Degenerative myelopathy

Clinical signs: Dogs with degenerative myelopathy (DM) show an insidious, progressive ataxia and paresis of the pelvic limbs ultimately leading to paraplegia and euthanasia (). If pelvic limb hyporeflexia is observed, reflecting nerve root involvement, the disease is termed canine degenerative radiculomyelopathy (CDRM) (). Although the German Shepherd Dog is the most commonly affected breed, degenerative myelopathy has been reported in other breeds and is emerging in a number of purebred dogs, such as the Pembrokeshire Welsh Corgi. Age of onset of neurological signs is usually 5 years or older with a mean age of 9 years; however, young dogs can be affected ().

Neuroanatomical localization is characterized by UMN signs to the pelvic limbs implying a spinal cord lesion between T3 and L3. DM can progress to paraplegia over 6-12 months after the suspected diagnosis. Pelvic limb tremors or muscle fasciculations are sometimes observed during stance and gait. Gait abnormalities progress from truncal ataxia to severe paraparesis, characterized by spasticity and long strides, typical of an UMN disease gait. Proprioceptive loss is recognized by toe dragging, knuckling deficits and ataxia, and pelvic limb dysfunction is usually asymmetrical. Thoracic limb function is usually spared except during end-stage disease. Spinal reflexes are usually present or exaggerated. Presence of a crossed extensor reflex is variable during flexor withdrawal evaluation. Decreased or absent patellar reflexes may indicate concurrent, or progression to, LMN involvement. Disuse atrophy of pelvic limb musculature develops gradually as paresis progresses. Control of urination and defecation may be affected late in the disease process. Lack of paraspinal hyperaesthesia is a key clinical feature of degenerative myelopathy. Nociception is usually unaffected.

Pathogenesis: The aetiology of degenerative myelopathy remains unknown. Griffiths and Duncan (1975) hypothesized degenerative myelopathy to be a ‘dying-back disease’ confined to the central nervous system (CNS), suggesting a toxic aetiology. Braund and Vandevelde (1978) refuted that hypothesis based upon morphometric data and instead suggested an inherited genetic cause. However, the late onset of the disease makes it difficult to collect data from parents and siblings to substantiate this theory. Recently, studies of the brain of DM affected dogs showed neuronal degeneration and loss in the red nucleus, lateral vestibular nucleus and in the dendate nucleus (). An immunological role was proposed based upon observations of depressed responses to thymus-dependent mitogens () and increased concentrations of circulating immune complexes (). A more recent study showed immunohistochemical evidence for immunoglobulin and complement deposition in the spinal cord of degenerative myelopathy dogs (). Although immune-related degenerative disease is a plausible theory, immunosuppressive therapies have shown no long-term benefits in halting the progression of degenerative myelopathy (). Decreased serum levels of vitamin E and B12 have also been proposed as causes of this disease but largely refuted by recent studies ().

Diagnosis: Tentative antemortem diagnosis is presently based upon ruling-out other diseases causing progressive myelopathy (). Common differentials include intervertebral disc disease, inflammatory disease and spinal cord neoplasia. Hip dysplasiaand degenerative lumbosacral stenosis (LSS) often can be confused with the diagnosis of degenerative myelopathy although the neurological findings are different if a careful examination is performed. It is not uncommon for degenerative myelopathy affected dogs to have coexisting neurological and orthopaedic diseases. Neurodiagnostic techniques for evaluation of spinal cord disease include CSF analysis, electrodiagnostic testing, myelography, CT and MRI. Definitive diagnosis of DM is determined postmortem by histopathological examination of the spinal cord (). Neuropathological lesions involve the spinal cord myelin and axons in all funiculi () but most extensively in the midthoracic region and are described as discontinuous, bilateral and asymmetrical ().

Treatment and prognosis: Until a cause of degenerative myelopathy is known it is difficult to recommend an appropriate treatment regimen. Aminocaproic acid, an antiprotease agent, has been advocated for long-term management of DM (); however, there have been no published clinical data to support drug efficacy. While treatment of vitamin deficiencies can resolve neurological disease in some animals, therapy with oarenteral cyanocobalamin or oral alpha-tocopherol did not affect neurological progression in a study of degenerative myelopathy affected dogs (). Combination therapies with an exercise regimen have also been advocated for treatment of degenerative myelopathy ().

Long-term prognosis is considered poor. Dogs often lose the ability to ambulate the pelvic limbs within – 5 months of diagnosis. The disease will eventually ingress to affect the thoracic limbs.