Degenerative Myelopathy (DM) is a progressive disease of the spinal cord in older dogs. It eventually causes hind limb weakness and paralysis. It is a condition that develops as a result of nerve function loss in the spinal cord.
 
The nerves are protected by a sheath called myelin. The myelin is an insulating sheath around peripheral nerves. In degenerative myelopathy, the myelin surrounding the nerves starts to breakdown and nerves begin to degenerate in the spinal cord. Both of these processes cause changes to nervous signals as they travel up and down the nerves.

Causes 

The cause of degenerative myelopathy is unknown. Dogs affected with degenerative myelopathy ultimately lose muscle mass from the disuse of their back legs and have difficulty getting up. Degenerative myelopathy is most often seen in German Shepards which have a genetic predisposition to DM, but many other breeds and mixes can be affected. Some breeds that have higher reported incidences of DM include: Welsh Corgis, Chesapeake Bay Retrievers, Irish Setters, Rhodesian Ridgebacks, and Boxers to name a few. The disease can present itself as early as 5 years of  age, but the average age of onset is generally between 9 and 11 years. It is more common in males than females.

Symptoms

Initially DM affects the hind legs and causes muscle weakness and lack of coordination. The affected dog will become off balance when walking, knuckle over or drag the feet. This can first occur in one hind limb and then affect the other. As the disease progresses, the limbs become weak and the dog begins to buckle and has difficulty standing. The weakness gets progressively worse until the dog is unable to walk at all.

Diagnosis

Diagnosis of degenerative myelopathy is generally made after a physical exam including a neurological exam done by the veterinarian. Diagnosis is also based on the history of the symptoms and, breed, age and health of the dog. Neurological procedures such as MRI are often inconclusive and are only helpful in ruling out other spinal problems. A myelogram (contrast dye study of the spine) may be performed to differentiate Degenerative Myelopathy from disc disease, tumors, and other progressive neurologic diseases. There is a DNA test now available through the Orthopedic Foundation for Animals (OFA) which identifies dogs that are carriers of the mutated gene, and those that are at risk for developing it.

Treatment

The treatments available are only aids in slowing the disease process, and as of now there is no cure. Physical therapy has been found to delay the progression of DM. Exercise may be a very important factor in maintaining as much mobility of the hind limbs as possible. Regular exercise such as walking and swimming can help keep muscles toned and maintain good circulation.