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



Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Thursday, May 31, 2012 9:49 am