Hereditary spinal muscular atrophy
With this motor neuron disease, homozygous-affected dogs display muscle motor unit dysfunction before motor terminal or axonal degeneration appear and this dysfunction is an important determinant of weakness in affected animals. The most important form of dysfunction is an inability to sustain motor unit force during repetitive activity. Interestingly, this dysfunction occurs to a significant extent in the absence of any electromyographic evidence of denervation.
Clinical symptoms include a progressive and profound hindleg weakness and hemiparesis which usually develops within the first 12 weeks of age with tetraparesis evident by 6 months of age. Clinical progression may develop slower in heterozygous dogs and these dogs may not develop tetraparesis until 1 - 2 years of age. Typically, the first signs of weakness appear in the tail extensor muscles, with neck extensor and paravertebral muscles exhibiting signs of weakness soon thereafter. Hindlimb reflexes are usually reduced although sensory reflexes are normal.
Radiographic, CT and MRi imaging are usually unrewarding and myographic studies and histological testing are required to definitely diagnose this condition.
Spinal cord lesions include chromatolysis, axonal swelling filled with neurofilaments and eventually motor neurone death. Abnormalities in glutamate metabolism are also reported with this condition.
Neurological examination, electrodiagnostic testing, and histologic studies indicates that the disease results from motor horn cell degeneration.
There is no known treatment for this condition which is unresponsive to medical therapy such as prednisolone.
Palliative responses may be observed with fampridine.
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