Degenerative myelopathy (DM) is an inherited, progressive disorder of the spinal cord that occurs secondary to a genetic mutation. Most commonly the mutation is in a gene known as the SOD-1 (superoxide dismutase) gene, however other mutations may occur. For example, Bernese Mountain dogs, which have the same clinical presentation as other breeds, have an alternate mutation. Additionally, most affected dogs have two abnormal copies of the gene (known as being homozygous) for the mutation; however, some homozygous dogs will not develop clinical signs. Heterozygotes (one normal copy, one abnormal copy) are also considered at-risk for developing the condition.
While more than 20 breeds of dogs have been confirmed to have DM, German Shepherds, Pembroke Welsh corgis, Boxers, Chesapeake Bay Retrievers, and Rhodesian ridgebacks are among the most common.
Although the age of onset may vary from 4-14 years, most dogs are around 8-years-old when signs appear. Affected dogs typically show a gradual onset (over a few weeks) of an uncoordinated gait (ataxia) in the rear limbs, followed by the onset of rear limb weakness (paraparesis) and knuckling-over. These symptoms are very similar to other conditions that occur in adult dogs such as intervertebral disc disease (IVDD) and some spinal cancers. One of the helpful differentiating factors is that DM is a non-painful condition. Dogs with DM are typically energetic and want to be active but are limited by the effects of the disease.
If your pet appears painful, there may be a coinciding orthopedic condition (arthritis, hip dysplasia).
A definitive diagnosis can only be established with a microscopic evaluation of tissue (known as histopathology). However, veterinarians have access to several tests that help to not only rule out other conditions but to provide evidence for a diagnosis of DM.
MRI is very helpful in ruling out causes of similar signs such as IVDD and spinal cancers. Newer MRI studies are also showing promise in helping diagnose DM. Patients with multiple disease processes can be confusing. Genetic testing and CSF results may help clarify how much each disease is contributing to the clinical picture.
Genetic testing is readily available through commercial labs and at home DNA test kits. The genetic testing provides one of three results: A/A (at risk/affected), A/N (at risk/affected, normal), and N/N (normal). It’s important to remember that even one abnormal copy poses a risk for disease. Keep in mind, most kits only test for the most common SOD-1 mutation, so a negative test does not necessarily rule out DM.
Most spinal fluid changes are non-specific though some special testing has shown promise in differentiating DM from other neurological conditions. Unfortunately, this test is not yet commercially available.
In general, a veterinary neurologist will evaluate the clinical exam findings in conjunction with MRI, spinal fluid, and genetic test results to determine the likelihood of DM as the underlying cause. Classically a patient with DM has a normal MRI, is homozygous for the mutation (at risk/affected), and has no evidence of significant inflammation or abnormal cells in the spinal fluid.
There is currently no cure for DM but physical rehabilitation has been shown to prolong and improve the quality of a dogs life. Compared to dogs that receive no physical rehabilitation, dogs that undergo ‘intensive’ therapy with a combination of active exercises, passive exercises, massage, hydrotherapy, and paw protection live more than three times as long (55 days vs 255 days). These types of physical rehabilitation tactics can be performed by a Board-Certified Veterinary Neurologist, a boarded specialist in physical rehabilitation and sports medicine or a general veterinary practitioner.
Use of a canine wheelchair is often a part of the long-term management process to help maintain mobility for these patients as they become progressively weaker. Many dogs are very happy to have the assistance of a cart to be able to fully enjoy their lives. Some dogs are a little more shy about adjusting but will often get the hang of it with positive affirmation and tasty rewards.
Researchers are actively working on gene therapy strategies to extend the quantity and quality of the lives of dogs with DM.
DM typically progresses over 3 months to 3 years, somewhat influenced by physical rehabilitation. Patients will become weaker in their pelvic limbs to the point that they can no longer move and are completely paralyzed. Incontinence may develop at this time, requiring manual bladder expression. Eventually the condition will progress to where the front legs become weak. If humane euthanasia is not elected, the brainstem and respiratory centers may become involved leading to difficulty breathing and death.
It may be helpful to keep track of good days and bad days to be able to somewhat objectively decide how your pet is coping with this disease. A quality of life scale such as the HHHHHMM Scale by Dr. Alice Villalobos can provide some guidelines. This can be answered monthly and compared to establish trends overtime.