Diagnosing Canine Degenerative Myelopathy
Canine degenerative myelopathy (DM) is a fairly uncommon but potentially fatal spinal cord disease that can be difficult to diagnose. A number of other disorders mimic the signs and symptoms of DM, including intervertebral disk disease, myelitis, degenerative lumbosacral stenosis, spinal cord neoplasia (cancer) and hip dysplasia, among others. A veterinarian presented with a dog showing hind limb lameness, lack of coordination, muscle wasting and partial paralysis must first rule out other causes of the dog’s condition before reaching a presumptive diagnosis of degenerative myelopathy. She will take a thorough history from the dog’s owner, focusing on when, where and how the dog’s symptoms first came to light. She will also perform a thorough physical examination, including a neurologic examination if she is skilled in that area, to try and localize which parts, if any, of the spinal cord have been damaged or are otherwise involved. The initial evaluation may include routine blood and urine assessment (complete blood count, serum chemistry panel and urinalysis). The results of those tests typically will be unremarkable if DM is the underlying cause of the dog’s condition.
A comprehensive neurological examination is critical to making a tentative diagnosis of degenerative myelopathy. Dogs with this disease are not painful, and a skilled veterinarian can localize their spinal cord lesions to the upper and lower back (thoracic and lumbar) areas. Survey X-rays of the chest and back (thoracolumbar radiographs) can be taken to screen for primary or metastatic cancer. The veterinarian will review the radiographs carefully, looking for any evidence of tumors along or around the spinal cord that may be contributing to the dog’s symptoms. Samples of cerebrospinal fluid can be analyzed for evidence of inflammation. Advanced imaging procedures, such as electromyography, myelography, nerve conduction studies, computed tomography (CT) scan and magnetic resonance imaging (MRI), can be performed alone or in combination to rule out other disorders. A test is now available to detect the presence of the genetic mutation that is responsible for causing degenerative myelopathy in dogs. Normal dogs will have two copies of the non-mutated gene; this is called being “homozygous” for the normal gene. Carriers will have one copy of the normal gene and one copy of the mutated gene, one coming from each parent; this is called being “heterozygous” for that gene. Dogs that have or are at risk for having DM will have two copies of the mutated gene; they will be “homozygous” for the mutated gene. All dogs with DM will have two of the abnormal genes, but not all dogs with two of the abnormal genes will develop DM.
In a living animal, degenerative myelopathy can only be diagnosed by ruling out other causes of progressive, irreversible paralysis. Unfortunately, the only definitive way to diagnose this disease is to examine the dog’s spinal cord under a microscope after the animal has died. This is done at a diagnostic pathology laboratory using a technique called histopathology.