Neurophysiology of Nerve Conduction Studies

12 Neurophysiology of Nerve Conduction Studies James B. Caress, Gregory J. Esper, and Seward B. Rutkove Summary The methodology for performing standard nerve conduction studies has been established by iden- tifying the most helpful and consistent physiological data obtainable while being constrained by a variety of technical and practical limitations. Nerve stimulation occurs underneath the negatively charged anode of the applied stimulator and simultaneous hyperpolarization of the nerve occurs beneath the positively charged cathode. Referential or bipolar recording techniques are used for all types of measurements. Sensory conduction studies can be performed either antidromically or ortho- dromically, although, for technical reasons, the former are usually preferred; the recorded sensory nerve action potential is made up of the simultaneous depolarization of all of the cutaneous sensory axons. In motor studies, the compound motor action potential is recorded from the motor point of the muscle of interest and represents the depolarization of the underlying muscle fibers rather than the nerve itself and is, thus, of considerably greater amplitude and duration. F-waves and H-reflexes rep- resent the two most commonly evaluated forms of late responses and assist with assessing the entire length of the neurons, from spinal cord to distal muscle. Key Words: Compound motor action potential; depolarization; late responses; nerve conduction study; sensory nerve action potential; stimulation. 1. PHYSIOLOGY OF STIMULATION Stimulators used in routine nerve conduction studies (NCS) have a cathode and an anode and are, therefore, bipolar. The cathode is negatively charged, whereas the anode is positively charged. The depolarization of axons occurs under the cathode because the negativity in the region of the cathode leads to a reduction in the potential difference between the inside and the outside of the cell (the inside of the cell is relatively negative at baseline). On the other hand, the extracellular environment under the anode is positively charged, leading to hyperpolarization of the underlying axons. Much of the current supplied by the stimulator travels in the very low resistance extracellular space because current follows the path of least resistance. The cross-sectional resistance of an axon will determine whether some of that current will enter and depolarize the nerve. Cross-sectional resistance is reduced as diameter of the axon increases, resulting in a greater area in which current can flow. The result relevant to NCS is that large axons will depolarize with relatively less stimulus current than smaller axons. Hence, with low stimulus intensities, large axons will be preferentially stimulated. From: The Clinical Neurophysiology Primer Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ 207 208 Caress, Esper, and Rutkove Fig. 1. A median sensory nerve action potential recorded from digit 2. The digitally averaged waveform is on top. 2. MEASUREMENT OF SENSORY POTENTIALS IN NCS Waveforms that are displayed during sensory NCS reflect the passage of current beneath surface electrodes that are at least a few millimeters distant from the current generators (that is, the depolarizing axons). Routine studies usually use a bipolar recording technique in which both the active and reference electrodes lie above the nerve. The potential measured by the active electrode is compared with that measured by the reference electrode, and both are compared with a ground electrode lying elsewhere on the patient. The reference electrode makes an important contribution to the observed compound sensory nerve action potential (SNAP) (Fig. 1). The nerve current is not directly measured in NCS but overlying loops of current in the soft tissue reflect the actual action potentials. This situation is referred to as volume conduction and is further described in Chapter 4. Temporal dispersion refers to the phenomenon that, as a sensory conduction study is performed over longer and longer segments of nerve, the recorded SNAP loses its sharpness (high amplitude and short duration) and becomes a broader, lower-amplitude potential. This occurs normally in any sensory NCS because individual sensory axons conduct at slightly different rates and because these slight differences are magnified when a NCS is performed over a large segment of nerve. In diseased states, such as polyneuropathies, temporal dispersion can be enhanced (abnormal temporal dispersion). Because sensory NCS are especially vulnerable to the effects of temporal dispersion in both health and disease, sensory studies are usually performed over shorter segments of nerve than motor studies, which tend to be less susceptible to these effects (described in Section 3). Sensory