Intervertebral Disk Disease (IVDD) is a common condition in dogs and people, much less common in cats.
As in people, the canine and feline disk (or disc) consists of an outer fibrous ring called the annulus fibrosus (annulus) and an inner, vanilla-pudding-like cushion called the nucleus pulposus (nucleus).
The normal, everyday function of the spinal disk is to act as a separation between spinal vertebrae, keeping the two bones apart from each other. Just as important, the disk acts as a cushion, absorbing the shock of activity from normal to vigorous.
On an X-ray (radiograph) a normal disk looks like a space between the bones.
Certain breeds of dogs are predisposed to IVDD. Generally, they encompass the chondrodysplastic breeds, those dogs with short, crooked legs. The dachshund is the poster child, followed closely in incidence by the poodle, beagle, basset hound, Shih Tzu, Pekingese and Bichon Frise.
Damage to the disk is usually categorized one of two ways.
Type I disk degeneration is most common in young dogs and trauma cases. In these cases there is a rapid deterioration of the annulus, sometimes in just a matter of weeks. Of course, if trauma is involved, such as an animal being hit by a car, the damage may occur in seconds. Today’s discussion, however, will be limited to naturally-occurring disk disease. Onset of signs may be sudden, even though the breakdown of the disk may have been progressing for a long time. That’s because signs are not usually apparent until the nucleus bulges sufficiently through or against a defective annulus to apply pressure to the spinal cord and/or nerve roots leaving the cord.
The spinal cord and nerve roots are surrounded by a layer known as dura mater. The name translates from Latin “hard mother,” indicating how tough the layer is in its job of protecting the very delicate spinal cord. The dura mater is heavily infiltrated with pain nerve fibers. When pressure is applied, such as from a bulging or ruptured disk, the dura mater is there not only to protect the cord, but also to let the brain know something is wrong. The pain we, or our pets, feel when a disk is damaged slows or even stops our activity. Naturally, ongoing activity would cause even more breakdown of the annulus and even more nucleus to put pressure on the spinal cord. Eventually, such pressure could damage the communication system of the cord permanently, leading to irreversible paralysis.
Radiographs of these patients may show a narrowing of space between two or more vertebrae. In the worst cases the space may be completely gone and the bones may appear to touch. If “plain” X-rays fail to show which disk(s) are involved, the patient may need a special dye injected into the space between the vertebrae and the spinal cord, then have more X-rays taken. This process is called a myelogram.
The other type of disk degeneration, logically, is called Type II. This occurs in older people and animals as a natural consequence of aging. Often when we radiograph dogs with Type II disk disease several disk spaces can be seen to have broken down long ago. The pet owners may or may not have noticed the previous episodes.
Treatment can range from ultra-conservative to surgical intervention.
The most conservative therapy consists of simple rest, called bed rest in humans and cage rest in animals. Such rest is so strictly enforced that a dog or cat is kept in a veterinarian’s hospital cage or a pet carrier at home, and allowed out only to use the bathroom. The concept is to restrict activity to the point that minimal activity puts minimal pressure on the disk(s), allowing it to heal on its own.
More commonly cage rest is accompanied by injectable and/or oral antiinflammatory medications. The purpose of medication is not to heal the disk, but to reduce the spinal cord’s natural tendency to respond to insult with swelling. Picture that the cord is “trapped” in the bony spinal canal, and there is only enough room for it and a tiny bit of fluid. Any swelling that occurs puts pressure on the bony vertebrae, which have no “give.” Therefore, the pressure exerted onto the dura mater results in even more pain. The greatest pressure is located where the disk is bulging or ruptured, but the cord can swell for its entire length.
In time, the body will send cells into the spinal canal to eat away the damaged disk and relieve the pressure.
However, in cases where the pressure has become so great that the patient is paralyzed or in substantial pain, he may not have time for “nature to take its course.” If pressure is allowed to continue for too long, eventually all or part of the cord will die. Therefore, surgery is indicated when the patient’s long-term outlook is in danger.
While there are a number of surgical approaches, the most common is hemilaminectomy, in which a portion of the wall of the vertebra is removed, allowing access for careful extraction of the disk material.
The vast majority of IVDD patients recover without surgery. However, we are indebted to the board-certified veterinary neurosurgeons who perform the operations without which some patients would never walk again.
See you Monday, Dr. Randolph.
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