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The
Median Nerve
Median Nerve
The sensory response can be recorded from the thumb, index, middle or
ring fingers (combination of median and ulnar fibers). The motor response
is recorded from the abductor pollicis brevis (APB) muscle. In our laboratory
we routinely study the index and middle finger. Record sensory responses
from both the second and third digits and the motor response from the abductor
pollicis brevis (APB). Palmar studies are described below.
Work with the index finger first. Place the active ring/wire electrode
over the middle of the proximal phalanx and the reference electrode over
the middle of the intermediate phalanx. Select a point 13 cm. proximal
to the active electrode over the median nerve at the wrist for stimulation.
Repeat the same process for the middle finger.
For the motor fibers, place the electrode over the motor point of the
APB, in the upper third of the thenar eminence close to the first metacarpal.
Place the reference electrode over the thumb and the ground in the palm,
and stimulate the nerve at the wrist between the flexor carpi radialis
and palmaris longus tendons.
Then stimulate the nerve above the elbow medial to the biceps tendon
over the brachial pulse. Be especially careful by slowly increasing
the current to ensure that the distal and proximal responses are similar
in shape and that the proximal amplitude's drop does not exceed 2 mV. Finally,
measure the distance between the proximal and distal stimulating points
and calculate the conduction velocity.
If the lesion is in the arm or brachial plexus, the median nerve is
also stimulated at the axilla and Erb's point. At the axilla, stimulate
the nerve with the stimulator held firmly against the uppermost medial
aspect of the arm where the nerve can be palpated medially to the
coracobrachialis.
Erb's point is located behind the sternocleidomastoid clavicular head
and just above the clavicle at the tip of the C6 transverse process (Figure
26). The distance from the axilla and Erb's point to the wrist is measured
with the arm abducted to 90 degrees.
Stimulation at the axilla or Erb's point is painful and induces much
limb displacement. Since the nerves are closely packed together at these
levels, avoid volume conduction from other simultaneously activated muscles
by gradually increasing the stimulus intensity and carefully comparing
the response obtained with the configuration of the distal response.
In addition to the above-mentioned studies, palmar nerve stimulation
with recording of the nerve action potential at the wrist can be very useful
in the diagnostic work-up of a carpal tunnel syndrome by showing selective
slowing of the nerve segment at the wrist.
The median nerve is stimulated in the palm of the hand with the stimulator
cathode over the thenar crease at the level of the second metacarpal and
the anode distal to it. The nerve response is recorded from the wrist (orthodromic).
Median
Entrapment, Compression or Injury Sites
The median nerve can be entrapped in the carpal tunnel at the wrist;
as it passes through the two heads of the pronator teres in the forearm (pronator syndrome); above the medial
epicondyle, by an anomalous ligament
between a bony spur and the medial epicondyle, the ligament of Struthers;
and infrequently at the brachial plexus level in thoracic outlet compression.
Its branch, the anterior interosseous nerve, can also be entrapped or involved
by injury at the forearm level causing weakness in the muscles innervated
by it. This nerve is not routinely studied by nerve conductions and lesions
to it are diagnosed by needle examination.
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