Dwarfed dogs: What exactly is going on in there? Part 2: Not the legs

Yesterday I wrote about how the growth plates in Cardigan legs are dysfunctional and don’t grow a normal length of bone. Today I want to look at the second reason we care about cartilage as breeders:

Because the bad cartilage isn’t just in the legs.

Dogs with chondrodystrophy or achondroplasia have bad cartilage EVERYWHERE. It has abnormal cell distribution and formation all over the place, from the front of the dog to the back. 

Dogs function perfectly well with bizarre cartilage, as long as no growth plates are actually injured and the cartilage doesn’t fail and cause arthritis, just about everywhere. The major exception is in one of the other places where the body has very, very important connective tissue: the spine.

As almost everyone knows, between the individual bones of the dog’s spine there are “discs” of squishy stuff. What does that have to do with cartilage? That the squishy stuff, which is basically like those (awesome!) 80’s bubble gum flavors with a soft outside and liquid inside, is a bit like very young baby cartilage. It’s not actually cartilage yet; it’s a fibrous sack with liquid in the center. That nucleus of liquid is what makes the spine such an amazing shock absorber; the bones can compress without touching each other and can be stretched apart without injury.

As dogs age, and this means ALL dogs, not just ours, that young baby cartilage (fibers and liquid) begins to age and dry out and turn into real cartilage. Most dogs are experiencing some lack of flexibility in the discs by the time they’re ten or so. Real cartilage is not as soft and bouncy as the disc used to be, so the spine becomes less able to flex. The now-cartilagenous discs begin to degenerate as the dog gets older and older, and so the bones (the individual vertebrae) grow little bony spines to try to stabilize the disc material. This is spinal arthritis or spondylosis.

Spinal arthritis is VERY normal in elderly dogs of all breeds. In most of them it just makes them say “Oh my achin’ back” in the same way that we do (or will eventually) as we age.

In an unlucky few the bony spines grow into the spinal cord or begin to squeeze some of the big trunk nerves that come off the spinal cord. This can lead to progressive weakening, instability, and paralysis (which, by the way, can mimic DM and is one of the many reasons you NEVER definitively diagnose DM without a necropsy).

In a few breeds, there is a tendency toward a severe form of spondylosis that occurs early in life and in the neck vertebrae. This causes progressive weakness and a “wobbly” gait as the spinal nerves are squished, so owners and breeders call it Wobblers. Wobblers is unfortunately more common than any of us would like to see it in Dobermans and Danes, and as of yet nobody knows what makes it happen. Bony growths are usually associated with injury, so we’ve all wondered whether these were dogs who had their necks traumatized at some point (Dobies and Danes have very long necks for their bodies), or if there is some familial relationship, but you don’t see it reliably passed along and predicting it is impossible.

Thankfully, most breeds don’t see any disc problems or bony changes in the vertebrae until they are advanced in age.

But… and you know there has to be a but… there is one class of breeds that has major disc damage at a very early age. And yes, you guessed it. We’re part of it.

Remember how the growth plates of the long leg bones in achondroplastic dogs age much more quickly than they do in longer-legged dogs? The same thing happens in the discs between the spinal bones.

In achondroplastic dogs, the liquid center of the discs doesn’t stay liquid. By the time the dog is six months old (yes, SIX MONTHS) the liquid center is being replaced by cartilage. This isn’t quite the same thing as what happens with longer-legged dogs; the liquid doesn’t gradually dry out like happens as those dogs age. In dwarfed dogs the liquid is actively replaced by cartilage. By twelve months the replacement is dramatic. By the time the dog is three years old, each disc has only a fraction of the liquid that should be in its center, and the center has actually begun to calcify (become like bone). The fibrous outer layer of the disc now has to absorb all the strain on each vertebra, and it is beginning to degenerate too. 

Eventually, in a lot of dogs, the fibrous outer layer of the disc gives way, letting the inner material get out. The fibers are thinnest right below the spinal cord, so when the outer layer ruptures it usually does it there. This allows the inner material (which is by this point bony cartilage) to squirt out. It either squirts out upward, squishing the big bundle of nerves that is the spinal cord, or it squirts out up and to the side, squishing one or more of the big nerves that come down off the spine.

That’s why you can have a dog go down with a complete rear-end paralysis, or it can seem to go down worse on one side or the other. 

The final strike against achondroplastic discs is that when the discs rupture, it’s usually catastrophic. In other breeds when elderly dogs get disc problems it’s often that the fibrous outer layer tears slightly, allowing only a small bulge of inner disc into the spine. In dwarfed dogs the fibrous layer fails completely, allowing a great deal of actual disc material to the nerve bundles. 

What happens next (as though enough hasn’t already happened!) is that the disc material that’s exploded out of the ruined fibrous layer can directly harm or cut the nerves of the spine. If it doesn’t harm the nerves, it often cuts off the blood and oxygen supply to the nerves by blocking or tearing blood vessels. Even if it avoids those two disasters, the disc rupture is interpreted by the body as a major wound and the body rushes all sorts of vasoconstrictors (signals that tell blood vessels to stop letting blood through) to the area, causing damage to the nerves’ blood supply.  And even of NONE of those things happen, within hours the body will try to respond to the damage by causng a huge amount of swelling in the area. Again, spinal cord death all too often results. 

If the dog has a very small rip in the disc with only a bulge of inner tissue, it can recover with rest and meds. Unfortunately, that kind of tear is not very common in dwarfed breeds. If there is a catastrophically ruptured disc but there’s no swelling around the disc and the spinal cord is normal, there’s a decent chance that surgery (to the tune of about four grand) can restore function. The surgeon carefully removes the disc material and the nerves can recover. However, a different disc will often herniate later, especially if multiple discs are showing calcification. If the ruptured disc has already caused swelling of the spinal cord, even with surgery the dog has the odds against him or her ever recovering the ability to walk.

Disc rupture occurs in dwarfed breeds most often in their middle years, from age 3 to age 8 or so. It is intensely and horribly painful, at least until the spinal cord dies. It’s a true emergency, and in our breed non-surgical methods are rarely effective.