Degenerative Myelopathy (DM) is a progressive neurological disease that affects the spinal cord, primarily in older dogs. It is characterized by a gradual loss of coordination and function in the hind limbs, leading to weakness and eventually paralysis. Crucially, the disease process of DM itself is considered non-painful, meaning the direct damage to the spinal cord does not register as pain to the dog. This distinction is important for owners, as the discomfort observed in affected dogs stems from secondary conditions and mobility challenges, not the neurological deterioration itself.
The Neurological Basis of Degenerative Myelopathy
Degenerative Myelopathy is fundamentally a disease of the white matter within the spinal cord, which functions as the body’s communication highway. The white matter consists of bundles of nerve fibers (axons) coated in a protective fatty layer (myelin sheath). These tracts are responsible for transmitting signals between the brain and the rest of the body, particularly those governing movement (motor function) and body awareness (proprioception).
The disease involves the gradual breakdown, or degeneration, of both the myelin sheath and the underlying axons, inhibiting the transmission of these crucial signals. Specifically, the pathways most affected are the ascending and descending tracts that control limb movement and the dog’s ability to know where its paws are in space. This is why dogs with DM exhibit symptoms like knuckling over their paws, stumbling, and hind-end weakness.
The reason the condition does not directly cause pain lies in the type of tissue damaged and the nature of the damage. DM is a slow degeneration, not an acute injury, inflammation, or compression of the nerve roots, which typically trigger pain signals (nociception). The spinal cord tissue itself contains very few pain receptors, meaning the slow breakdown of the nerve fibers does not generate a pain response. This contrasts with painful conditions like intervertebral disc disease, where compression and inflammation activate pain pathways.
Sources of Secondary Discomfort and Pain
While the spinal cord degeneration is painless, the resulting loss of mobility and coordination places immense strain on the rest of the dog’s body, leading to significant secondary discomfort. The most common source of pain is orthopedic strain from compensating for the weakened hindquarters. Dogs with DM shift their weight forward onto their front legs and shoulders, overworking these joints and muscles to maintain balance and movement.
This unnatural gait and the constant instability can exacerbate conditions such as osteoarthritis or hip dysplasia, which are inherently painful. The compensating limbs, particularly the forelimbs, may develop muscle soreness and inflammation from being chronically overworked. Furthermore, the lack of use in the hind limbs causes rapid muscle atrophy, which can lead to stiffness and discomfort in the surrounding joints and soft tissues.
Another significant source of discomfort arises from the physical consequences of impaired movement and recumbency. Dogs that drag their paws due to proprioceptive deficits often develop chronic abrasions and sores on the tops of their feet, which can become irritated or infected. As the disease progresses and the dog spends more time lying down, prolonged pressure on bony prominences can lead to the formation of painful pressure sores. Frustration and anxiety associated with losing independence also represent a form of distress often interpreted as physical pain.
Managing Mobility and Comfort
Addressing the secondary sources of pain and discomfort is the primary goal of care for dogs with Degenerative Myelopathy.
Physical Therapy
Physical therapy plays a central role in managing DM by maintaining muscle mass and joint flexibility. Techniques like hydrotherapy, which involves exercise in water, allow the dog to move its limbs without the full impact of gravity, helping to preserve muscle tone. Passive Range of Motion (PROM) exercises are also performed, where the owner gently moves the dog’s joints through their natural arc to prevent stiffness and contracture. A customized regimen, often including therapeutic exercises and massage, helps manage compensatory soreness in the forelimbs and shoulders.
Assistive Devices
To support the dog’s daily mobility and reduce orthopedic strain, various assistive devices are employed. Supportive harnesses, particularly those that support the hindquarters, allow owners to lift and stabilize the dog during walking and standing. As weakness progresses, a rear-support wheelchair, or cart, can provide independence and allow the dog to exercise without placing undue stress on its joints.
Environmental Modifications
Environmental modifications are important for comfort and safety. Placing non-slip surfaces, such as rugs or mats, over slick floors provides traction and reduces the effort and instability required for walking. Ramps should be installed to eliminate the need to navigate stairs or step up onto furniture, which can be physically taxing and increase the risk of injury.
Pharmaceutical Management
Pharmaceutical management involves the use of medications like Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) or Gabapentin. These are used not for the DM itself, but to manage the pain from co-existing conditions like arthritis and the musculoskeletal discomfort caused by compensatory movement.