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