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NERVE
CONDUCTION STUDIES Basic
Nerve Conduction Studies
The basic concept of nerve stimulation is this: When a nerve is electrically
stimulated a reaction should occur somewhere along the nerve itself. With
appropriate recording electrodes, this reaction can be seen and the time
relationship between the stimulus and the response can be identified. In
this section we will consider first the mechanical and procedural part
of the EMG and then the types of findings and information that can be obtained.
Procedural
Grounding
Grounding is essential for obtaining a response that is relatively free
of artifact. Always first apply the ground lead to the patient. Furthermore,
never apply more than one ground to the patient at any time. The presence
of multiple grounds from different electrically powered devices can form
"ground loops", which are potentially dangerous electrical circuits
from one ground to another.
Place the ground between the stimulating electrode and the active recording
electrode and when possible at an equal distance between stimulating and
recording electrodes.
Usually the ground is a metal plate that is much larger than the recording
electrodes and provides a large surface area of contact with the patient.
Sometimes, though, it may be an uninsulated needle inserted into the patient's
skin.
Stimulation
Normally a peripheral nerve can be easily stimulated if the stimulus
source can be applied near the nerve. Thus, most nerve stimulation is done
to segments of nerve that lie close to the skin surface. Because of the
need for proximity, the number of nerves accessible to stimulation and
the locations of the stimulation of that nerve are limited. To stimulate
nerves deep to the skin you must use an insulated needle electrode with
its uninsulated tip lodged near the nerve.
The stimulating electrodes are normally two metal or felt pad electrodes
placed about 1.5 to 3 cm. apart. Locate the nerve and place the cathode
(black, conventionally) toward the direction in which the nerve is to conduct.
Raise the current until a maximal response is obtained and then by 25 to
50 percent more to insure that the response is truly maximal. Factors that
cause difficulties in stimulating a nerve include the following: improper
electrode placement, edema, obesity, thick calloused skin, faulty electrodes,
movement of the stimulating electrode and electrode cream bridge from stimulating
to recording electrode and between them and the ground electrode. Regenerating
nerves and those nerves with pathological changes are also more difficult
to stimulate.
Helpful in overcoming difficulties in nerve stimulation is increasing
the duration of the stimulus. Such increase causes some additional pain
to the patient but can overcome increased tissue resistance in edema, obesity,
and the like. Another useful procedure is to place a bare-tip insulated
needle electrode near the nerve as the cathode with a surface electrode
as the anode. The stimulating electrode's cathode should always be moved
about until the largest response is obtained.
The sites of stimulation depend on the nerve's anatomy. Some nerves
may only be accessible at one point whereas others may be stimulated at
three or four points along their course. In simple nerve conduction studies,
we usually use two stimulus sites, but other types of tests may only require
one site. The potential sites for stimulation will be discussed with each
nerve.
Recording
Recording electrodes are placed according to the type of response being
studies. If the objective is to record a motor response, then place the
active electrode over the belly of the muscle being activated. This placement
should be over the motor point to give an initial clear negative deflection
(upward) in the response. If a sensory nerve is being tested, place the
active electrode over the nerve itself to record the nerve action potential.
Place the reference electrode distally.
Most motor recording electrodes are surface disc electrodes about .5-1
cm in diameter. Needle electrodes can be used in specific instances when
amplitude measurements are not necessary.
Sensory recording electrodes are usually surface electrodes, with flat
buttons, spring clips, or rings most frequently used. However, bare-tip
insulated needle electrodes placed close to the nerves are used by many
investigators.
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