Degenerative Myelopathy

TL;DR. Degenerative myelopathy is a slow, painless degeneration of the spinal cord in older dogs that gradually causes hind-limb weakness and loss of coordination, ultimately leading to paralysis.

Degenerative myelopathy typically begins as mild, painless weakness in the hind legs of older dogs.

What is it?

Degenerative myelopathy (DM) is a slowly progressive, nonpainful degenerative disorder of the spinal cord's white matter. To understand this disease, it helps to think of the spinal cord as a major fiber-optic cable transmitting signals between the brain and the rest of the body. The "white matter" of the spinal cord contains these vital communication pathways, known as axons, which are insulated by a protective fatty sheath called myelin.

In dogs affected by degenerative myelopathy, this insulation begins to break down, and the underlying nerve fibers slowly die off. This widespread myelin and axon loss occurs primarily in the middle to lower back region of the spinal cord, medically referred to as the mid to caudal thoracic spinal cord. As these pathways degrade, the brain can no longer reliably send signals to or receive signals from the hind limbs.

This disruption in communication leads to two primary neurological deficits: proprioceptive ataxia (a wobbly, uncoordinated walk due to the dog not knowing where its feet are in space) and upper motor neuron spastic paresis (weakness characterized by stiff, poorly controlled movements). Because the disease targets the nerve pathways rather than the surrounding bones or joints, it is entirely painless. However, the progressive loss of mobility is deeply challenging for both pets and their owners.

While degenerative myelopathy is primarily a canine disease, cats are occasionally listed as susceptible. However, feline cases are extraordinarily rare. The vast majority of clinical research, diagnostic protocols, and management strategies are based on canine medicine, and any guidance for cats is extrapolated from these canine standards.

Causes & risk factors

Degenerative myelopathy is an inherited genetic disorder. Research has linked the disease to a mutation in the superoxide dismutase 1 (SOD1) gene. This gene is responsible for producing an enzyme that helps clear harmful free radicals from cells. When the gene is mutated, abnormal proteins accumulate within the motor neurons of the spinal cord, leading to cellular stress and the eventual breakdown of the myelin sheath and axons.

Because it is a genetic condition, certain breeds are highly predisposed to developing the disease. The most commonly affected breeds include:

• German Shepherd Dogs
• Pembroke Welsh Corgis
• Boxers
• Chesapeake Bay Retrievers
• Rhodesian Ridgebacks

Age is another critical risk factor. Degenerative myelopathy is a disease of mature and senior pets. It rarely shows clinical signs before a dog reaches 8 years of age, with most dogs being diagnosed between 9 and 11 years old. Both male and female dogs are affected equally.

Signs to watch for

The signs of degenerative myelopathy begin subtly and are often mistaken for normal aging, arthritis, or hip dysplasia. Recognizing the specific neurological nature of these signs is key to seeking early veterinary intervention.

Cardinal Signs

• Upper motor neuron (UMN) paraparesis: Progressive weakness in both hind limbs, characterized by a stiff, weak, or dragging gait.
• Absence of localizable spinal pain: Unlike a herniated disc or spinal tumor, DM does not cause pain when the vet palpates the spine.
• Ataxia of the rear limbs: A wobbly, uncoordinated walk where the hindquarters sway from side to side.

Common Signs

• Wearing of the dorsal nail surfaces: The tops of the toenails on the hind paws become worn down or bleeding because the dog drags its feet.
• Toe scuffing: A distinct scraping sound as the dog walks on hard surfaces.
• Long-strided gait: A floating, exaggerated stride in the hind legs as the dog attempts to compensate for coordination loss.
• Normal to increased rear limb reflexes: When the vet tests the knee-jerk reflex, the response is intact or even hyperactive, which distinguishes DM from diseases affecting the lower motor neurons.
• Abnormal postural reactions: The dog fails to quickly correct its foot placement when its paw is turned upside down.
• Loss of proprioception: The dog loses the subconscious awareness of where its limbs are positioned.
• Knuckling: Standing or walking on the tops of the paws rather than the pads.

Knuckling, or standing on the tops of the paws, is a common early sign of proprioceptive loss.

Rare Signs

• Flaccid paresis with muscle atrophy and loss of reflexes in all four limbs: This occurs only in the very late, terminal stages of the disease. If a dog is kept alive through intensive nursing care long after losing the ability to walk, the degeneration eventually spreads forward to the neck and brainstem, causing limp paralysis, severe muscle wasting, and loss of reflexes in all four limbs.

How vets diagnose it

Degenerative myelopathy is a diagnosis of exclusion. This means there is no single, non-invasive test that can definitively diagnose DM in a living dog. Instead, your vet must systematically rule out all other potential causes of spinal cord dysfunction, such as herniated discs (intervertebral disc disease), spinal tumors, infections, or inflammatory diseases.

