Premature disc degeneration is a disease in which a dog's vertebral discs begin to degenerate more rapidly than could be naturally expected. Degeneration might effect either the discs themselves in that they can become calcified and less mobile, or the spaces between the discs when cartilaginous cushions begin to deteriorate. It is classified under the generalized heading of premature aging and has many profound effects upon a dog's overall health, causing at best chronic pain and at worst loss of motor function.
Premature disc degeneration usually begins to occur in dogs that are between three and five years old, and seems to affect the discs in the cervical region of the spine most often. Depending on which discs are affected, the problem manifests itself visually in a variety of different ways. For example, the deterioration of certain discs might cause scoliosis, which is a condition in which one of your dog's hips or shoulder blades is visibly shorter than the other, producing a lopsided effect and extreme pain. Another possible visual confirmation of premature disc degeneration is kyphosis, which is where the spine forms a noticeable hump, producing a hunchback-like appearance that affects your dog's ability to walk. Depending on where the degenerating discs are located, you might even notice abnormal changes in the structure of your dog's ribcage. They might protrude out at a sharper angle or even begin to angle in. Any of the above cases should be treated as an emergency and medical attention is necessary.
Premature disc degeneration is diagnosed by taking an extensive series of radiographs of the spine. If this is done regularly, before any of the above problems begin to manifest, you might notice an early warning sign such as a single calcified disc or a narrowing of the spaces between discs. These should be taken very seriously as if they're caught early, you might be able to escape the most problematic symptoms of premature disc degeneration with a well-timed preventative surgery.
The only treatment for premature disc degeneration is surgery, and even then, success is not always guaranteed. Generally speaking, the sooner in the process of degeneration that the surgery is performed, the better the chances of a full recovery are. In any case, the surgery is quite extensive and is not usually recommended for breeds that don't perform well under anesthesia. Speak with your vet about other less invasive treatments if this is the case with you and your dog.
The main thing to remember about premature disc degeneration is that it's a strongly progressive disease. It will not stop advancing on its own without treatment, and the longer you wait before doing something, the worse the prognosis will become. If you wait too long, damage will move from the discs and cartilage cushions to the spinal cord itself. At that point, irreversible nerve damage will have occurred and severe, permanent injuries and the possibility of paralysis are almost certain.