Neurophys - EMG

  1. What non-pathologic factors can a affect nerve conduction velocities?
    • Tempetature
    • Age (CV 1/2 normal at birth while Amp dec c/ age)
    • Length of segment and height
    • Gender
  2. What is an F-wave?
    • Response elicited from any motor nerve with supramaximal stimulation
    • Created by antidromic conduction which triggers a back-firing response
    • Does not involve a synapse
    • Abnormalities are seen as an early pathologic finding in demyelinating neuropathies like AIDP / CIDP
  3. What is an H Reflex?
    • Can only be elicited in the tibial nerve in response to a submaximal stimulus
    • It is the electrical equivalent of the ankle jerk reflex (S1) and measures the integrity of the entire reflex arc
    • Monosynaptic in the cord
  4. What are the two basic patterns of pathology in a nerve conduction study?
    • Axonal
    • Demyelinating
  5. What are the features of axonal loss?
    • Reduced CMAP (motor) and SNAP (sensory) amplitudes
    • Mildy Reduced CV due to a loww of the fastest conducting fibers (not below 75% of normal)

    There is no change in the distal motor latency, F-wave, conduction block, or abnormal temporal dispersion.
  6. What are some pathologic processes which might cause an a pattern of axonal loss on nerve conduction study?
    • Axonal Neuropathies
    • Nerve Trauma
    • Motor Neuron Diseases
    • Radiculopathies (CMAPs only)
    • Secondary axonal loss in a primarily demyelinating process
  7. What are the features of demyelination?
    • Substantial increases in CV (conduction velocity) which may be noted as:
    • - Decreased CV in motor units (<70% nl)
    • - Prolonged distal motor latencies (>130% nl)
    • - Prolonged F-waves (>130% nl)

    Conduction Block / Abnormal Temporal Dispersion
  8. Define Conduction Block ...
    • A decrease in the amplitude of a proximal response relative to that of the distal response without an increase in the response duration.
    • Reflects the block of conduction through a subset of motor axons in the nerve resulting from segmental demyelination.
    • Must be at least a 30% decrement but often exceeds 50%
  9. What NCS phenomenon is depicted here ...
    Image Upload 1
    Conduction Block

    There is a reponse to distal but not proximal stimulation indicating a segmental, demyelinating process.
  10. What is Temporal Dispersion?
    • An abnormal prolongation of the motor response.
    • Reflects the desynchronization of components of the response due to different conduction velocities along the length of the nerve
    • >30% increase in dispersion is typically abnormal
  11. What NCS phenomenon is depicted here ...
    Image Upload 2
    Temporal Dispersion
  12. After an acute nerve injury, which amplitudes (SNAP or CMAP) are affected first?

    • Normal for 2-3 days
    • Decline over 3-6 days
    • Disappear by 7 days
  13. After an acute nerve injury, how long does it take to seen the maximal extent of axonal loss?
    10 days

    • Maxiaml decline in CMAPs are seen at 5-7 days
    • Maximal decline in SNAPs are seen at 7-10 days
  14. What is the sequence of changes after an cute nerve lesion?
    • Conduction Block across the site of injury (T0)
    • No conduction across the site of injury (T0)
    • Reduced Amplitudes on distal stimulation (T+7-10)
    • Acute Denrvation (fibrillation portentials - T+14-35)
    • Renervation (T+70-84)
  15. Define the term Insertional Activity?
    It is the motor respnse generated by passing a needle through the muscle and typically last <20 ms
  16. What can cause an increased insertional activity?
    Acute Denervation
  17. What can cause decreased insertional activity?
    • Inexcitable Tissue (fibrosis, fatty replacement, ischemia)
    • Physiologic Contractures (McArdle's, PFK)
    • During an attack of periodic paralysis
  18. What are examples of abnormal sponaneous muscle activity?
    • From Muscle:
    • Fibrillation Potentials
    • Positive Sharp Waves
    • Complex Repetative Discharges
    • Myotnic Discharges

    • From Motor Neuron:
    • Fasciculation Potentials
    • Myokymia
    • Neuromyotonia
    • Tremor
    • Cramp Discharges
  19. What are Fibrillation Potentials?
    • Spontaneous firing of a single muscle action potential
    • Diphasic with initial positivity (downward)
    • Appear 2-4 weeks following:
    • - Denervation
    • - Neuropathy
    • - Myopathy
  20. Image Upload 3What EMG finding is demonstrated here?
    Fibrillation Potentials
  21. What are Complex Repetative Discharges?
    • A train of grouped muscle fiber action potentials
    • Fire in a regular, lock-step, drumming fashion
    • Begin and end abruptly
    • Seen in any muscle disease
  22. What are myotonic discharges?
    • These are repetative discharges provoked by passage of the needle through the muscle or percussion
    • Train of muscle fiber action potentials with a waxing and waning frequency / amplitude ("dive bomber")
    • Seen in hereditary myotonias, periodic paralysis, and some toxic / metabolic myopathies
  23. Image Upload 4What EMG phenomenon is depicted here?
    Complex Repetative Discharges
  24. Image Upload 5What EMG phenomenon is depicted here?
    Myotonic Discharges
  25. What are Fisciculation Potentials?
    Spontaneous motor neuron discharge leading to the appearance of an action potential (MUAP) but slower

    • Many causes:
    • - Neurogenic Diseases
    • - Cholinesterase Inhibitors
    • - Benign Fascics / Cramps-Fasciculation Syndrome
  26. What is Myokymia?
    • Repeating discharges of groups of MUAP’s which have the appearance of grouped fasciculations.
    • Bursts repeat at regular or semiregular intervals
    • Variety of causes:
    • - Facial: MS, Brainstem Lesions
    • - Limb: radiation plexopathies
    • - General: Isaac's SYndrome
  27. What is a normal conduction velocity in an upper extremity in an adult?
    50 msec
  28. What is a normal conduction velocity in the lower extremity of an adult?
    40 msec
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Neurophys - EMG