INTRODUCTION

Between each of the vertebrae (bones of the spine) lies an intervertebral disk. This disk is composed of a soft tissue, called fibrocartilage that allows movement and acts as a shock absorber.

Normal wear and tear with aging results in deterioration of these disks (disk degeneration). Although some older patients can show clinical signs, more often the deterioration progresses without any problems or symptoms. In some pets, this deterioration is accelerated and middle-aged patients can show clinical signs.

TYPE I DISK DEGENERATION
Two groups of patients are most commonly affected. The first group is typically small breed dogs that are usually between the ages of 3 and 9 years. The center of the intervertebral disks, the nucleus pulposus, becomes calcified and rigid losing its normal elasticity. With a mild traumatic event, and occasionally even normal movement, the disk can rupture and the calcified center be rapidly extruded or herniated. If the disk ruptures to the sides or bottom, the result may only be transient pain lasting for a few hours to a few days. It may even be unnoticed by many owners. However, if the disk ruptures above, the calcified contents are extruded into the spinal canal and can impact on the spinal cord.

This results in severe pain and varying degrees of paralysis. Immediate treatment is necessary or complete and permanent paralysis may occur. Surgery is necessary if medication alone does not work or if the signs are severe.

TYPE II DISK DEGENERATION

The second groups of patients most commonly affected with disk degeneration are large breed dogs, usually 4 to 9 years of age. In these patients the disk degeneration results in a slowly protruding or bulging disk.

The spinal cord may become compressed over the course of many months and thus symptoms may be gradual and be mistaken for arthritis. Treatment should be started before irreversible damage to the spinal cord has occurred. Again, the decision for medical treatment or surgery will depend on the severity of the symptoms and where in the neck or back the problem is occurring.

TRAUMATIC (TYPE III) DISK DEGENERATION

Major traumatic events may also result in rupture of deteriorated or even normal disks in any patient. Treatment in patients with traumatic disk rupture may be the same as for those patient with Type I or Type II disk degeneration.

DIAGNOSIS
Although we can be suspicious that disk degeneration is the cause of the clinical signs, a myelogram or MRI is required to be certain of the diagnosis. A myelogram is a special x-ray study, performed under general anesthesia, where a dye is injected through a spinal tap needle and x-rays are taken. This dye highlights the spinal cord and the tops of the disks. The procedure localizes which disks are protruding, bulging, or extruding and thus causing the compression and swelling of the spinal cord.

TREATMENT
If surgery is indicated, the procedure will depend on where the disk problem is located. If the degenerated disk is in the neck, the disk is removed from the spinal canal from the front (underside) of the neck through a procedure we call a "slot".

Degenerate disks in the lower back cannot be removed in this way because of all the abdominal organs. In these cases, a "laminectomy" or "hemilaminectomy" is performed. A window is made in the top of the bones of the spine, and the degenerate disk is carefully removed and the compression relieved.

In some patients with calcified disks that have not yet herniated, a prophylactic (preventative) procedure called "fenestration" may be recommended. In fenestration, a small window is cut in the side or bottom of the calcified disks. Some of the calcified disk material can then be removed, the remainder, if ever extruded, should not extrude into the spinal canal. The procedure cannot be performed on all the disks thus only those disks considered to be at higher risk are operated upon.

AFTER-CARE AND PROGNOSIS
Patients are generally hospitalized for about 1 week. They must be confined to a crate or pen until instructed differently by the doctor. If permanent, irreversible damage has not already occurred, patients may begin to show improvement within a few days to a few weeks after surgery. Physical therapy consisting of slings, carts, exercises and swimming may be important for recovery of strength and coordination. Patients may continue to improve for up to 6 months after treatment.

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