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Technical, Physiological and Anatomic Considerations in Nerve Conduction Studies

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Technical, Physiological and Anatomic Considerations in Nerve Conduction Studies 13 Technical, Physiological and Anatomic Considerations in Nerve Conduction Studies James B. Caress Summary Nerve conduction studies and their interpretation are subject to a variety of factors. First, technical factors including submaximal stimulation, environmental electrical noise, inaccurate placement of the recording electrodes, and stimulus artifact can substantially interfere with accurate recording of nerve and muscle responses. Second, physiological factors, such as the effects of body height and age, can cause profound variation in all nerve conduction parameters, and studies require interpretation keep- ing these individual variations in mind. Another physiological factor is temperature, in which cooling can produce a variety of changes, including slowing of conduction velocity and increase of response amplitude. Anatomic factors are also important, the most common being the Martin–Gruber anasto- mosis, usually presenting with a reduction in response amplitude with proximal stimulation of the ulnar nerve, and the second most common being the presence of an accessory peroneal nerve. Paying constant attention to all of these details is a critical element to the accurate performance and interpre- tation of nerve conduction studies. Key Words: Conduction velocity; distal latency; Martin–Gruber anastomosis; nerve conduction study; stimulation; temperature. 1. INTRODUCTION Nerve conduction studies (NCS) are considerably more technically demanding than EMG. Correct interpretation of NCS data is dependent on re-creating the conditions that prevailed when the normal data tables were generated. This means that close attention to temperature and electrode placement are crucial. Physiological potentials are tiny in comparison with ambient electrical noise and must be amplified many times for evaluation. Artifacts may be similarly amplified and need to be recognized and minimized. 2. SKIN PREPARATION Good skin preparation is vital to performing NCS. The skin acts as a barrier to the meas- urement of electrical signals of interest and the effect of this barrier can be minimized by application of conductive gel that provides a low-resistance pathway to the electrode. Thick or edematous skin adds additional distance between the recording electrodes and the signal generator (i.e., the nerve or muscle), resulting in lower amplitudes. When the skin is callused, there may be different amounts of resistance to measuring the charge over the skin between the active and reference recording electrodes. Impedance is the term used to describe resistance From: The Clinical Neurophysiology Primer Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ 217 218 Caress to current flow in NCS. The impedance of recording electrodes must be similar, or ambient electrical noise will appear different at the electrodes. When this occurs, 60-Hz artifact or a broad stimulus artifact may obscure the waveforms. Skin lotions or perspiration can provide a conductive medium from the stimulator to the recording electrodes and can also result in a broad stimulus artifact. Soap and water cleansing is usually effective, but alcohol and acetone may be used to obtain a clean surface. 3. STIMULUS ARTIFACT Spread of excess current along the skin and deeper tissues results in a stimulus artifact. Modern instruments use a variety of technical mechanisms to minimize this, but, even so, stimulus artifact can obscure potentials with very short latencies, especially palm-to-fingers or palm-to-wrist recordings. Reducing stimulus duration and intensity can reduce stimulus artifact, but it is imperative to obtain supramaximal responses. Placing the ground electrode between the stimulating and recording electrodes can reduce stimulus artifact by providing a low-impedance pathway for excess current to flow through. Rotating the anode of the stimu- lator while leaving the cathode in place can minimize the stimulus artifact. Increasing the dis- tance between stimulating and recording electrodes or decreasing the distance between the active and reference recording electrodes will reduce stimulus artifact at the cost of lengthen- ing the latency and, possibly, reducing the amplitude of the waveforms. 