INTERVERTEBRAL DISK DISEASE
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.”
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 types of disease that can afflict the intervertebral disk causing the disk to press painfully against the spinal cord: 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 rocklike 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 of the spinal cord further above.
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 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
Cervical (neck) disk herniations occur in 15% of disk herniation patients
(C2 - C3 (between the 2nd and 3rd cervical vertebrae)
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).
When presented with a patient with spinal weakness, it is important to determine whether the problem involves spinal cord compression. This is important because compressive lesions can benefit from surgery/anti-inflammatories while other diseases cannot. A compressive lesion in the spinal cord does not have to be a disk herniation; it could be a vertebral fracture or dislocation, a tumor, or a disk infection. A non-compressive lesion will not benefit from surgery. Such lesions 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
While advanced modes of imaging such as MRI and CT scanning are not available to most veterinary hospitals, plain radiography usually is. It 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
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).
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:
LOSING THE ABILITY TO WALK IS AN EMERGENCY!
Page posted: 8/27/2010