Thoracolumbar Intervertebral Disc Disease

Intervertebral disc disease is the most common neurologic syndrome seen in the dog. Disc degeneration has been reported in 84 breeds with particular susceptibility in certain small breeds. These breeds (Dachshund, Pekinese, Poodle, Beagle, etc.) have characteristic skeletal changes that predisposes the discs to change at a very early age.

Intervertebral discs act as cushions between the vertebrae and function as the shock absorbers of the spine. A normal disc has two regions: a resilient gelatinous nucleus in the center and an outer fibrous ring that encircles the nucleus (see Fig.1). A degenerative disc loses its resiliency when its jelly-like center calcifies and takes on a gritty, hardened consistency. No longer able to cushion the vertebrae, the center is predisposed to bulging and to rupture (extrusion), resulting in pressure on the spinal cord, pain, and paralysis.

Mild disc rupture may cause back pain while a more moderate rupture causes weakness and a wobbly gait (see Fig. 2). If a large amount ruptures, or if the disc ruptures quickly and causes spinal cord swelling, the pressure can result in a potentially life threatening paralysis (see Fig. 3).

Diagnosis

A tentative diagnosis of thoracolumbar intervertebral disc disease is made on the history and neurologic examination. Radiographs (X-rays) can reveal the presence of degenerative, calcified discs and may outline narrowed disc spaces with evidence of extruded (ruptured) calcified disc material in the spinal canal. A definitive diagnosis may require a myelogram. A myelogram (a contrast dye study of the spine) is used to confirm and document not only the location of the ruptured disc but also the amount of spinal cord swelling. The myelogram is a common and safe diagnostic procedure when performed with care and under the proper conditions.

An individual’s prognosis depends on many factors:

• The severity of neurologic dysfunction

• The number of previous episodes of back pain

• The amount of disc material that has ruptured

• The degree of accompanying spinal cord swelling

• How quickly the disc ruptured (minutes to over several days)

• The length of time the disc has been ruptured

• The overall physical condition of the patient

This means that paralysis is not the only factor in the individual patient’s prognosis for recovery.

In general, the ability to perceive deep pain in the rear limbs and tail area remains the key prognostic indicator. If paralysis is present, how quickly they went down and how quickly they may have lost deep pain perception are the keys to determining if permanent damage has occurred. Therefore, the neurologic status and radiographs (x-rays) are used to determine the severity of each individual’s condition and, subsequently, the best treatment.

Treatment

Individuals experiencing their first episode of back pain with minimal neurologic dysfunction may be treated medically. The medications include corticosteroids to relieve the cord swelling and pain caused by intense inflammation. Patients with recurring painful episodes or significant neurologic deficits are candidates for a hemilaminectomy. This procedure removes one wall of the vertebrae allowing the surgeon to delicately extract the disc material from the spinal canal without injuring the spinal cord (see Fig. 4). With pressure removed from around the cord, neurologic function may then begin to return.

A second procedure is then performed to remove the center of the adjacent degenerative discs. This procedure can include up to six intervertebral discs and involves cutting a window in the outer fibrous ring of the discs followed by extraction of the calcified, degenerative centers. This fenestration of the disc centers should prevent recurrence of any disc ruptures, while allowing normal, pain free motion at each disc site. As the resected center of each disc center scars, there is little to no effect on back mobility (see Fig. 5).