MOTOR CONDUCTION STUDIES - PARAMETERS

When a motor or mixed nerve is stimulated and recording is made by placing electrodes over a muscle supplied by that nerve, the recorded potential is known as compound muscle action potential or CMAP. There are three main parameters of CMAP, which are routinely evaluated during motor nerve conduction studies. They are latency, amplitude and conduction velocity.

LATENCY
This is the time in milliseconds between nerve stimulation and initial deflection from baseline. It reflects the time required for action potential to travel along the fastest-conducting axons to activate the muscle fibers.

• The latency includes not only the time taken for impulse to travel along the nerve till it reaches nerve terminal, but also the time taken for neuromuscular junction transmission and muscle activation.
  
• Whenever possible, the nerve is stimulated at two points: a distal point near the recording site (distal latency) and a more proximal point (proximal latency).
  
• Prolonged latencies are usually taken as evidence of demyelination.

AMPLITUDE
This is usually measured as height in millivolts of CMAP, from the baseline to the negative peak.

• CMAP amplitude is a semiquantitative measure of the number of axons conducting between the stimulating and the recording points.
  
• Decreased CMAP amplitudes usually suggest either axon loss or conduction block from demyelination located b/w the stimulation site and recorded muscle. But it can be due to reasons other than motor nerve dysfunction (e.g. neuromuscular junction, muscle fiber etc).

CONDUCTION VELOCITY

Measurement of differences in distance and latency b/w proximal and distal stimulation sites allows calculation of conduction velocity in the segment of nerve b/w the site of stimulation and is expressed in meters per second.

• Normal conduction velocities are from 40-50 m/sec in the legs and from 50-70 m/sec in the arms.
  
• Motor conduction velocity can not be calculated by performing a single stimulation. This is because, the latency of compound muscle action potential reflects transmission across nerve, junction and muscle, measurement of true conduction velocity across the nerve will necessarily require stimulation at two points.
  
• Decrease in conduction velocities is usually taken as sign of demyelination.
  
  

Reference:

1. Kimura J. Electrodiagnosis in disease of nerve and muscle: Principles and Practice, New York: Oxford V. Press, 3rd edition
   
2. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders, Boston: Butterworth-Heinemann