What is Degenerative Myelopathy?
Canine Degenerative Myelopathy
A
dog with degenerative myelopathy often stands with its legs close
together and may not correct an unusual foot position due to a lack of
conscious proprioception
Canine degenerative myelopathy, also known as chronic degenerative radiculomyelopathy, is an incurable, progressive disease of the canine spinal cord that is similar in many ways to amyotrophic lateral sclerosis.
Onset is typically after the age of 7 years and it is seen most
frequently in the German shepherd dog, Pembroke Welsh corgi, and boxer
dog, though the disorder is strongly associated with a gene mutation
in SOD1 that has been found in 43 breeds as of 2008, including the wire
fox terrier, Chesapeake Bay retriever, Rhodesian ridgeback, and Cardigan
Welsh corgi.[1][2] Progressive weakness and incoordination of the rear
limbs are often the first signs seen in affected dogs, with progression
over time to complete paralysis. 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.
Contents:
- 1 Testing
- 2 Genetics
- 3 Symptoms
- 4 Causes
- 5 Treatment
- 5.1 Exercise
Testing:
The Orthopedic Foundation for Animals has a DNA saliva test to screen
for the mutated gene that has been seen in dogs with degenerative
myelopathy. Now that a test is available the disease can be bred out of
breeds with a high preponderance. The test is only recommended for
predisposed breeds, but can be performed on DNA from any dog on samples
collected through swabbing the inside of the animal's cheek with a
sterile cotton swab or through venipuncture.
The
test determines whether the mutated copy of SOD1 is present in the DNA
sample submitted. It must be interpreted with caution by a veterinary
clinician in combination with the animal's clinical signs and other lab
test results.
The results reported are:
- Normal
/ Normal (N/N, or 'clear'): The dog does not have the mutation and is
extremely unlikely to develop degenerative myelopathy. There have been
cases in which dogs that tested clear were found to have DM upon
necropsy. This information was given to Dr Keller from the OFA. Dr
Coates performed necropsy. It is important to note the OFA statement on
their website that states "Recent evidence suggest that there are other
causes of DM in some breeds".
- Normal / Abnormal (N/A or
'carrier'): The dog has one mutated copy of the gene (is heterozygous)
and is a carrier but will not have degenerative myelopathy though there
has now been several cases of Carriers developing DM. It will be
possible for it to pass the mutation to offspring. A thorough
examination of the dog's pedigree and DNA testing should be undertaken
prior to breeding a dog with this result.
- Abnormal /
Abnormal (A/A or 'At Risk'): The dog has two copies (is homozygous) for
the mutation and is at risk for degenerative myelopathy.
Genetics Breeding risks for degenerative myelopathy can be calculated using the Punnett Square:
- If both parents are clear (N/N) then all of the puppies will be clear
- If one parent is a carrier (N/A) and one is clear (N/N) then roughly 50% of the puppies will be clear and 50% will be carriers
- If
both parents are carriers (N/A) then roughly 25% will be clear (N/N),
50% will be carriers (N/A), and 25% will be At risk (A/A)
- If one parent is clear (N/N) and one parent is affected (A/A) then all puppies will be carriers (N/A)
- If
one parent is a carrier (N/A) and one is at risk (A/A) then roughly 50%
of the puppies will be carriers (N/A) and 50% will be At risk (A/A)
- If both parents are At risk (A/A) then all puppies will be At risk (A/A)
Symptoms 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 and paralysis. Eventually
cranial nerve or respiratory muscle involvement necessitates euthanasia
or long term palliative care.
Progression
of the disease is generally slow but highly variable. The animal could
be crippled within a few months, or may survive as long as three years
or more.
Causes: The etiology of
this disease is unknown. Recent research has shown that a mutation in
the SOD1 gene is a risk factor for developing degnerative 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 tumors can cause compression of the spinal
cord with similar signs to degenerative myelopathy.
Treatment: Degenerative
myelopathy is an irreversible, progressive disease that cannot be
cured. There are no treatments that have been clearly shown to stop or
slow progression of DM.
Exercise:
Exercise has been recommended to maintain the dog's ability to
walk. Physiotherapy may prolong the length of time that the dog remains
mobile and increase survival time. Canine hydrotherapy (swimming) may
be more useful than walking. Use of a belly sling or hand-held harness
allows the handler the ability to support the dog's hind legs for
exercising or going up and down stairs.A 2-wheel dog cart, or "dog
wheelchair" can allow the dog to remain active and maintain its quality
of life once signs of weakness or paralysis of the hind limbs is
detected.
- 5.1 Exercise