Common pitfalls & best practices
Common pitfalls & best practices
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.
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.
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.
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).
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.