responses can be recorded with an orthodromic or an antidromic technique. In orthodromic studies, the recording electrodes are proximal to the stimulation site, and, in antidromic studies, stimulation is proximal to the recording position. In the hands, ring electrodes are used around the fingers to stimulate (orthodromic) or record (antidromic). Although orthodromic Nerve Conduction Studies 209 Fig. 2. A median compound muscle action potential. Recordings made with wrist and elbow stim- ulating APB, abductor pollicis brevis. responses reflect more accurately the physiology of sensory responses via afferent conduction, upper extremity orthodromic responses are usually considerably smaller than antidromic responses. 3. MEASUREMENT OF MOTOR POTENTIALS IN NCS The active electrode is placed over the motor point of the muscle, where the majority of the motor axons synapse with the end plates of the muscle fibers. Rather than measuring a compound nerve action potential, the summation of the muscle fiber action potentials is measured. This is called the compound motor action potential (CMAP) (Fig. 2). In this case, there is no leading edge of a dipole to cause an initial downward deflection, because the depolarization initiates directly beneath the recording electrode. The influx of Na+ at the muscle end plates generates extracellular negativity that is displayed as an abrupt upward deflection from the baseline. The waveform returns to the baseline as the resting membrane potential of the muscle fibers is reestablished. The distal latency is the time at which the depolarization of the fastest nerve fiber is recorded; this response is routinely recorded but does not directly measure the conduction velocity, as is the case with sensory recordings. This is because of the additional time it takes for acetylcholine to traverse the synapse, to attach to receptors, and to generate muscle fiber action potentials. Conduction velocity is calculated only in the proximal segments of nerve, by subtracting the distal latency and distance from the values obtained with proximal stimulation. However, by establishing normal values for distal laten- cies for specific distances, meaning can be given to the measurement that can assist in the evaluation of a variety nerve disorders, including distal compression neuropathies, such as carpal tunnel syndrome (median neuropathy at the wrist). One point that is frequently overlooked is that the recorded CMAP does not reflect muscle contraction. The CMAP is a purely electrical signal from the summation of muscle fiber action potentials, which occurs well before the actual contraction of the muscle takes place. 210 Caress, Esper, and Rutkove The visible muscle contraction requires Ca2+ release cross-linking of actin and myosin fibers (so-called excitation–contraction coupling), which occurs much more slowly and is not measured in routine NCS. Also, as in sensory NCS, the reference electrode is not electrically inac- tive, and it contributes to the waveform, even though it is often placed over a bone, a region generally considered electrically silent. This is particularly clear during ulnar or tibial motor NCS, where the waveform characteristically exhibits a double-peaked negative phase. The second peak results from activity recorded at the reference electrode. Similar to sensory responses, normal temporal dispersion of the responses does occur because smaller motor neurons will conduct at slower velocities than larger, causing the recorded CMAP to spread out (increase in duration and decrease in amplitude) the longer the segment of nerve studied. However, the effects are not nearly as dramatic as those observed in sensory studies, because the duration of the CMAP is much larger than of the SNAP (~6 times longer). Hence, small increases in duration are not be readily apparent, and the amplitude of the response generally declines only modestly with increasing distance. 4. NEUROPHYSIOLOGY OF THE LATE RESPONSES 4.1. F-Waves The “F” stands for foot, because these responses were first recorded from intrinsic foot mus- cles. On stimulation of a single motor axon, the wave of depolarization will travel distally to be recorded as part of the CMAP, or “M-wave,” but also will travel proximally to its anterior horn cell (AHC). Retrograde depolarization of AHCs will result in regeneration of an action potential at the axon hillock in a small subset of neurons (~5–10%), which then travels back down the motor axon to the innervated muscle, recorded at the electrodes as the F-wave (Fig. 3). Although, for theoretical reasons, it is desirable to reverse the polarity of the stimulator, placing the anode distally and cathode proximally to avoid “anodal block,” this remains more of a theoretical

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