Your vet will begin with a thorough physical and neurological examination. To narrow down the possibilities, they will recommend several diagnostic tests:

• Spinal radiographs (X-rays): While standard X-rays of the spine are typically normal in dogs with DM, they are essential to rule out arthritis, bone infections, or obvious bone tumors.
• Myelography or MRI: These advanced imaging techniques are the gold standard for ruling out compressive spinal diseases. A leading veterinary internal medicine reference notes:

  "Myelography or MRI must be performed to rule spinal cord compression or focal spinal cord neoplasia. Normal spinal radiographs, a cytologically normal CSF, and normal spinal cord imaging in an older dog with slowly progressive UMN signs to the pelvic limbs warrant [a presumptive diagnosis of degenerative myelopathy]."

• Cerebrospinal fluid (CSF) analysis: A sample of the fluid surrounding the spinal cord is collected and analyzed. In dogs with DM, the fluid is cytologically normal, though a slight increase in protein concentration is occasionally identified.
• DNA test for the SOD1 mutation: A simple DNA swab or blood test can determine if a dog carries the mutated gene. However, a positive test only indicates that the dog is at risk of developing DM; it does not prove that the dog's current mobility issues are caused by DM, as many dogs with the mutation never develop clinical signs.

Advanced imaging like MRI is crucial to rule out other causes of spinal cord compression.

Treatment options

Currently, there is no cure for degenerative myelopathy, and no treatment can reverse the degeneration of the spinal cord. The goal of therapy is to support the dog's quality of life, maintain mobility for as long as possible, and manage secondary complications.

Medications and Supplements

Your vet may discuss several medical therapies aimed at supporting nerve health or reducing cellular stress, though clinical evidence of their efficacy is limited:

• Aminocaproic Acid: This medication acts as a fibrinolysis inhibitor and antiprotease. It is sometimes prescribed in the hope of slowing down the inflammatory or degradative processes within the spinal cord.
• Acetylcysteine: Known primarily as an antidote and mucolytic, this compound is sometimes used for its potent antioxidant properties to protect nerve cells from free-radical damage.
• Glucocorticoids: These anti-inflammatory corticosteroids may be used in short, conservative courses if there is suspected concurrent inflammation, though long-term use is avoided as they can accelerate muscle wasting.
• Vitamin B Complex: This nutritional supplement is commonly administered to support overall nerve function and metabolism.
• Omega-3 Fatty Acids: These nutraceuticals provide natural anti-inflammatory support for the nervous system and joints.

Physical Rehabilitation

Active physical therapy is the single most effective intervention for dogs with degenerative myelopathy. Studies have shown that dogs receiving regular physical rehabilitation (such as underwater treadmill therapy, stretching, and targeted strengthening exercises) maintain their ability to walk significantly longer than those receiving no therapy.

Prognosis

The long-term prognosis for dogs with degenerative myelopathy is poor, as the disease is relentlessly progressive. However, because the condition is entirely painless, dogs can maintain a high quality of life during the early and middle stages of the disease.

As the disease progresses, the timeline to loss of mobility varies by breed:

• Large breeds (such as German Shepherds and Boxers): Most affected large-breed dogs progress from mild coordination loss to being completely unable to walk (nonambulatory) within 6 to 9 months of the onset of clinical signs.
• Pembroke Welsh Corgis: This breed tends to experience a slower progression, with a median time of 18 months before becoming nonambulatory.

If owners choose to manage the disease long-term using mobility aids like custom canine wheelchairs and harnesses, the disease will eventually progress to the front limbs. Over time, this leads to widespread denervation, limp paralysis in all four legs, muscle wasting, and eventually, difficulty swallowing or breathing. Most owners elect humane euthanasia when the dog loses its ability to stand or when maintaining hygiene and mobility becomes too difficult.

Prevention

Because degenerative myelopathy is an inherited genetic disorder, it cannot be prevented through lifestyle changes, diet, or exercise. The only way to prevent the disease is through responsible breeding practices.

Breeders of highly predisposed breeds should perform DNA screening for the SOD1 mutation on all potential breeding dogs. Dogs that carry two copies of the mutated gene should not be bred, or should only be bred to dogs that are completely clear of the mutation, to ensure that future generations do not inherit the double mutation required to develop the disease.

When to call your vet

You should schedule an appointment with your vet if you notice your older dog scuffing its hind toes, walking with a wobbly or swaying gait, or showing mild weakness when rising.

You must seek immediate veterinary care if your dog suddenly loses the ability to walk, cries out in pain, pants heavily, or experiences a rapid decline in mobility over the course of a few hours or days. Degenerative myelopathy is a very slow, painless disease. A sudden loss of mobility or signs of pain indicate a different, potentially treatable emergency, such as a ruptured spinal disc or a spinal blood clot, which requires urgent intervention.

For specific breeds

• German Shepherd Dogs: This breed is the classic representative for degenerative myelopathy. They tend to show a relatively rapid progression (6 to 9 months) and, due to their large size, managing their mobility loss requires significant physical effort from owners.
• Pembroke Welsh Corgis: Corgis are unique in that they tolerate the disease for much longer, often maintaining mobility with the help of carts for up to 18 months or more. Because they are low to the ground and lightweight, they are excellent candidates for custom canine wheelchairs.
• Boxers and Rhodesian Ridgebacks: These breeds often present with a long-strided, floating gait in the hind legs before developing obvious weakness. Like German Shepherds, their large size makes early mobility planning essential.

Sources

• Internal Medicine, 5th Edition, page 1100.