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Nerve Conduction Concepts & Fundamentals
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.
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.
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 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.