Introduction

Intervertebral disc disease (IVDD) is a common cause of neurological dysfunction seen in small animal practice. Symptoms can range from pain and discomfort to proprioceptive delays, ataxia, paresis, and paralysis. Certain breeds are at significantly higher risk of developing IVDD. These high-risk breeds include the Dachshund and Pekingese. Two types of intervertebral disc disease are seen, Hansen type 1 and Hansen type 2 disc disease.

Thoracolumbar Disc Disease

Thoracolumbar disc disease accounts for approximately 85% of disc disease seen in dogs. The remaining cases are mainly cervical disc disease. The breeds most commonly affected are the Dachshund (majority of cases), Pekingese, Spaniel, and Staffordshire bull terrier. The highest incidence is seen between the ages of 4–6 years for chondrodystrophic breeds vs. 8–10 years for non-chondrodystrophic breeds. Anatomically, the highest risk discs for IVDD are T12/13; T13/L1 and L1/2 (70% of IVDD).

The clinical presentation/symptoms of dogs with disc disease are dependent on the location of the disc extrusion/protrusion, the dynamic force of compression, and the duration of the spinal cord compression. Clinical symptoms include spinal/back pain, conscious proprioception deficits, ataxia/paresis and paralysis (with or without deep pain sensation). The presence or absence of deep pain sensation is a vital prognostic indicator. Dogs with hindlimb paralysis will also have urinary incontinence.

Anatomy of the Intervertebral Disc

The intervertebral discs are located between adjacent vertebral endplates. The two parts of the disc are the annulus fibrosus and the nucleus pulposus. The annulus is the peripheral ring which surrounds the centrally located nucleus. The annulus consists of parallel layers of collagenous connective tissue and is attached to the vertebral endplate. The nucleus is eccentrically located (i.e., the annulus is thinner dorsally and thicker ventrally and laterally [2–3 times thicker]). The nucleus is gelatinous in consistency. The intervertebral disc has a poor blood supply.

Pathophysiology of Intervertebral Disc Disease

The major function of the disc is that of a "shock absorber" between the vertebral bodies. The ability to perform this function effectively is dependent on the water-binding qualities of the nucleus, the proteoglycan content of the nucleus, and the structure and integrity of the annulus. With normal age-related changes there is a decrease in the proteoglycan content, the disc starts dehydrating and the pliability/elasticity of the annulus decreases. These changes result in a compromised ability to perform its main function (shock absorption).

Two types of disc degeneration are possible, chondroid metaplasia and fibroid metaplasia. Both of these result in changes to the biochemistry of the disc with resulting changes to the biomechanical properties of the disc.

Chondroid metaplasia is seen in chondrodystrophic breeds and is characterized by:

 Starts at 8 months to 2 years of age

 Seventy-five percent of discs show signs of degeneration by 1 year

 Decreased proteoglycans and water content

 Increased collagen content

 Calcification within the nucleus

 Decreased " shock absorbing" ability

 Acute mechanical failure (with or without trauma)

 Highest incidence at 2–7 years

Fibroid metaplasia is seen in non-chondrodystrophic breeds and is characterized by:

 Slower onset

 Rarely calcify

 Nucleus maintains a more gel-like consistency

 Bulging dorsal annulus (i.e., disc protrusion as apposed to a disc extrusion)

 Highest incidence at 8–10 years

Disc disease may be classified as either a Hansen type 1 or a Hansen type 2. Hansen type 1 IVDD is typically an acute mechanical failure with rupture of the dorsal annulus fibrosus and extrusion of nucleus pulposus. It is characterized by free disc material within the vertebral canal and may be explosive. It is seen in chondrodystrophic breeds and typically between 2 and 7 years of age with the highest incidence at 4–6 years. A Hansen type 1 IVDD is by definition a disc extrusion.

Hansen type 2 IVDD is often more chronic in nature and is characterized by a bulging of the dorsal annulus. Hansen type 2 IVDD may occur in chondrodystrophic and non-chondrodystrophic breeds. No free disc material occurs within the vertebral canal and Hansen type 2 IVDD is therefore a disc protrusion as apposed to a disc extrusion.

Clinical Presentation of IVDD

The clinical signs associated with IVDD are highly variable and dependent on the volume of extruded disc material (degree of spinal cord compression) as well as the rate at which the compression occurs (acute vs. chronic).

When the spinal cord is compressed, there is a predictable loss of function. The first sign will be loss of proprioception followed by a loss of motor function. This will be followed by loss of superficial sensation and lastly by loss of deep pain sensation.

It is for this reason that the assessment of deep pain sensation is vital as it is a very important prognostic indicator for recovery following surgical treatment.

Diagnosis and Treatment of IVDD

Diagnosis of IVDD is dependent on either myelography or MRI. These modalities allow identification of spinal cord compression by the prolapsed or extruded disc. Treatment is dependent on the clinical symptoms. Animals showing signs of spinal pain only may be treated medically and monitored closely. If there is any deterioration or a lack of improvement, surgical management should be advised.

Surgical management is advised for all cases showing neurological deficits (delayed proprioception, ataxia, paresis, and paralysis). Surgical decompression via a pediculectomy or hemilaminectomy is advised. Fenestration of the affected disc as well as adjacent high-risk discs is advised. Fenestration has been shown to reduce the risk of disc extrusion in the future.

Conclusion

Intervertebral disc disease (IVDD) is a common neurological disease encountered in small animal practice. Chondrodystrophic breeds are at significantly higher risk of developing IVDD than other breeds. The breeds most commonly presented for IVDD are the Dachshund and the Pekingese. Within these breeds there seems to be certain genetic lines that are at higher risk.