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Basic Nerve Conduction Studies
After the history is taken and a directed physical examination is performed, every study begins with nerve conduction studies (NCSs). The needle electromyography (EMG) examination is performed after the NCSs are completed because the findings on the NCSs are used in the planning and interpretation of the needle EMG examination that follows.
Peripheral nerves usually can be easily stimulated and brought to action potential with a brief electrical pulse applied to the overlying skin. Techniques have been described for studying most peripheral nerves. In the upper extremity, the median, ulnar, and radial nerves are the most easily studied; in the lower extremity, the peroneal, tibial, and sural nerves are the most easily studied (see Chapter 10, Chapter 11 ). Of course, the nerves selected for study depend on the patient’s symptoms and signs and the differential diagnosis. Motor, sensory, or mixed nerve studies can be performed by stimulating the nerve and placing the recording electrodes over a distal muscle, a cutaneous sensory nerve, or the entire mixed nerve, respectively. The findings from motor, sensory, and mixed nerve studies often complement one another and yield different types of information based on distinct patterns of abnormalities, depending on the underlying pathology.
Motor Conduction Studies
Motor conduction studies are technically less demanding than sensory and mixed nerve studies; thus, they usually are performed first. Performing the motor studies first also has other major advantages. It is not uncommon for the sensory responses to be very low in amplitude or absent in many neuropathies. Performing the motor studies first allows one to know where the nerve runs, where it should be stimulated, and how much current is needed and also gives some information about whether the nerve is normal or abnormal. On the other hand, if the sensory study is done before the motor study, one might spend a lot of unnecessary time stimulating and trying to record a sensory response that is not present. For example, imagine a patient with a moderately severe median neuropathy at the wrist who is sent for an electrodiagnostic (EDX) evaluation. If the median motor study is performed first, the correct stimulation site can be confirmed, the amount of current needed to stimulate the median nerve will be known, and one will also know that the median nerve is abnormal before doing the median sensory study. Then, when performing the median sensory study, one is confident of where to stimulate the nerve and how much current is needed. In this case, if no sensory response is present, one can have a high degree of certainty that the response is truly absent and move along to the next nerve to be studied. However, if the sensory conduction study is done first and is absent, it will not be as obvious if the absent response is due to a technical problem or is truly absent. One can waste a lot of time unnecessarily trying to figure this out. Do the motor conduction study first; your study will be more efficient, and the patient will tolerate the study much better.
Motor responses typically are in the range of several millivolts (mV), as opposed to sensory and mixed nerve responses, which are in the microvolt (μV) range. Thus, motor responses are less affected by electrical noise and other technical factors. For motor conduction studies, the gain usually is set at 2–5 mV per division. Recording electrodes are placed over the muscle of interest. In general, the belly-tendon montage is used. The active recording electrode (also known as G1) is placed on the center of the muscle belly (over the motor endplate), and the reference electrode (also known as G2) is placed distally, over the tendon to the muscle ( Fig. 3.1 ). The designations G1 and G2 remain in the EMG vernacular, referring to a time when electrodes were attached to grids (hence the G) of an oscilloscope. The stimulator then is placed over the nerve that supplies the muscle, with the cathode placed closest to the recording electrode. It is helpful to remember “black to black,” indicating that the black electrode of the stimulator (the cathode) should be facing the black recording electrode (the active recording electrode). For motor studies, the duration of the electrical pulse usually is set to 200 ms. Most normal nerves require a current in the range of 20–50 mA to achieve supramaximal stimulation. As current is slowly increased from a baseline 0 mA, usually by 5–10 mA increments, more of the underlying nerve fibers are brought to action potential and, subsequently, more muscle fiber action potentials are generated. The recorded potential, known as the compound muscle action potential (CMAP), represents the summation of all underlying individual muscle fiber action potentials. When the current is increased to the point that the CMAP no longer increases in size, one presumes that all nerve fibers have been excited and that supramaximal stimulation has been achieved. The current is then increased by another 20% to ensure supramaximal stimulation.
