IVDD in dogs is primarily caused by premature degeneration of the cushioning discs between the vertebrae, and genetics are the biggest driver. A specific gene mutation found in short-legged breeds causes their spinal discs to dry out and harden as early as one year of age, making them vulnerable to rupture. But genetics aren’t the only factor. The type of IVDD, how it develops, and what triggers an episode all vary depending on a dog’s breed, age, and physical activity.
The Genetic Mutation Behind Most Cases
The strongest known cause of IVDD is a genetic mutation called an FGF4 retrogene, located on chromosome 12. This mutation is responsible for the trait known as chondrodystrophy: the combination of shortened legs and abnormal spinal discs that defines breeds like Dachshunds, French Bulldogs, Corgis, and Basset Hounds. A second, related FGF4 retrogene on chromosome 18 causes short legs alone without the disc abnormalities.
In dogs carrying the chromosome 12 mutation, the soft, gel-like center of the spinal disc transforms into stiff, cartilage-like tissue very early in life. In 75 to 90 percent of chondrodystrophic dogs, this transformation is already complete by the time the dog turns one. The hardened disc material loses its ability to absorb shock, and the tough outer ring of the disc weakens. Over time, the rigid center can burst through the weakened outer layer and press into the spinal cord.
The mutation acts as what geneticists call a “gain of function” allele, meaning it produces extra activity from a growth factor protein. The exact chain of events from extra protein to damaged discs isn’t fully mapped, but the end result is clear: breeds with this mutation develop disc disease at dramatically higher rates. About 15 percent of all Dachshunds will develop IVDD during their lifetimes, the highest rate of any breed.
How Type I and Type II IVDD Differ
Not all disc disease works the same way. The two main forms, called Hansen Type I and Type II, have different causes, affect different dogs, and progress on very different timelines.
Type I: Disc Extrusion
This is the form driven by the genetic mutation described above. The hardened disc center loses water content and becomes mineralized, then ruptures through the outer disc wall and explodes into the spinal canal. This can happen suddenly, sometimes during a jump off the couch or a twist during play. The impact on the spinal cord comes from both the physical compression of the disc material and the concussive force of the rupture itself. Type I is the most common form in chondrodystrophic breeds and typically strikes younger dogs, often between ages 3 and 7.
Type II: Disc Protrusion
Type II develops in non-chondrodystrophic breeds, usually dogs 7 years of age or older. Instead of a sudden rupture, the disc center gradually bulges outward while the outer wall remains intact but weakened. This slow protrusion pushes against the spinal cord over weeks or months. Because the compression builds gradually, many Type II cases are initially asymptomatic. Larger breeds like German Shepherds and Labrador Retrievers are more commonly affected. The underlying process here is age-related wear rather than a genetic defect in the disc itself.
Acute Non-Compressive Extrusion
A less common third type occurs when a small amount of disc material fires out at very high velocity, hitting or even penetrating the spinal cord. Unlike Type I, this material doesn’t stay in place and compress the cord. Instead, it causes bruising and bleeding. These episodes tend to happen during high-energy activities like running, jumping, or roughhousing, and the onset is immediate. This type can affect any breed.
Why Certain Breeds Are So Vulnerable
Breed is the single biggest predictor of IVDD risk. Dachshunds top the list, but the condition is common across all chondrodystrophic breeds. French Bulldogs, Beagles, Shih Tzus, Pekingese, Cocker Spaniels, and Corgis all carry elevated risk. These breeds share the chromosome 12 FGF4 mutation that causes early disc degeneration.
The risk isn’t just about having short legs. Some short-legged breeds carry only the chromosome 18 mutation, which shortens the limbs without affecting the discs. It’s specifically the chromosome 12 variant that damages disc structure. Genetic testing can now identify which mutation a dog carries, which is useful for breeders trying to reduce IVDD prevalence in future generations.
Non-chondrodystrophic breeds aren’t immune. German Shepherds, Dobermans, and other large breeds can develop Type II disease as they age. The mechanism is different, driven by decades of mechanical stress rather than a genetic flaw in the disc, but the result is similar: disc material pressing on the spinal cord.
Physical Triggers and Lifestyle Factors
Genetics load the gun, but physical activity often pulls the trigger. In dogs with already-degenerated discs, a sudden movement can cause the final rupture. Jumping off furniture, climbing stairs, rough play, or even an awkward landing during a walk can be enough. Many owners describe their dog “suddenly” going down, but the disc deterioration had been building for months or years before that moment.
Excess body weight is widely cited as a risk factor, and it makes intuitive sense: more weight means more compressive force on the spine. However, at least one study found that body condition score was not statistically associated with the severity grade of IVDD at diagnosis. This doesn’t mean weight is irrelevant to spinal health, but it does suggest that the genetic component overwhelms lifestyle factors in determining who gets the disease. A lean Dachshund is still at far higher risk than an overweight Labrador.
Repetitive spinal stress likely contributes as well. Dogs that frequently jump on and off beds, navigate steep stairs daily, or engage in high-impact activities place repeated strain on already-compromised discs. This is why veterinarians often recommend ramps and activity restrictions for high-risk breeds.
How Severity Is Graded
Once IVDD strikes, the severity depends on how much spinal cord damage has occurred. Veterinarians use a five-grade scale based on what the dog can still do:
- Grade 1: The dog is in pain but can still walk normally or nearly normally.
- Grade 2: The dog can walk but staggers, crosses legs, or places paws upside down.
- Grade 3: The dog cannot walk without help but can still make deliberate leg movements.
- Grade 4: The dog is paralyzed with no deliberate leg movement, though can still feel deep pain in the toes.
- Grade 5: Full paralysis with no deep pain sensation in the affected legs, plus loss of bladder and bowel control.
This grading matters because it directly predicts recovery. Dogs with grades 1 through 3 have roughly 80 percent recovery rates even without surgery, and up to 95 percent with surgical intervention. Grade 4 dogs recover about 64 percent of the time without surgery and 90 percent with it. Grade 5 is where the outlook changes sharply: only about 10 percent recover without surgery, and even with surgery the success rate drops to 50 to 60 percent. Up to 17.5 percent of grade 5 dogs develop progressive myelomalacia, a painful and untreatable deterioration of the spinal cord that worsens over the first week.
What Happens Inside the Disc
A healthy spinal disc has two parts: a soft, water-rich center that acts like a shock absorber, and a tough, fibrous outer ring that holds it in place. In Type I disease, the center loses its water content and key structural molecules, becoming stiff and mineralized. As this happens, the outer ring weakens. The disc essentially turns from a flexible cushion into a brittle structure sitting under constant pressure. When the outer ring finally tears, the calcified center material shoots into the spinal canal.
In Type II disease, the center doesn’t mineralize the same way. Instead, it gradually becomes more fibrous and loses its ability to stay contained. The outer ring stretches and bulges rather than tearing. The compression on the spinal cord builds slowly, which is why symptoms in Type II cases tend to creep in over time rather than appearing overnight.
The location of the affected disc also shapes the symptoms. Disc herniations in the mid-to-lower back affect the hind legs and bladder control. Cervical (neck) disc disease causes neck pain and can affect all four legs. About 65 percent of IVDD cases occur in the thoracolumbar spine, the section between the ribcage and lower back, which bears significant mechanical load during movement.