What is Degenerative Myelopathy?
Degenerative myelopathy is a progressive disease of the spinal cord in older dogs. The disease has an insidious onset typically between 8 and 14 years of age. It begins with a loss of coordination (ataxia) in the hind limbs. The affected dog will wobble 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. The clinical course can range from 6 months to 1 year before dogs become paraplegic. If signs progress for a longer period of time, loss of urinary and fecal continence may occur and eventually weakness will develop in the front limbs. Another key feature of DM is that it is not a painful disease.
What causes Degenerative Myelopathy?
Degenerative myelopathy begins with the spinal cord in the thoracic (chest) region. If we look under the microscope at that area of the cord from a dog that has died from DM, we see degeneration of the white matter of the spinal cord. The white matter contains fibers that transmit movement commands from the brain to the limbs and sensory information from the limbs to the brain.
This degeneration consists of both demyelination (stripping away the insulation of these fibers) and axonal loss (loss of the actual fibers), and interferes with the communication between the brain and limbs. Recent research has identified a mutation in a gene that confers a greatly increased risk of developing the disease.
How is degenerative myelopathy clinically diagnosed?
Degenerative myelopathy is a diagnosis of elimination. We look for other causes of the weakness using diagnostic tests like myelography and MRI. When we have ruled them out, we end up with a presumptive diagnosis of DM. The only way to confirm the diagnosis is to examine the spinal cord under the microscope when a necropsy (autopsy) is performed. There are degenerative changes in the spinal cord characteristic for DM and not typical for some other spinal cord disease.
What else can look like degenerative myelopathy?
Any disease that affects the dog’s spinal cord can cause similar signs of loss of coordination and weakness. Since many of these diseases can be treated effectively, it is important to pursue the necessary tests to be sure that the dog doesn’t have one of these diseases. The most common cause of hind limb weakness is herniated intervertebral disks. The disks are shock absorbers between the vertebrae in the back. When herniated, they can cause pressure on the spinal cord and weakness or paralysis. Short-legged, long back dogs are prone to slipped disks. A herniated disk can usually be detected with X-rays of the spine and myelogram or by using more advanced imaging such as CT scan or MRI. Other diseases we should consider include tumors, cysts, infections, injuries and stroke. Similar diagnostic procedures will help to diagnose most of these diseases. If necessary, your veterinarian can refer you to a board certified neurologist who can aid in diagnosing degenerative myelopathy. A directory to a neurologist near you can be found at American College of Veterinary Internal Medicine website under the "Find a Specialist Near You" link.
How do we treat degenerative myelopathy?
There are no treatments that have been clearly shown to stop or slow progression of DM. Although there are a number of approaches that have been tried or recommended on the internet, no scientific evidence exists that they work. The outlook for a dog with DM is still grave. The discovery of a gene that identifies dogs at risk for developing degenerative myelopathy could pave the way for therapeutic trials to prevent the disease from developing. Meanwhile, the quality of life of an affected dog can be improved by measures such as good nursing care, physical rehabilitation, pressure sore prevention, monitoring for urinary infections, and ways to increase mobility through use of harnesses and carts.
Champion Agility Ridgeback Shawna with Degenerative Myelopathy in dog wheelchair
Understanding the DNA Test for Degenerative Myelopathy
We have discovered a mutation in a gene which is associated with development of degenerative myelopathy (DM). In that gene, the DNA occurs in two possible forms (or alleles). The “G” allele is the predominant form in dogs that seldom or never develop DM; you can think of it as the “Good” allele. The “A” allele is more frequent in dogs exhibiting clinical signs of DM; you can think of it as the “Affected” allele.
Summary: “A” allele is associated with DM; “G” allele is not associated with DM.
Since an individual dog inherits two alleles (one from the sire and one from the dam) there are three possible test results: two “A” alleles; one “A” and one “G” allele; and, two “G” alleles.
Summary: Test results can be A/A (affected/at risk), A/G (carrier), or G/G (normal) .
Microscopic examination of a section of spinal cord (following euthanasia) is the "gold standard" for diagnosing and confirming DM. We do not have the opportunity to examine cord samples from all the dogs that have died or been euthanized due to DM, but for those cords submitted for evaluation, and where the cellular changes have been consistent with a diagnosis of DM, the dogs have had a DNA test result of A/A in all but 2 individuals. There is additional work being done to better understand these 2 exceptions, but it is clear that the vast majority of real DM cases do have the A/A test result.
Summary: Dogs that test A/G or G/G are very unlikely to develop DM. Dogs that test A/A are likely to develop clinical signs of DM at some point as they age. Additional research now in progress is focused on understanding why some A/A dogs show clincal signs of DM at 7 or 8 years of age while others only begin to show clinical signs at 14 or 15yrs or older, or may die from some other cause without developing recognized clinical signs of DM.
The “A” allele is very common in some breeds. In these breeds, an overly aggressive breeding program to eliminate the dogs testing A/A or A/G might be destructive to the breed as a whole because it would eliminate a large fraction of the high quality dogs that would otherwise contribute desirable qualities to the breed. Nonetheless, DM should be taken seriously. It is a fatal disease with devastating consequences for the dogs and a very unpleasant experience for the owners who care for them. Thus, a realistic approach when considering which dogs to select for breeding would be to consider dogs with the A/A or A/G test result to have a fault, just as a poor top-line or imperfect gait would be considered faults. Dogs that test A/A should be considered to have a worse fault than those that test A/G. Dog breeders could then continue to do what conscientious breeders have always done: make their selections for breeding stock in light of all of the dogs’ good points and all of the dogs’ faults. Using this approach over many generations should substantially reduce the prevalence of DM while continuing to maintain or improve those qualities that have contributed to the various dog breeds.
Summary: We recommend that dog breeders take into consideration the DM test results as they plan their breeding programs; however, they should not over-emphasize this test result. Instead, the test result is one factor among many in a balanced breeding program. It is important for breeders and owners to keep in mind all the traits present in an individual dog, and not to simply breed a test result. We encourage breeders to DNA test potential breeding stock and consider the results of the DM test as part of their evaluation and decision-making process.
Figure 1a & b- Breeds with > 50 individuals tested
If allele frequency = 0.5, half of all individuals have the allele; if frequency = 1, all animals have the allele.