The CMAP is a biphasic potential with an initial negativity, or upward deflection from the baseline, if the recording electrodes have been properly placed with G1 over the motor endplate. For each stimulation site, the latency, amplitude, duration, and area of the CMAP are measured ( Fig. 3.2 ). A motor conduction velocity can only be calculated after two sites, one distal and one proximal, have been stimulated.
Latency
Latency is the time from the stimulus to the initial CMAP deflection from baseline. Latency represents three separate processes: (1) the nerve conduction time from the stimulus site to the neuromuscular junction (NMJ), (2) the time delay across the NMJ, and (3) the depolarization time across the muscle. Latency measurements usually are made in milliseconds (ms) and reflect only the fastest conducting motor fibers.
Amplitude
CMAP amplitude is most commonly measured from baseline to the negative peak and less commonly from the first negative peak to the next positive peak. CMAP amplitude reflects the number of muscle fibers that depolarize. Although low CMAP amplitudes most often result from loss of axons (as in a typical axonal neuropathy), other causes of a low CMAP amplitude include conduction block from demyelination located between the stimulation site and the recorded muscle, as well as some NMJ disorders and myopathies.
Area
CMAP area also is conventionally measured as the area above the baseline to the negative peak. Although the area cannot be determined manually, the calculation is readily performed by most modern computerized EMG machines. A negative peak CMAP area is another measure reflecting the number of muscle fibers that depolarize. Differences in CMAP area between the distal and proximal stimulation sites take on special significance in the determination of conduction block from a demyelinating lesion (see the “Conduction Block” section).
Duration
CMAP duration usually is measured from the initial deflection from baseline to the first baseline crossing (i.e., negative peak duration), but it also can be measured from the initial to the terminal deflection back to baseline. The former is preferred as a measure of CMAP duration because when CMAP duration is measured from the initial to terminal deflection back to baseline, the terminal CMAP returns to baseline very slowly and can be difficult to mark precisely. Duration is primarily a measure of synchrony (i.e., the extent to which each of the individual muscle fibers fire at the same time). Duration characteristically increases in conditions that result in slowing of some motor fibers but not others (e.g., in a demyelinating lesion).
Conduction Velocity
Motor conduction velocity is a measure of the speed of the fastest conducting motor axons in the nerve being studied, which is calculated by dividing the distance traveled by the nerve conduction time. However, motor conduction velocity cannot be calculated by performing a single stimulation. The distal motor latency is more than simply a conduction time along the motor axon; it includes not only (A) the conduction time along the distal motor axon to the NMJ, but also (B) the NMJ transmission time and (C) the muscle depolarization time. Therefore, to calculate a true motor conduction velocity without including NMJ transmission and muscle depolarization times, two stimulation sites must be used, one distal and one proximal.
When the nerve is stimulated proximally, the resulting CMAP area, amplitude, and duration are, in general, similar to those of the distal stimulation waveform. The only major difference between CMAPs produced by proximal and distal stimulations is the latency. The proximal latency is longer than the distal latency, reflecting the longer time and distance needed for the action potential to travel. The proximal motor latency reflects four separate times, as opposed to the three components reflected in the distal motor latency measurement. In addition to (A) the nerve conduction time between the distal site and the NMJ, (B) the NMJ transmission time, and (C) the muscle depolarization time, the proximal motor latency also includes (D) the nerve conduction time between the proximal and distal stimulation sites ( Fig. 3.3 ). Therefore, if the distal motor latency (containing components A+B+C) is subtracted from the proximal motor latency (containing components A+B+C+D), the first three components will cancel out. This leaves only component D, the nerve conduction time between the proximal and distal stimulation sites, without the distal nerve conduction, NMJ transmission, and muscle depolarization times. The distance between these two sites can be approximated by measuring the surface distance with a tape measure. A conduction velocity then can be calculated along this segment: (distance between the proximal and distal stimulation sites) divided by (proximal latency minus distal latency). Conduction velocities usually are measured in meters per second (m/s).
