IVDD (intervertebral disc disease) happens when the cushioning discs between a dog’s vertebrae break down and push into the spinal canal, compressing the spinal cord. This degeneration can begin surprisingly early, sometimes within the first year of life in certain breeds, and it involves a cascade of structural, biochemical, and genetic factors that weaken discs long before symptoms ever appear.
What a Healthy Disc Looks Like
Each intervertebral disc has two main parts. The center, called the nucleus pulposus, is a gelatinous, bean-shaped mass with an extremely high water content, up to 88% in young dogs. That water is held in place by large, electrically charged protein complexes that create osmotic pressure, essentially pulling water in and keeping the disc plump and shock-absorbent. Surrounding this gel center is the annulus fibrosus, a tough outer shell made of organized layers (lamellae) of fibrous tissue arranged like the rings of a tree trunk.
Together, these two structures let the disc absorb compressive forces from running, jumping, and everyday movement while keeping the spinal cord safely cushioned inside the vertebral canal. When either component starts to deteriorate, the entire system fails.
How Discs Degenerate Over Time
The core problem in IVDD is that the gel center of the disc dries out and hardens. Those water-retaining protein complexes break down, the disc loses its ability to hold moisture, and the once-supple center becomes stiff and sometimes calcified. In chondrodystrophic breeds (the short-legged dogs like Dachshunds, Corgis, and French Bulldogs), this hardening process begins remarkably early, starting from birth to one year of age. In non-chondrodystrophic breeds, the degeneration is slower and tends to show up later in life.
As the center hardens, the outer shell also weakens. Small separations develop between its layered fibers, potentially worsened by repeated minor trauma from normal activity. The disc can no longer distribute forces evenly, and the altered mechanics accelerate the damage in a self-reinforcing cycle.
Type I: Sudden Rupture
In a Type I disc event (historically called Hansen Type I), the calcified, dehydrated center of the disc suddenly bursts through the weakened outer shell and explodes into the spinal canal. This is an acute, often dramatic event. One moment a dog is playing or jumping off the couch; the next, they may be unable to walk. The force of the extruded material slamming into the spinal cord causes immediate compression and often significant inflammation.
Type I events are strongly associated with chondrodystrophic breeds and typically strike young adult dogs. In Dachshunds, the overwhelming majority of these extrusions (92%) occur in the thoracolumbar region, the stretch of spine between the mid-back and lower back. Other breeds show more variation. French Bulldogs, for example, experience cervical (neck) disc extrusions in about 30% of cases.
Type II: Slow Protrusion
Type II disc disease looks different. Instead of the center rupturing through the outer shell, the degenerative material gradually infiltrates between the fibers of the annulus, causing the outer wall itself to thicken and bulge into the spinal canal. This protrusion develops slowly, pressing against the spinal cord over weeks, months, or even years. Dogs with Type II disease often show gradually worsening coordination or stiffness rather than a sudden crisis.
Because the compression builds incrementally, the spinal cord can partially adapt, which is why some dogs function surprisingly well despite significant narrowing of the spinal canal. But that adaptation has limits, and minor additional trauma can suddenly push a dog from “a little wobbly” to seriously impaired.
The Genetic Factor Behind High-Risk Breeds
The reason certain breeds are so vulnerable comes down to a specific genetic insertion. Researchers identified a copy of a growth factor gene (called an FGF4 retrogene) on chromosome 12 that is responsible for both the short-legged body shape and the predisposition to Type I IVDD across all affected breeds. Dogs carrying this gene had an odds ratio of 51.23 for developing IVDD, meaning they were roughly 51 times more likely to be affected than dogs without it.
A second, related FGF4 insertion on chromosome 18 was previously linked to the short-legged appearance in breeds like Basset Hounds and Corgis. But the chromosome 12 variant is the one most directly tied to disc calcification and disease. Dachshunds carry both, which helps explain why they face a 10 to 12 times higher relative risk of IVDD than other breeds, with an estimated 19% to 24% showing clinical signs during their lifetime.
What Happens to the Spinal Cord After Compression
The initial disc herniation is only the beginning. Once disc material compresses the spinal cord, a wave of secondary injury processes kicks in that can be just as damaging as the compression itself. Blood vessels in the spinal cord go into spasm, cutting off oxygen supply. The affected tissue swells. Cells release excessive amounts of signaling chemicals that overstimulate and kill neighboring nerve cells, a process called excitotoxicity. Calcium floods into cells, triggering enzymes that break down tissue from the inside.
The spinal cord is especially vulnerable to this kind of damage because nerve tissue has high energy demands, relatively weak built-in defenses against oxidative stress, and large amounts of fatty insulation (myelin) that is easily damaged by inflammatory byproducts. In the most severe cases, this secondary injury can spiral into a condition where spinal cord tissue liquefies and the damage spreads up and down the spine beyond the original injury site, with dramatically worsened markers of oxidative stress and inflammation at the center of the lesion.
How Severity Is Assessed
Veterinarians grade IVDD on a scale based on neurological function, ranging from pain with no nerve deficits at the mildest end to complete paralysis with loss of deep pain perception at the most severe. The critical dividing line is whether a dog can still feel a firm pinch on their toes (deep pain perception). Dogs that retain this sensation, even if they cannot walk, generally have a good to excellent prognosis for recovering the ability to walk again, depending on treatment. About 30% of dogs in surgical studies present as paralyzed but with intact deep pain perception. Roughly 16% present with no deep pain perception at all, which carries a significantly more guarded outlook.
Where IVDD Occurs Along the Spine
Across all breeds, about 73% of disc extrusions occur in the thoracolumbar spine, the section roughly from the last few ribs to the lower back. About 15% affect the cervical (neck) region. The exact distribution varies by breed. Dachshunds are overwhelmingly affected in the thoracolumbar area. French Bulldogs and English Cocker Spaniels show a broader spread, with a notably higher percentage of cervical cases.
The thoracolumbar region is vulnerable in part because of the mechanical transition between the relatively rigid thoracic spine (stabilized by the ribs) and the more flexible lumbar spine. This junction absorbs significant bending and rotational force, placing extra stress on the discs in that area. Type I disease in young adult dogs is typically first recognized in this region, with breed-specific differences in the exact age of onset.
How IVDD Is Detected
MRI is the most sensitive imaging tool for identifying disc herniation, with a detection rate of 98.5% in one comparative study. It is also more accurate than CT at pinpointing the exact location of compression and distinguishing between an extrusion (Type I) and a protrusion (Type II). CT without contrast dye still performs reasonably well at 88.6% sensitivity and is faster, which can matter in emergency situations. In practice, the choice often depends on availability, the dog’s stability, and whether fine detail about the type of herniation will change the treatment plan.