Clinical Demonstration:
Nerve Conduction Studies and Electromyography


C. Chalk
p 1 2 3 4 quiz

 

Motor Nerve Conduction
Motor nerve conduction is evaluated by recording the compound muscle action potential (CMAP) associated with a mechanical contraction of a given muscle (twitch), in response to electrical stimulation of the motor nerve fibers supplying that muscle.  The CMAP is the sum of all the action potentials occurring individually in the contracting muscle fibers.
The CMAP is recorded from a pair of electrodes taped to the skin overlying the muscle; because the CMAP is small (millivolts) and brief (milliseconds), it must be amplified electronically and displayed on an oscilloscope.

 

The electrical shock is delivered from a hand held stimulator at sites where the nerve of interest is close to the surface (to allow as small a shock as possible to be used). With the electrodes in place, and amplifier and oscilloscope turned on, a single shock is given. At the beginning of the oscilloscope trace, one sees the shock marker (stimulus artifact), followed in a few milliseconds by the CMAP. The time between the shock and the beginning of the CMAP (the latency, in ms) and the size of the CMAP (CMAP amplitude, in mV) are measured from the oscilloscope.


Stimulation at wrist; Recording at abductor pollicis brevis
Subject sample values, measured from the oscilloscope for stimulation at the wrist (1st trace on scope):

Latency:  3.6 ms

CMAP Amplitude: 10 mV

 

 

The procedure is then repeated, stimulating the nerve at a second more remote site, and again recording the latency and CMAP amplitude.


Why is it necessary to use two stimulation sites? The time between the shock and the appearance of the CMAP (the latency) comprises three components: (1) time for action potentials to travel down the nerve, (2) time to cross the neuromuscular junction,and (3) time for muscle action potentials to disperse throughout the muscle. Only the first component is relevant to calculating nerve conduction velocity. Including components (2) and (3) would introduce a small systematic error, but these are constants, and can be removed by subtracting the distal stimulation site latency from the proximal site latency.

 



Stimulation at elbow; Recording at abductor pollicis brevis
Subject sample values, measured from the oscilloscope for stimulation at the elbow (2nd trace on scope):

Latency: 7.0 ms

CMAP Amplitude: 10 mV

 

 

  For an avi video (7 Meg) demonstrating the above procedure, click here

  For a streaming version (Real Player required), click here

 

Conduction Velocity Formula
The distance between the two sites of stimulation is measured. 

The motor conduction velocity (in m/s) between the two sites of stimulation is calculated by:

Conduction Velocity  = 

        Distance/(Latency A - Latency B)

 

Subject sample values:

Distance:  22 cm (220 mm)

Conduction Velocity:  220/(7 - 3.6) = 64.7 mm/ms or m/s

   

Sensory Nerve Conduction
Sensory nerve conduction is evaluated in a slightly different way. Instead of recording the response of a muscle, the sensory nerve action potential (SNAP) - in response to electrical stimulation - is recorded from the nerve itself, with recording electrodes again placed on the surface of the skin. The SNAP is the sum of all the action potentials generated in sensory nerve fibres by the electrical shock. The SNAP is about 1000 times smaller than the CMAP, making sensory nerve conduction technically more difficult to measure. In order to ensure that one is recording action potentials from sensory nerve fibres only, the recording electrodes must be placed along a branch of the nerve which contains only sensory fibres.

As with motor conduction studies, the SNAP amplitude (in microvolts) and latency (in ms) between the shock and the SNAP are recorded using two sites of stimulation. The sensory conduction velocity (in m/s) is calculated using the equation above.



Stimulation at wrist: Recording at index finger
Subject sample values, measured from the oscilloscope for stimulation at the wrist (1st trace on scope):

Latency:  2.8 ms

SNAP Amplitude: 70  V



Stimulation at elbow; Recording at index finger
Subject sample values, measured from the oscilloscope for stimulation at the elbow (2nd trace on scope):

Latency: 7.3 ms

SNAP Amplitude: 35 V

Note that, strictly speaking, calculation of  sensory nerve conduction velocity does not require the step of subtracting the distal site latency from the proximal latency, as neither neuromuscular junction or muscle conduction times are involved. The calculation is carried out this way mostly as a matter of convention.
Subject sample values:

Distance:  28 cm (280 mm)

Conduction Velocity:  280/(7.3 - 2.8) = 62 mm/ms or m/s

 

  For an avi video (6 Meg) demonstrating the above procedure, click here

  For a streaming version (Real Player required), click here

 

>Click here to continue