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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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