally better not to extend hospitalization beyond seven days, because regaining the ability to walk partly depends on exercise and motivation. Since motivation is such an important part of the recovery process, your veterinarian may encourage you to visit your dog, starting a few days after surgery.
If paralysis was present before surgery, your dog may not be able to walk when they are discharged from the hospital. You will be given detailed instructions on the procedures that should be performed. Recovery is dependent on four factors: whether there was permanent damage before surgery, whether the surgery relieved the pressure quickly enough, whether physical therapy can be performed at home, and whether the dog is motivated to recover.
Can my dog rupture a disc again?
Yes, a dog can rupture a disc again; however, not all disc ruptures require surgery.
What other conditions could be mistaken for intervertebral disc disease?
Conditions that may mimic intervertebral disc disease include:
Spinal shock. Spinal shock 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 there may be a full recovery. Recovery from spinal shock generally occurs within a few hours to a few days.
Fibrocartilaginous embolism. 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 dies, resulting in sudden-onset 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 embolism is based on specific clinical signs and an abnormal MRI; however, early in the condition, an MRI may show no visible changes. Unfortunately, a definitive diagnosis can only be made by performing a spinal cord biopsy after death.
Degenerative myelopathy. In this condition, the spinal cord is slowly dying. Degenerative myelopathy results in progressive paralysis that begins with the dog dragging its rear feet as it walks, a sign called "knuckling over." Dragging the rear feet results in those toenails being worn down, because they drag on 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 usually occurs in large dog breeds, especially German shepherds.
Degenerative myelopathy is often called "German Shepherd Dog Disease" or "Alsatian Disease." There is no treatment, and it ultimately leads to loss of urine and bowel control. Diagnosis of degenerative myelopathy is based on clinical signs, breed, and a normal MRI. Confirmation requires a biopsy of the spinal cord at time of death.
