DEGENERATIVE MYELOPATHY by ANA PAOLA DINIZ
D egenerative Myelopathy (DM) is a slowly progres- sive spinal cord disorder that resembles Amyo- trophic Lateral Sclerosis (ALS or Lou Gehrig’s Disease) in people. The inevitable result for dogs with DM is paraplegia. DM affects primarily older dogs, with symptoms typically beginning at eight years of age or older. Back in the day (when I was just a pup) we referred to this disease as German Shepherd Myelopathy because we thought it was unique to this breed. We now know that DM occurs in many purebred and mixed breed dogs. The breeds most commonly affected include the German Shepherd, Pembroke Welsh Corgi, Cardigan Welsh Corgi, Boxer, Borzoi, Rhodesian Ridge- back, American Eskimo Dog, Bernese Mountain Dog, Golden Retriever, Great Pyrenees, Kerry Blue Terrier, Poodle, Pug, Shetland Sheepdog, Soft Coated Wheaten Terrier, Wire Fox Terrier and Chesapeake Bay Retriever. DM symptoms progress slowly over the course of months to even years. From beginning to end, DM affected dogs typically remain alert and animat- ed. The symptoms typically progress as follows: INITIAL • Loss of coordination (ataxia) in the hind legs • Dragging the hind feet causing wearing down of the toenails. • Hind end weakness (difficulty climbing stairs, jumping up into the car, going for walks) INTERMEDIATE • Knuckling of hind feet (weight bear- ing on the tops of the feet rather than their undersides) • Difficulty supporting weight with hind legs • Inability to walk without support • Urinary and/or fecal incontinence ADVANCED • Paraplegia (paralysis of hind legs). • Weakness in front legs. Although this degenerative process is not painful, affected dogs can devel- op discomfort because of overuse of other body parts attempting to compen- sate for the hind end weakness.
DM causes degenerative changes within spinal cord axons, structures that transmit information back and forth between the brain and the rest of the body. These degenerative changes begin in the thoracolumbar region of the spinal cord, the portion that lines up with the end of the rib cage. This explains why the hind limbs are more severely affected. Given enough time, the disease progresses toward the head end of the body, causing loss of front leg function as well. DM is an inherited disease. In 2008 a group of researchers reported through Texas A&M University that a genetic mutation on the SOD1 gene is a major risk factor for the development of DM. Their study involved Boxers, Pembroke Welsh Corgis, German Shepherds, Chesapeake Bay Retrievers and Rhode- sian Ridgebacks. The researchers discovered that DM has a recessive mode of inheritance. In order for a dog to be affected, the mutation must be inherited from both dam and sire. What remains unknown is why some dogs who have this “dou- ble mutation” never develop symptoms of DM. Testing is available to determine an individual dog’s SOD1 mutation status.
This test is available through the Ortho- pedic Foundation for Animals (OFA). All that is required is a blood sample or cheek swab. This DNA test identifies dogs that are normal (have two normal copies of the gene), those who are carriers (have one normal copy of the gene and one mutated copy), and those who are at risk for development of DM (have two mutated copies of the gene). It is impor- tant to remember that DNA testing does not diagnose DM. This is because not all dogs with two mutated copies of the gene go on to develop DM. Responsible breeders utilize DNA testing for DM to help assess whether or not a particular dog is suitable for breeding purposes. If contemplating purchasing a pup of an at-risk breed, it is important to request DM test results for the dam and sire of the litter of inter- est. It is also reasonable to have the pup- py tested prior to purchase, although, if the parents have been tested and have “normal” results, this is unnecessary. DM is a “rule out diagnosis”. What this means is that a presumptive diag- nosis can only be made by ruling out other causes of spinal cord disease (e.g., herniated intervertebral disk, tumor, infection, trauma). The only way to
“DM SYMPTOMS PROGRESS SLOWLY OVER THE COURSE OF MONTHS TO EVEN YEARS. FROM BEGINNING TO END, DM AFFECTED
DOGS TYPICALLY REMAIN ALERT AND ANIMATED.”
72 • S how S ight M agazine , M arch 2018
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