INTERVERTEBRAL DISK DISEASE

Lower back spinal column of a dog showing the vertebrae and disk spaces in between them
(original graphic by marvistavet.com)
 

WHAT IS A DISK?

Most people are aware of the fact that the backbone is not just one long, tubular bone. The backbone (or spine) is actually made of numerous smaller bones called “vertebrae” which house and protect the spinal cord. The fact that numerous vertebrae make up the spine allows for flexibility of the back. The vertebrae are connected by special joints called “intervertebral disks.”

 (Photocredit: TBjornstad via Wikimedia Commons (modifications made in labeling)

There are 7 cervical (neck) vertebrae, 13 Thoracic (chest) vertebrae, 7 lumbar (lower back) vertebrae, 3 sacral vertebrae (which are fused), and a variable number of tail vertebrae.

TYPE I AND TYPE II DISK DISEASE

There are two main types of disease that can afflict the intervertebral disk causing the disk to press painfully against the spinal cord, nerve roots and tender dorsal longitudinal ligament: Hansen Type I Disk Disease and Hansen Type II Disk Disease. In Type I Disk Disease, the nucleus pulposus becomes calcified (mineralized). A wrong jump by the patient causes the rock-like disk material to shoot out of the annulus fibrosus. If the disk material shoots upward, it will press painfully on the ligament above and potentially cause compression and bruising (contusion) of the spinal cord. If it shoots sideways, it will smack into the nerves as they exit the spinal cord. Either way it is suddenly sharply painful with varying degrees of reduced nerve function.

Type II Disk Disease is a much slower degenerative process. Here the annulus fibrosus collapses and protrudes upward creating a more chronic problem with pain and spinal cord compression. The disk material impinges on the nerves and spinal cord over months or even years.

The condition where disk material presses against the ligament above and spinal cord is called “Disk Herniation.”

The most common sites of disk herniation are T11 - T12
(between the 11th and 12th thoracic vertebrae)
and L2 - L3 (between the 2nd and 3rd lumbar vertebrae).

Cervical (neck) disk herniations occur in 15% of disk herniation patients
but 80% of these patients are dachshunds, beagles, or poodles.

(C2 - C3 (between the 2nd and 3rd cervical vertebrae)
is the most common site of disk herniation in the neck.)

The area of the back or neck just over the disk involved is generally painful (whereas many degenerative spinal processes are not painful). In milder cases, pain at the site of the disk may be the only symptom. As inflammation increases in the spinal cord, neurologic deficits can occur. Disk herniations in the neck tend to have more pain and less dysfunction.

The first thing to go is what is called “conscious proprioception.” This concept refers to the ability to perceive where one’s feet are and orient them properly. The examiner will turn the foot over so that the top of the foot is on the ground and will see if the patient replaces the foot in the proper position. Dogs with poor proprioception will scuff their toes or even occasionally fail to flip their feet while walking and will walk on the tops of their feet. Nerves responsible for conscious proprioception are located on the outside of the spinal cord so when the disk herniates, these nerves feel the pressure first.

After that go the nerves for voluntary motion (including voluntary control of urination/defecation), followed by the nerves for superficial pain perception and ultimately the nerves for deep pain perception (usually tested by applying a strong pinch to the toe).

DIAGNOSTICS

When presented with a patient with spinal weakness, it is important to determine whether the problem involves actual compression of the spinal cord. This is important because compression can benefit from surgery/anti-inflammatories while other spinal diseases cannot. Compression in the spinal cord does not have to be from a disk herniation; it could be a vertebral fracture or dislocation, a tumor, or a disk infection. Any of these things could be putting pressure on the spinal cord or nerves and if there is a physical compression, the compression, at least in theory, could be relieved with surgery.

A non-compressive disease will not benefit from surgery. Such non-compressive diseases include: spinal degeneration, spinal infection or inflammation, demyelination injuries, or fibrocartilaginous embolism.

Step One: The Neurologic Examination

By testing different reflexes, the doctor can localize the area of the spinal cord that is affected. This might be the cervical area (neck), the thoracolumbar area (where the chest and abdomen come together), the lumbar (lower back), or the sacral area (where the tail starts). The thoracic area of the spinal cord is usually spared in disk disease because the ligaments connecting the ribs to the back provide extra protection for the spinal cord.

Step Two: Plain Radiographs

 
Calcified collapsed L1-2 disk space towards the left of the image.
Compare to the normal adjacent disk spaces between the other lumbar vertebrae to the right.
(original graphic by marvistavet.com)

While advanced modes of imaging such as MRI and CT scanning are not available to most veterinary hospitals, plain radiography usually is. Radiography is also relatively inexpensive compared to other forms of imaging. The first step of imaging typically involves plain radiographs to rule out obvious spinal issues. Broken bones or dislocations are generally obvious. Calcified disks and disk space collapse can often be seen. The location of a disk herniation can be determined in 50-75% of disk cases. Radiography of the neck requires general anesthesia or sedation to get proper relaxation of the muscles.

Step Three: Advanced Imaging

If surgery is being considered, then it becomes necessary to identify the exact disk space involved so that the surgeon will know where to cut. Classically, myelography has been the next step. Myelography requires general anesthesia and the injection of iodine based dye around the spinal cord. The image of the dye can be seen to narrow at the area of spinal cord compression identifying the area of compression in 85-95% of cases. The patient typically then goes directly to surgery without even waking from anesthesia. As a general rule, there is not much point in performing a procedure to specifically localize a compressive lesion unless surgery is being considered.

Myelogram showing dye column obstructed
between the 5th and 6th cervical vertebrae (where arrow is)

As CT becomes more available, a scan is often performed in addition or instead of a myelogram. This gives an even more accurate image of the disk herniation and its location (as well as whether there is more than one disk involved).

TREATMENT

Once it is clear from the plain radiographs and neurologic examination that the patient has disk disease, the decision must be made as to whether or not surgery should be pursued. Spinal surgery is very expensive and requires a long recovery period but may be the best choice if the dog is to regain normal function. There are some general rules that are typically applied in making this decision:

• If the dog cannot walk, surgery affords the best chance at recovery.
• If the dog can walk, medical (non-surgical) treatment is a reasonable choice but this also depends on how much pain the patient is in and how long-standing signs are and what sort of treatments have been unsuccessful in the past.
• The longer the neurologic deficits have been going on, the poorer the results of treatment.
• If the dog cannot walk but deep pain is present in the limbs, there is a 83-90% success rate for recovery with surgery.
• If the dog has been unable to walk, has no deep pain perception in the limbs but has only been down less than 48 hours, success with surgery drops to 50%. After 48 hours in this situation, prognosis is much worse and it may not be worth considering surgery.
• If the dog cannot walk, medical management may still have success though surgery is definitely more likely to yield success.

LOSING THE ABILITY TO WALK IS AN EMERGENCY!
DO NOT WAIT TO SEE IF IT GETS BETTER; GO TO THE VET AT ONCE!

 Page posted: 8/27/2010
Page last updated: 3/18/2022