It is essential to note that both latency and conduction velocity reflect only the fastest conducting fibers in the nerve being studied. By definition, conduction along these fibers arrives first and thus it is these fibers that are the ones measured. The many other normal slower conducting fibers participate in the CMAP area and amplitude but are not reflected in either the latency or conduction velocity measurements.
Sensory Conduction Studies
In contrast to motor conduction studies, in which the CMAP reflects conduction along the motor nerve, NMJ, and muscle fibers, in sensory conduction studies, only nerve fibers are assessed. Because most sensory responses are very small (usually in the range of 1–50 μV), technical factors and electrical noise assume more importance. For sensory conduction studies, the gain usually is set at 10–20 μV per division. A pair of recording electrodes (G1 and G2) are placed in line over the nerve being studied, at an interelectrode distance of 2.5–4 cm, with the active electrode (G1) placed closest to the stimulator. Recording ring electrodes are conventionally used to test the sensory nerves in the fingers ( Fig. 3.4 ). For sensory studies, an electrical pulse of either 100 or 200 ms in duration is used, and most normal sensory nerves require a current in the range of 5–30 mA to achieve supramaximal stimulation. This is less current than what is usually required for motor conduction studies. Thus, sensory fibers usually have a lower threshold to stimulation than do motor fibers. This can easily be demonstrated on yourself; when slowly increasing the stimulus intensity, you will feel the paresthesias (sensory) before you feel or see the muscle starts to twitch (motor). As in motor studies, the current is slowly increased from a baseline of 0 mA, usually in 3–5-mA increments, until the recorded sensory potential is maximized. This potential, the sensory nerve action potential (SNAP), is a compound potential that represents the summation of all the individual sensory fiber action potentials. SNAPs usually are biphasic or triphasic potentials. For each stimulation site, the onset latency, peak latency, duration, and amplitude are measured ( Fig. 3.5 ). Unlike motor studies, a sensory conduction velocity can be calculated with one stimulation site alone, by taking the measured distance between the stimulator and active recording electrode and dividing by the onset latency. No NMJ or muscle time needs to be subtracted out by using two stimulation sites.
Onset Latency
The onset latency is the time from the stimulus to the initial negative deflection from baseline for biphasic SNAPs or to the initial positive peak for triphasic SNAPs. Sensory onset latency represents nerve conduction time from the stimulus site to the recording electrodes for the largest cutaneous sensory fibers in the nerve being studied.
Peak Latency
The peak latency is the time from the stimulus to the midpoint of the first negative peak. When sensory NCSs were first developed, peak latencies were classically used instead of onset latencies. In the past, peak latencies offered several advantages, some of which are still as valid today as when they were first developed. The peak latency can be ascertained in a straightforward manner; there is practically no interindividual variation in its determination. In contrast, the onset latency can be obscured by noise or by the stimulus artifact, making it difficult to determine precisely. In addition, for some potentials, especially small ones, it may be difficult to determine the precise point of deflection from the baseline ( Fig. 3.6 ). These problems do not occur in marking the peak latency. Normal values exist for peak latencies for the most commonly performed sensory studies.
However, peak latencies have two distinct disadvantages. Most important, the peak latency cannot be used to calculate a conduction velocity. The population of sensory fibers represented by the peak latency is not known, in contrast to the onset latency, which represents the fastest conducting fibers in the nerve being studied. Second, normal values for peak latencies are dependent on using standard distances. This can be problematic especially in the upper extremities in patients with particularly large or small hands.
Both onset and peak latencies can be used in the analysis of sensory NCSs. In the examples given in this text in the later clinical chapters, both values are measured and reported. If the potential has an abrupt, consistent, and easily marked onset latency, the onset latency is the preferred measurement. However, in cases where the onset latency is obscured, or not precise, peak latency is better used.
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