Intervertebral disk disease
Anatomy of the vertebral column and spinal cord: The vertebral column, or backbone, is made up of a series of small bones, the vertebrae. These bones surround and protect the spinal cord, the large collection of nerves through which information is transmitted between the body and brain. The spinal cord must be intact and undamaged in order to feel sensations (including touch and pain), and for normal movement of the body and limbs. The individual vertebrae are separated by intervertebral disks. These soft tissue structures allow for normal movement between the vertebrae, and also act as shock absorbers. The structure of these disks is important in understanding this disease. Each disk is composed of an outer layer, which is fibrous, and an inner portion, which is a jelly-like mass. The outer fibrous layer (called the annulus fibrosus) acts to contain the inner jelly-like portion (the nucleus pulposus) and prevent it from bulging outwards.
Intervertebral disk disease (IVDD) occurs when the jelly-like inner layer protrudes, or herniates, into the vertebral canal and presses on the spinal cord. Compression of the spinal cord may be minimal (causing mild back or neck pain) to severe (causing paralysis, loss of sensation, and lack of bladder and bowel control) and may be irreversible.
Problems are most commonly seen in the thoracolumbar (lower back) region of the spine, and are also seen in the cervical (neck) region. IVDD is seldom a concern in the thoracic (upper back) region, because extra ligaments connect the ribs and help to reinforce the annulus fibrosus.
Type I IVDD primarily occurs in chondrodystrophoid (or chondrodysplastic) breeds such as the bassett hound, dachshund and corgi. These dogs were bred to have short, thick legs, and although this is normal for the breed, it is basically the result of abnormal development of cartilage. The intervertebral disks in these dogs gradually become more like cartilage than fibrous tissue; this increases the risk of rupture of the annulus fibrosus and herniation of the nucleus pulposus into the vertebral canal, causing IVDD. Disk herniation in these dogs occurs at a relatively young age (3 to 6 years), commonly occurs at several sites in the back, and causes intense pain.
Fibrous degeneration of disks (without calcification) occurs in older dogs of all breeds, but generally only causes problems in large-breed dogs. This is Type II IVDD; protrusion of disk material (without complete rupture of the annular layer as in Type I) usually only occurs at 1 disk and the clinical signs (pain, weakness or paralysis) generally develop more slowly and are less severe than with Type I IVDD.
With occasional exceptions, the Doberman pinsher is the only nonchondrodysplastic large breed dog to be affected by Type I IVDD, commonly in the neck region (cervical vertebral instability, or Wobbler's).
The clinical signs vary, depending on whether there is type I or II IVDD, where in the spine the herniation occurs, and the extent of the protrusion of disk material into the spinal canal. Signs with type I generally develop more quickly (over hours or even minutes) and are more severe. Depending on the location and extent of the pressure on the spinal cord, there may be pain in the neck region, lack of pain sensation in the limbs, and weakness or paralysis in any or all limbs. This is a very serious disease, which may result in permanent hindlimb paralysis and incontinence if left untreated. While confinement may help initially in some cases, surgery is often required to relieve pressure on the spinal cord. The outcome will depend on a number of factors (including the severity of clinical signs, whether pain perception is still present, and length of time between the onset of clinical signs and surgery, to name a few). While surgery may restore the animal to normal function, it is not always successful.
With type II IVDD, clinical signs usually develop slowly over months. Again there may be pain, and weakness or paralysis in any or all limbs.
Your veterinarian will strongly suspect spinal cord compression due to intervertebral disk disease if your chondrodystrophoid-breed dog presents with neck or back pain, +/- hindlimb weakness or paralysis. Your dog may also become incontinent. Your veterinarian will examine your dog, and perform a neurologic exam to test your dog’s reflexes and the function of nerves. The absence or in some cases, heightening, of reflexes will help to determine where the pressure on the spinal cord is occurring. Plain x-rays of the vertebral column are taken of suspect areas, to locate abnormalities of the vertebrae or mineralization of the intervertebral discs. A specialized radiographic technique called myelography is necessary to pinpoint the areas of spinal cord compression, if surgical repair is being considered. In order to do a myelogram, your dog is anesthetized, dye is injected into the spinal canal, and x-rays are taken which will show the exact location(s) of spinal cord compression. A spinal tap, also performed under general anesthesia, is done to obtain a sample of cerebrospinal fluid and rule out other causes of your dog's spinal cord disease. Your veterinarian may refer you to a veterinary referral centre for these specialized radiographic techniques, and for possible surgery.
Treatment depends on several factors, including the extent and duration of clinical signs, and the findings on neurologic and radiographic examination.Surgery is the only way to remove disc material which is compressing the spinal cord, but surgery is not necessarily the first treatment of choice. Medical treatment may be successful when there is only mild to moderate pain, and no weakness or paralysis. Medical, or conservative, treatment consists of anti-inflammatory medication in combination with strict cage rest (your dog will be confined to a cage, and let out only to urinate and defecate while on a leash). If your dog's condition worsens, or there is no sign of improvement within a week or so, surgical treatment should be considered.
Surgery will be recommended, and will be necessary, if your dog shows more severe neurologic signs, or if your dog has repeated episodes of pain and muscle weakness. Where there is paralysis and loss of deep pain sensation, surgery to remove the pressure on the spinal cord should be performed within 24 hours, or there is likely to be permanent nerve damage.
The prognosis (prospect for recovery) after surgery ranges from fair to good, depending on factors such as the severity of clinical signs before surgery, the rapidity with which the signs developed, and the length of the interval between the onset of problems and surgery. Where there has been paralysis and loss of deep pain perception for more than 24 hours, surgery is not likely to result in improvement.
Once your veterinarian has examined your dog, s/he will discuss treatment options with you. If surgery appears to be the best option, s/he will likely discuss with you referral to an appropriate surgical facility, because of the specialized radiographic and surgical techniques required. With either surgical or medical treatment, intensive nursing care will be required.
Although signs with type I IVDD usually progress more rapidly, this is not always the case. Also, there may be periods of improvement and subsequent deterioration over weeks or months (perhaps due to extrusion of small amounts of disk material over time). Neurologic deficits depend on the location of the injury. The main clinical sign with type I cervical spine disk disease is pain; less commonly there are proprioceptive deficits and tetraparesis or tetraplegia. Signs with type I thoracolumbar disease are generally more severe, and range from abdominal or back pain, to complete paraplegia and lack of deep pain perception.
Clinical signs with type II disease generally progress more slowly; paraparesis or tetraparesis are the most common findings. As with type I disease, deficits may be asymmetric.
It is difficult to make breeding recommendations because, although the condition is common in some breeds, a mode of inheritance has not been determined. It is best to avoid breeding affected animals, and to carefully scrutinize their close relatives before including them in a breeding programme.
FOR MORE INFORMATION ABOUT THIS DISORDER, PLEASE SEE YOUR VETERINARIAN.
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Bennett D, May C. 1995. Joint diseases of dogs and cats. In EJ Ettinger and EC Feldman (eds) Textbook of Veterinary Internal Medicine, p. 2032-2077. WB Saunders Co., Toronto.
Stigen O, Christensen K. 1996. Calcification of intervertebral discs in the dachshund: an estimation of heritability. ACTA Veterinaria Scandinavica 34(3): 357-361.