Neurophysiology Tests

Electromyography (EMG)

  • Direct measurement of the electrical activity in a given muscle
    • Insertion of an electrode into a specific muscle
    • Used to establish if a particular muscle is innervated

Nerve Conduction Studies (NCS)

  • Used to measure electrical activity along a sensory, motor, or mixed nerve
    • Motor nerves – antegrade
    • Sensory or Mixed nerves – retrograde

Definitions

Latency

  • The time in ms for conduction between two points along a nerve

Amplitude

  • The extent of the conducted signal (millivolts)
  • Indicates how many of the nerve fibers are working

Velocity

  • The speed/distance between two points
  • Latency and velocity measure the quality of nerve conduction
  • Amplitude measures the quantity of nerve conduction

Factors Affecting Nerve Conduction

  • Age of person
    • Correlates with the degree of myelination
    • Slower in very young or elderly
  • Temperature
    • Slows with decreasing temperature (optimum 30°C)
  • Limb
    • Upper limb conduction is faster than lower limb
  • Location in the nerve
    • Proximal conduction is faster within a given nerve
  • The nerve itself
    • Different nerves conduct at different velocities

Motor Nerve Conduction

  • Measured by supramaximal stimulation of a motor nerve
  • Recording of the generated waveform with a needle in a muscle
    • e.g., APB for CTS
  • Measurement is called:
    • CMAP, MUAP, or M wave
    • Compound motor action potential / Motor unit action potential
    • All mean the same thing
  • Latency, amplitude, and velocity of the CMAP are all calculated
  • Motor conduction velocity (MCV) is calculated separately
    • By measuring latencies at different sites and the distance between the two sites

Sensory Nerve Conduction

  • Measured by supramaximal stimulation of a sensory nerve
  • Usually in a retrograde fashion
  • Provides the Sensory Nerve Action Potential (SNAP)
  • Latency, amplitude, and velocity are all calculated
  • Sensory Conduction Velocity (SCV) is calculated separately
    • Same method as MCV

Measures of Proximal Conduction

F Wave Response

  • When a motor nerve is supramaximally stimulated to measure the CMAP, a retrograde (antidromic) conduction also occurs
  • Since this does not cross synapses, it behaves like an echo
  • Represents stimulation of anterior horn cells at the root level
  • Characteristics:
    • Longer latency and smaller amplitude than CMAP
    • Called the F wave
  • Clinical significance:
    • Absence or reduction may indicate a root level problem
    • Especially if distal conduction (CMAP) is normal
    • Muscles are innervated by multiple roots, so a single root injury may not significantly affect it
    • More useful for diagnosing multi-root problems
      • e.g., Plexopathies, Guillain-Barré Syndrome

H Reflex

  • Effectively an electrical deep tendon reflex
  • Elicited by submaximal stimulation of muscle stretch receptors, which return
    (afferent) to the anterior horn cells
  • Results in a motor response via a monosynaptic reflex, which is recorded
  • Difficult to measure, so not widely used
  • Absence is noted in:
    • Radiculopathy
    • Polyneuropathy
    • Elderly

Somatosensory Evoked Potentials (SSEP)

  • Used for spinal cord monitoring and investigation of proximal lesions
  • Evoked potentials from the central nervous system recorded at a peripheral site
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