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TROUBLESHOOTING
No Response
Sometimes when trying to do nerve conduction studies you will get no
response. Because such nonresponse can result from many causes, a careful
step-by-step analysis of the nerve stimulation technique is necessary.
With motor nerve stimulation, you should see a visible muscle contraction
(though in severe neuropathic disease the contraction may be minimal).
If none is seen follow these steps:
 Check
to be sure the stimulation is delivering an impulse. Most patients will
feel the stimulus, but you can check it with your finger while turning
up the voltage. If no stimulus is being delivered, then check the switches
to see if they are on: remove the stimulator wires from their sockets and
reinsert them properly. Next, check the stimulator wires for a defect,
first visually then electrically with an ohmmeter to determine whether
the wire has continuity. If after following these steps you find nothing
amiss, then the problem lies within the stimulator, which must be tested
out by the electronics service man.
If
the stimulator is found to be working, then check the anatomical location
of the stimulation electrodes. Occasionally a beginner will place the electrodes
in the wrong area or over the wrong nerve.
If
the stimulating electrodes are in the proper position, then check for the
amount of cream under the anode and cathode. Too much cream or sweating
will create a cathode-anode bridge and will render nerve stimulation impossible.
Try drying the skin with alcohol or ether. Little or no cream will deliver
a submaximal stimulus strength.
If
the stimulating electrodes are in the proper position, then raise the stimulus
strength to the full output of the stimulator. If there is no response,
increase the duration of the stimulus; next, bring the stimulus to full
strength. This procedure is often necessary in the extremely obese persons
or in those with edema, severe nerve disease, or regenerating nerves.
Muscle
Contraction But No Evoked Response
Check
the switch controlling the input on the preamplifer to be sure it is in
the "on" position.
Confirm
that the recording electrodes are over the end-plate area of the muscle
being stimulated. If you still get no response.
Remove
excessive cream, which can cause an active bridge to the reference electrode
and will result in either a very small or no response. Add cream wherever
it is insufficient under the recording electrode. (Insufficient cream can
have the same effect as too much cream.)
Check
the recording electrodes and connecting wires with an ohmmeter for their
integrity or replace them with new electrodes.
On
a multichannel EMG machine, if you still get no response, check the connections
between the appropriate preamplifer and amplifier.
Check
the ground lead, for often when the ground is not in contact, the trace
on the CRT will be off the screen.
Assure
that the trace is centered on the screen by checking the appropriate channel
selection on the CRT.
Set
the adequate CRT sweep speed so that the expected response is on the screen
(Try using a slower sweep speed to see if the response is off the screen).
In
the event that the response is of low voltage, increase the gains on the
amplifier.
Stimulus
Artifact
If the record shows a large stimulus artifact, look into these possibilities:
The
ground is not functioning (sensory potential with loose ground on the left, motor on the right). Be sure that the electrode
paste is adequate and that the ground is on tightly and located in the
right place, preferably near but not touching the recording electrodes
or between the stimulating and recording electrodes, and the electrode
wire is tested with an ohmmeter to assure its continuity.
A
recording electrode is defective (Sensory potential with loose active electrode on the left, motor on the right). Again, be sure the
electrode paste is adequate, the electrodes are on tightly, and the electrode
and wire are checked with an ohmmeter for a defect. Defective electrodes
should be changed. The electrodes and their wires should also be tested
with an ohmmeter.
Check
the stimulating electrodes to assure that there is no electrode paste bridge
between the electrodes.
If
the above measures do not help, try using needle recording electrodes.
Make
sure recording and stimulation electrode connection cables are not crossed
and touching.
Abnormal
Recorded Potential
If the recorded potential is abnormal in its voltage, follow these
steps:
Move
the stimulating electrodes in small increments until the best response
is obtained. Be sure that the stimulus strength is supramaximal (submaximal
stimulus may appear to give a decremental type of response, especially
if the stimulator is not directly over the nerve).
Check
the recording electrodes to assure they are over the appropriate muscle
or nerve and that the amount of electrode paste is adequate to avoid a cream bridge effect (see below).
Initial
Positive Deflection
If the evoked response seen on the cathode ray tube has an initial
positive deflection, do the following, except for the posterior tibial
nerve, where recording from the abductor hallucis (AH) usually results
in an initial positive deflection.
Move
the active recording electrode about until it is over the motor point of
the muscle.
Make
sure that the appropriate nerve is being stimulated and that there is not
a spill over to another, faster conducting nerve (which can be checked
by stimulating that other nerve).
Consider
whether a crossover is present that would stimulate more remote muscles
sooner than the one being tested (see page 59).
Check
for reversed electrode connections to preamplifier input jacks.
 Table
of Contents Chapter
Index 
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