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The
Radial Nerve
Radial Nerve
Routinely, only the sensory fibers of the radial nerve are studied because
the distal muscles innervated by this nerve are in the forearm, where the
chances of volume conduction from other muscles are great. In radial nerve
lesions, however, the motor fibers are studied, with the response recorded
from the extensor difitorum communis.
Record the sensory response with the active electrode placed over the
web space between the first and second metacarpals and the reference over
the thumb. Stimulate the radial nerve at a point 10 cm. proximal to the
active electrode over the edge of the radius. To overcome the patient's
difficulty for relaxing the thenar muscles, ask him to loosely grasp a
piece of gauze or a tennis ball with his fingers, with his hands held perpendicular
to the bed.
To study the motor fibers, place the active electrode over the extensor
digitrm communis, easily located by asking the subject to extend his fingers
at the metacarpophalangeal joint. Then place the reference electrode over
the wrist and stimulate the radial nerve at the elbow with the stimulator
probes firmly pressed against the lateral aspect of the biceps tendon.
Next, stimulate the nerve at the spiral groove between the medial and lateral
heads of the triceps muscle. Calculate the conduction velocity between
the spiral groove and elbow. Because the distances used are small and the
nerve segment under study proximal, the calculated conduction velocities
tend to be faster than those obtained for the median and ulnar nerves.
In more proximal radial nerve lesions (Saturday night palsy and brachial
plexus lesion), you can stimulate the radial nerve at both the axilla and
Erb's point and calculate the conduction velocity on each one of these
segments.
In radial nerve lesions with conduction blocks at the spiral groove,
you can generally locate the site of the block by moving the stimulator
probes down in small increments along the spiral groove, until a motor
response can be obtained.
Radial
Entrapment, Compression or Injury Sites
The superficial radial sensory branch can be injured in the forearm
along its course; the posterior interosseous branch may be entrapped or
injured at its origin; the radial nerve is subject to compression or trauma
at the spinal groove; or shortly after it emerges from the brachial plexus,
at the axilla.
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