4. RECORDING ELECTRODES Recording electrodes used in routine NCS include disc electrodes, ring electrodes, and bar electrodes that connect two discs at a fixed distance. Disc electrodes are commonly used to record motor potentials and nondigital sensory potentials. The discs are connected to separate wires that allow the distance between the active and reference electrode to be altered. Standard interelectrode distance is 3 to 4 cm, but there are occasions when adjustments must be made because of anatomic or physiological conditions. Any variation from standard positioning can change the characteristics of the recorded potentials, therefore, understanding the underlying electrophysiology is helpful in interpreting the data when alterations are necessary. NCS and EMG electrodes record all potentials using a differential amplifier system that magnifies differences between the active and reference electrodes at each point in time. Identical signals appearing simultaneously at both electrodes undergo common mode rejec- tion and are not displayed in the waveform. This is an ideal situation to minimize noise, but can also reduce physiological potentials. In sensory NCS, the active and reference recording electrodes are placed linearly along the course of the nerve, approx 3 cm apart. It is optimal for the traveling depolarized zone to pass completely under the active electrode before beginning to pass under the reference electrode, because this maximizes the displayed amplitude. Interelectrode differences of less than 3 cm may result in reduction of the amplitude. Increasing interelectrode difference beyond 3 to 4 cm does not increase the amplitude further and improves the chance of introducing electrical noise that may appear different at the two points. When the interelectrode distance is neces- sarily altered (as occurs with an amputation of a distal phalanx or to avoid bandages or sim- ilar obstacles), these concepts need to be kept in mind because normal values depend on specific interelectrode distances. Amplitudes vary inversely with the recording electrode’s distance from the nerve, thus, it is advisable to take steps to minimize edema, particularly Technical Factors 219 when recording from the sural, superficial peroneal, and radial nerves with disc electrodes. Placement of subdermal monopolar needle electrodes can circumvent this problem. In motor NCS, recording electrodes are placed in the “belly-tendon” arrangement. The active electrode is placed directly over the spot where most of the axons terminate in the neu- romuscular junction, called the motor point. When the nerve is stimulated, proper placement over the motor point will result in an initial negative (upwards) deflection from the baseline. If the electrode is not over the motor point, depolarization adjacent to the electrode results in a “positive dip” with an initially positive deflection. This downward deflection is a volume- conducted response akin to the positive deflection at the onset of a triphasic sensory wave- form. If a positive dip is noted, the active electrode should be moved until an initial negative deflection is observed. On occasion, repositioning the electrodes cannot correct a positive dip because of unusual anatomy (particularly tibial motor potentials) or because of severe neuro- muscular pathology. The reference electrode is placed distally over a tendinous region. Older texts have stated that the reference electrode is electrically inactive or “indifferent” and, thus, some practition- ers may be unaware that the reference electrode makes important contributions to the meas- ured motor potential. This is most apparent with ulnar motor studies recording from abductor digiti quinti (ADQ). The ulnar motor waveform characteristically exhibits a double-peaked morphology in its negative phase, and the second peak is usually of higher amplitude. In an elegant study, Kincaid et al. demonstrated that the reference electrode generates the second peak. When the reference electrode was moved to the contralateral index finger, the second peak was not recorded. In median nerve studies, the contribution of the reference electrode has a more subtle impact. The effect of the reference electrode should be kept in mind when double-peaked waveforms appear in unusual situations or when inclusion of the second peak would produce seemingly inaccurate results. 5. VIRTUAL CATHODE A tenet of NCS is that depolarization occurs directly under the cathode of the stimulator but, with overzealous attempts to maximize the amplitude of potentials, excess current traveling in the overlying superficial tissues may result in depolarization distal to the cathode. When this occurs, the cathode is “virtually” in a different place than its actual anatomic location. This can be readily observed by keeping the stimulator in place and increasing the intensity beyond maximal values (see Fig. 1). The amplitude stays relatively stable but the distal latency is shortened. The erroneously short latency may mask pathological slowing. Virtual cathode can easily occur when the nerve is quite deep to the stimulator. For example, during ulnar motor NCS at the below-elbow site, the deep location of the nerve results in high-intensity stimula- tion to elicit the supramaximal response. Depending on the anatomy of the individual person,
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