Degenerative myelopathy (progressive rear limb weakness or paralysis) was first described as a specific neurologic disease in 1973. The age at onset may be five to fourteen years. It has been reported in almost all large breeds of dogs with the disease appearing with relative frequency in German Shepherds, Welsh Corgis, Irish Setters, and Chesapeake Bay Retrievers.
The disease has an insidious onset typically around 9-11 years of age. The classic presentation is a painless, slowly progressive rear limb weakness or paralysis. There may be discomfort due to arthritis in the hip or lower lumbar (lower back) area, but this usually improves with activity. Over days, weeks, or months the dog becomes progressively weaker as is evidenced by shuffling of the rear limbs and lack of coordination. The affected dog will wobble when walking, knuckle over or drag their feet, and may cross the feet. As the disease progresses, the limbs become weak and the dog begins to buckle at the knees and have difficulty standing. The weakness gets progressively worse until the dog is unable to walk. They may ultimately lose continence and function in the front limbs as well. Finally, full paralysis coupled with fecal and urinary incontinence develops.
Causes of DM
The exact cause of DM is unknown. Genetics probably plays an important role in the disease since it is common only in certain breeds of dogs and follows a stereotyped pattern. Various infectious, immune mediated, and nutritional theories have been investigated, but no definitive cause has been found.
Current research points to an immune mediated basis for degenerative myelopathy. It is hoped that research involving the antigens present in the immune complexes formed in degenerative myelopathy will lead to a blood test for this condition.
A diagnosis of degenerative myelopathy is based on a history of progressive spinal ataxia and weakness that may be waxing or steadily progressive. The tentative diagnosis is supported by neurological findings of widespread thoracolumbar (back) spinal cord dysfunction. Laboratory findings are generally within normal limits except for an elevated cerebral spinal fluid (CSF) protein. A myelogram (contrast dye study of the spine) must be performed to differentiate degenerative myelopathy from disc disease, tumors, and other progressive neurologic diseases.
DM is a diagnosis of elimination. That is we look for other causes of the weakness and when we have ruled them out, we end up with a tentative diagnosis of DM. The only way to confirm that suspicion, however, is to examine the spinal cord under the microscope when a necropsy (post-mortem exam) is performed. There are characteristic degenerative changes in the spinal cord, which tell us this is DM and not some other spinal cord disease.
If we look under the microscope at that area of the cord from a dog who has died from DM, we see degeneration of the white matter of the spinal cord. The white matter contains the 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 fibers themselves).
Other Disease That Can Cause Similar Symptoms
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 tests necessary 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 (slipped disks). The disks are shock-absorbers between the bones of the back. When they herniate, they can cause pressure on the spinal cord and weakness or paralysis. All of the short-legged dogs (Welsh Corgi, Dachshund, Basset Hound, etc.) are prone to slipped disks. A slipped disk can usually be detected with special X-rays of the spine (myelogram), but sometimes more advanced studies such as a CT or MRI scan are necessary. Even dogs with severely slipped disks can often be helped with surgery if diagnosed early.
Infections of the spinal cord can also cause weakness or paralysis. A spinal tap, usually taken at the time the myelogram is performed, can help detect inflammation of the spinal cord. Other diseases we consider include tumors, cysts, injuries and strokes. The combination of myelogram, spinal tap, and possibly CT or MRI allow us to diagnose most of these diseases.
Included among measures that can improve the quality of life for the dog are good nursing care, physical therapy, pressure sore prevention, aggressive treatment of urinary infections, and sometimes carts or harnesses to improve mobility.
Below are four specific additional suggestions for the management of degenerative myelopathy and care of an affected dog:
1. Exercise - Exercise is extremely important in maintaining the well being of affected dogs, maximizing muscle tone, and maintaining good circulation and conditioning. This is best achieved by an increasing schedule of alternate day exercise. Since many dogs have lost muscle tone prior to their diagnosis, it is important to gradually build up their level of activity. The goal is to do aerobic exercise for thirty minutes twice a week and one hour once a week. This can begin with walking and gradually progress to a faster pace. While not all patients can reach this goal, it is important to strive to do so. Running loose on the owner’s property is not adequate exercise. Regular periods of programmed, continuous exercise are required. It is equally important that the patient with degenerative myelopathy be allowed to rest on the day exercise is not scheduled. This will allow strained muscles and tendons to heal and will increase muscle strength. The dogs do not have to be confined but merely discouraged from any strenuous exercise on days of rest. Consistent, controlled building of muscle tone through exercise will help delay the progression of degenerative myelopathy.
2. Supportive measures - Vitamin support may be useful in delaying degenerative myelopathy symptoms. Some recommend that patients receive 200 IU of vitamin E daily and one high potency B vitamin (B-1) twice daily. Synthetic vitamins are cheaper and just as effective as natural vitamins. No other supplementation of a balanced diet is needed or indicated in the treatment of degenerative myelopathy. Because vitamin E at the levels recommended is a nonsteroidal antiinflammatory agent, concurrent use of aspirin-like drugs is not recommended. Should aspirin-like drugs be required to treat arthritis in degenerative myelopathy patients, daily vitamin E supplementation is reduced to 100 IU.
3. Medication - Aminocaproic acid (Amicar) may prevent progression or result in clinical remission of degenerative myelopathy in over 15 to 20% of patients. It is given orally at 500 mg every eight hours (three times daily). Since the pill form of Amicar has become expensive, we recommend giving aminocaproic acid as a solution using the generic product. The aminocaproic acid can be mixed with a vitamin/mineral supplement to provide a palatable solution for oral usage. The only side effects that have been attributed to aminocaproic acid have been occasional gastrointestinal irritation. This presents a problem in only a few patients, usually those with preexisting gastrointestinal problems. In some dogs, vitamin supplements can cause excessive flatulence, necessitating that they receive the aminocaproic acid solution without the vitamin/mineral supplement.
4. Minimization of stress - Degenerative myelopathy progresses at different rates in each animal. Stress plays a role in its advancement. Minimizing stressful situations is important. While anesthesia does not appear to cause problems with degenerative myelopathy, even minor invasive surgical procedures can result in a marked increase in clinical signs. Therefore, we recommend caution in considering major surgery in patients with degenerative myelopathy. The worsening caused by surgical stress can be irreversible.
Despite the very best and most up-to-date care, the outlook for a dog with DM is still grave. There are no treatments that have been clearly shown to stop or slow the progression of the disease. There are a number of approaches that have been tried, and we continue to look for new treatments to improve lessening of symptoms and general quality of life.