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Electromyography



- See:
     - Nerve Injury:
     - Nerve Menu
     - outside links:
             - The Expert Electromyographer
             - EMG Table of Contents

- EMG Findings in Specific Conditions:
    - Normal Study:
             - normal insertional activity;
             - silent rest activity;
             - biphasic and triphasic potentials;
             - complete interference;
    - Neuropraxia
             - normal insertional activity;
             - silent rest activity;
             - no biphasic and triphasic potentials;
             - interference: none
    - Axonotmesis:
             - increased insertional activity;
             - rest activity: fibrillations & positive sharp waves;
             - no biphasic and triphasic potentials;
             - interference: none
    - Neurotmesis
             - increased insertional activity;
             - rest activity: fibrillations & positive sharp waves;
             - no biphasic and triphasic potentials;
             - interference: none
    - Axonal Neuropathies:
             - increased insertional activity;
             - rest activity: fibrillations & positive sharp waves;
             - no biphasic and triphasic potentials;
             - interference: incomplete;
    - Demyelinating Neuropathies
             - normal insertional activity;
             - silent rest activity;
             - no biphasic and triphasic potentials;
             - interference: incomplete;
    - Anterior Horn Disease
             - increased insertional activity;
             - rest activity: fibrillations & positive sharp waves;
             - large polyphasic contractions;
    - Myopathic Disorders
    - Neuropathic Disorders
    - Peripheral Neuropathy


- Indications for EMG:
    - EMG studies are highly sensitive, but results are nonspecific;
    - to evaluate motor neuron dysfunction;
    - confirme & extension of clinical examination;
    - useful in localizing level of specific lesion, distinguishing between partial and complete lesions, differentiating primary muscle or nerve 
            pathology, & evaluating malingerers;
    - useful for diff dx & in presence of coexisting disease;
    - radiculopathy due to dz of C-spine, diffuse peripheral neuropathy, or proximal median neuropathy can pose clinical questions that electrodiagnostic testing can answer;
           - no other test has a higher diagnostic accuracy in patients w/ final diagnosis of carpal tunnel syndrome;


- Technique:
     - small needle is inserted into muscle to record electrical activity of several neighboring motor units;


- Specific Measurements:
    - Rest Activity:
    - S wave:
          - occurs when action potentials travel from the point of stimulation of peripheral nerve to the spinal cord and back to the muscle;
          - another factor that may lead to normal EMG in presence of compressive radiculopathy is overlapping motor innervation of single muscle;
    - Nerve Entrapment:
          - Motor Conduction Latency:
          - Sensory latency:
    - F wave:
          - F wave is often measured to supplement routine nerve conduction studies because the F wave permits evaluation of the proximal segments of peripheral nerves;
          - F waves are valuable in evaluating disorders involving the nerve roots, plexuses and the proximal segments of peripheral nerves;
          - determine of F wave latencies is particularly valuable in evaluating patients with demyelinating paolyradiculopathies;
    - Number of motor units under voluntary control;
    - Duration and amplitude of each motor unit potential



Clinical and electromyographic study of seven spastic children with internal rotation gait