What is degenerative myelopathy?
Degenerative myelopathy (DM), also known as chronic degenerative radiculomyelopathy (CDRM), is a disease affecting the spinal cord, resulting in slowly progressive hind limb weakness and paralysis. The symptoms result from degeneration of the white matter of the spinal cord. DM is similar to some of the forms of human amyotrophic lateral sclerosis (ALS) more commonly known as Lou Gehrig’s Disease.
The exact cause of DM is unknown. In its early stages, the symptoms of DM resemble those of osteoarthritis (arthritis), which often occurs secondary to hip dysplasia in many large breed dogs, making diagnosis challenging.
In later stages of the disease, the progressive weakness and ataxia (wobbling, stumbling) distinguish it from osteoarthritis of the hip joints. Other considerations for this condition include spinal injuries, spinal tumors, lumbosacral stenosis, fibrocartilaginous embolism, myasthenia gravis, and discospondylitis.
What are the clinical signs?
Early clinical signs include:
- The hind paws "knuckle" or turn under so that the dog walks on its knuckles, especially when turning.
- The dog's hindquarters appear to sway when standing still.
- The dog falls over easily when pushed from the side.
- The hind feet seem to scrape the ground when walking and sometimes the top surface of the feet become hairless and irritated from repeated trauma.
- The dog has difficulty getting up from a lying position.
As the condition progresses and the spinal cord deteriorates, these symptoms worsen, eventually progressing to paralysis of the hind end.
What breeds are most commonly affected?
A genetic mutation, SOD-1, has been identified as a major risk factor for DM. DM is considered a disease of middle-aged to older dogs including German Shepherds, German Shepherd crosses, Siberian Huskies, and Collies. Several other breeds have been identified as at risk for developing DM, including Bernese Mountain Dog, Boxers, Chesapeake Bay Retrievers, Golden Retrievers, Kerry Blue Terriers, Miniature Poodles, Nova Scotia Duck Tolling Retrievers, Pembroke Welsh Corgis, Pugs, Rhodesian Ridgebacks, Standard Poodles, Welsh Corgis, Welsh Corgi Cardigans, Welsh Corgi Pembrokes, and Wirehaired Fox Terriers).
At what age does DM typically occur?
The condition is most common in middle-aged to older dogs, with a range from 4-14 years. It has been reported in young dogs on rare occasions.
What causes DM?
The exact cause of degenerative myelopathy is unknown although a genetic mutation is highly suspected. DNA testing through the Orthopedic Foundation for Animals can identify:
- dogs that are clear of DM (two normal copies of the gene),
- dogs that are carriers (one normal copy of the gene and one abnormal copy), and
- dogs at a much higher risk for developing DM (two copies of the mutated gene).
"At-risk breeds should be tested for the SOD-1 mutation prior to breeding to avoid passing on the abnormal gene."
Degenerative myelopathy is an autosomal recessive genetic disease, meaning inheriting one or two copies of the gene (SOD1), increases the risk of developing the disease; however, not all dogs who carry even both genes will develop DM. Other factors, both genetic and environmental, are believed to contribute to developing DM. There is still much to be discovered about DM in dogs and its causes. At-risk breeds should be tested for the SOD-1 mutation prior to breeding to avoid passing on the abnormal gene.
How is DM diagnosed?
This disease will be suspected on the basis of breed, medical history, physical examination, and diagnostic tests. X-rays and other spinal imaging techniques will rule out other problems such as hip dysplasia and chronic arthritis, most often during the initial stages of DM. If one of these other problems is also present, it can contribute to the patient's loss of function of the hindquarters. Other tests that may be conducted include cerebrospinal fluid (CSF) analysis, tissue biopsies and neuromuscular tests such as spinal cord evoked potentials. DNA testing for the SOD-1 mutation is recommended in any at-risk breed displaying clinical signs consistent with DM. Histopathology of the spinal cord is required for definitive diagnosis of DM. The diagnosis of DM is a challenging, often a time-consuming endeavor requiring many tests before a definitive diagnosis is made.
Are there bowel and bladder problems?
Bladder and bowel function are generally normal in the initial stages of degenerative myelopathy. However, as the disease progresses to paralysis, the dog may develop urinary and fecal incontinence.
Is the condition painful?
Most affected dogs do not seem painful, just very weak. If the dog appears to be painful, there may be another condition such as arthritis complicating the condition.
Is treatment possible?
There is no effective treatment for degenerative myelopathy at present. Treatment of other concurrent problems such as arthritis or hip dysplasia may provide some relief from pain or discomfort. It is important to avoid obesity, so diet and exercise (walking and swimming) are vital components of treatment. The goal is to maintain the dog on its feet for as long as possible. Physical therapy has been shown to prolong quality of life and preserve muscle mass. Any dog with DM should be kept as physically active as possible for as long as possible. The progression of clinical signs has been shown to be slowed with a combination of epsilon-aminocaproic acid, N-acetylcysteine, prednisone, vitamins B, C, and E, and exercise therapy.
What is the prognosis?
Unfortunately, degenerative myelopathy is a progressive, incurable disease. Although bladder and bowel control are not affected initially, as the spinal cord continues to degenerate, the dog's mobility becomes severely restricted and it has more difficulty controlling urination and defecation. The nature and temperament of the affected dog determines whether or not a mobility aid such as a paraplegic cart will improve its quality of life in the short term. However, the inevitable progression of this degenerative condition means that the symptoms will worsen over time. Your veterinarian will assess your dog's circumstances and help you determine the most appropriate treatment options for your pet.