Canine degenerative myelopathy (also known as chronic degenerative radiculomyelopathy) is a progressive disease of the spinal cord in older dogs. The disease has an insidious onset typically between 7 and 14 years of age. It begins with a loss of coordination (ataxia) in the hind limbs. As of July 15, 2008 the mutated gene responsible for DM has been found present in 43 breeds including German Shepherds, Boxers, Chesapeake Bay Retrievers, Rhodesian Ridgebacks, and both breeds of Welsh Corgis. The disease is chronic and progressive, and resulting in paralysis.
The myelin is an insulating sheath around neurons in the spinal cord. One proposed cause of degenerative myelopathy is that the immune system attacks this sheath, breaking it down. This results in a loss of communication between nerves in lower body of the animal and the brain.
The Orthopedic Foundation For Animals has a DNA test that can be purchased to see if their dog carries the mutated genes for DM. The test is for any dog but is only recommended for certain breeds. The test consists of a cheek swab (using something similar to a Q-tip to swab the inside of the cheek to submit for testing).
The test checks for the mutated gene that will tell if your dog may be affected by degenerative myelopathy, if it is a carrier, or if it is unaffected.
The results are:
- Normal / Normal (N/N) – this means your dog does not have the mutated strain and it will not develop degenerative myelopathy. (clear)
- Normal / Abnormal (N/A) – this means your dog is a carrier of the gene but will not develop degenerative myelopathy. (carrier)
- Abnormal / Abnormal (A/A) – this means that your dog is affected with degenerative myelopathy and may develop degenerative myelopathy (affected). Not all dogs with A/A results will develop DM.
Degenerative myelopathy initially affects the back legs and causes muscle weakness and loss, and lack of coordination. These cause a staggering affect that may appear to be arthritis. The dog may drag one or both rear paws when it walks. This dragging can cause the nails of one foot to be worn down. The condition may lead to extensive paralysis of the back legs. As the disease progresses, the animal may display symptoms such as incontinence and has considerable difficulties with both balance and walking. If allowed to progress, the animal will show front limb involvement and extensive muscle atrophy. Eventually cranial nerve or respiratory muscle involvement necessitates euthanasia. Progression of the disease is generally slow but highly variable. The animal could be crippled within a few months, or may survive up to three years or more.
The aetiology of this disease is unknown. Recent research has shown that a mutation in the SOD1 gene is a risk factor for developing degenerative myelopathy in several breeds. Mutations in SOD1 are also associated with familial amyotrophic lateral sclerosis (Lou Gehrig’s disease) in people.
Known causes of spinal cord dysfunction should be excluded before accepting the diagnosis of degenerative myelopathy; disc disease (protrusions) or spinal cord tumours can cause compression of the spinal cord with similar signs to degenerative myelopathy.
Degenerative myelopathy is a non-reversible, progressive disease that cannot be cured. There are no treatments that have been clearly shown to stop or slow progression of DM.