Degenerative Myelopathy (DM), sometimes referred to as CDRM (Chronic Degenerative RadiculoMylopathy), is a fatal, neurodegenerative disease characterized by a general lack of coordination of muscle movements in the pelvic limbs. Dogs diagnosed with DM have axonal (a part of a nerve cell, or neuron) and myelin (an electrically insulating layer around an axon, also known as the myelin sheath) degeneration at all levels of the spinal cord.
As motor neurons degenerate, they can no longer send electric impulses to the muscle fibers that normally result in muscle movement. When muscles no longer receive the messages from the motor neurons that they require to function, the muscles begin to atrophy (become smaller). Limbs begin to look "thinner" as muscle tissue atrophies. In the later stages of the disease, limbs may become totally paralyzed.
Age at Onset: DM affects adult dogs typically over the age of 8 years. DM has been detected in dogs as young as 3 years.
Breeds Affected: The German Shepherd dog (GSD or Alsatian) is the breed most commonly affected however it is also seen in many other breeds.
Symptoms: DM progresses slowly, so sudden onset of disease symptoms may indicate another cause for these signs. Although variable in presentation and course, generally, Dogs with DM typically require mobility assistance within 9 months of the first onset of symptoms. Symptoms can progress to paralysis in 3-6 months when untreated.
Genetics of DM: To date, a single gene called SOD1 (superoxide dismutase 1) is associated with DM. One study showed that dogs that had mutations in both copies of their SOD1 genes were strongly associated with the clinical diagnosis of DM. Although all dogs clinically diagnosed with DM under strict guidelines had mutations in both SOD1 genes, not all of the dogs carrying two copies of the mutated SOD1 gene displayed symptoms of DM. Thus, DNA tests analyzing the SOD1 gene alone cannot definitively determine whether a patient will present with degenerative myelopathy.
Canine DM is similar to Amyotrophic lateral sclerosis (ALS) in Humans: Similar to DM in dogs, ALS, also known as Lou Gehrig's Disease, in humans refers to a progressive neurodegenerative disease that affects nerve cells in the brain and the spinal cord. Early symptoms of ALS often include increasing muscle weakness, especially involving the arms and legs, speech, swallowing or breathing. With voluntary muscle action progressively affected, patients in the later stages of the disease may become totally paralyzed.
ALS is directly hereditary in only a small percentage of families. Although there is likely a genetic predisposition involved in 90% of adult-onset ALS, no family history of ALS exists with these patients, and they present as an isolated case. This is called sporadic ALS (SALS) because of the lack of direct inheritance in a family. In 10% of persons with ALS have a close second family member with ALS, which is referred to as familial ALS (FALS). Currently the best tool to distinguish between SALS and FALS is the family history.
The SOD1 gene in dogs and humans: Just as the SOD1 gene plays as role in canine DM, the human version of the SOD1 gene has been implicated in FALS. Changes in the SOD1 gene on chromosome #21 have been found in about 20% of families with FALS.
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