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EMG of Myopathic Disorders

- Discussion:
    - as general rule, an EMG of patient who has myopathic disease shows a decreased amplitude & duration of response, compared w/ that of unaffected pt,
          while an EMG of pt who has neuropathic disorder shows an increased amplitude and duration of response;
          - EMG studies are generally useful for excluding neurogenic disorders and confirming either active or inactive myopathy;
    - short duration (consisent finding), low-amplitude polyphasic units (which is variable) on voluntary activation;
    - appearance of small amplitudes (variable) & short duration (consistent) and polyphasic waves, are related to a loss of muscle fibers within the motor units;
          - there is rapid recruitment of motor units w/ complete interference pattern of reduced amplitude on weak effort;
    - increased spontaneous activity w/ fribrillations, complex repetitive discharges, and positive sharp waves;
          - fibrillations w/ positive sharp wave can be seen in inflammatory myopathies, muscular dystrophies, and some toxic mypathies;
          - this EMG pattern occurs in a variety of acute, toxic, and active myopathic processes and should not be considered diagnostic for the inflammatory myopathies;
   - mixed myopathic and neurogenic potentials (polyphasic units of short and long duration) may also be present as a consequence of the regeneration of muscle
          fibers and chronicity of the disease