The spine is made up of block bones called vertebrae. Dogs have seven cervical (neck), thirteen thoracic (chest), seven lumbar (back), three fused sacral (tail bone) and a variable number of tail vertebrae. Each vertebra is made up of a body, the lamina, which houses the spinal cord, and the facets that are partially responsible for the connection from one vertebra to the next (figure 1).
Intervertebral discs are situated between the vertebrae. In dogs and cats the disc sits just beneath or ventral to the spinal canal. The discs are comprised of a fibrous outer ring called the annulus fibrosis and an inner gel called the nucleus pulposes. The purposes of the discs are to act as both connectors and shock absorbers between each vertebra.
In general IVDD manifests by two different pathologic processes. In toy breed dogs the disc may start to degenerate and dehydrate as early as one year of age. As the disc loses its resiliency and the outer layer weakens, the inner nucleus is at risk to displace upward resulting in spinal cord trauma and compression (figure 2).
This process often involves a rapid burst of the disc resulting in acute clinical signs. In large breed dogs the pathologic process usually involves a gradual break down and fibrous repair of the annulus fibrosis resulting in gradual disc bulging and insidious or slow onset of clinical signs.