The conduction times that are utilized in clinical work are the latency from the stimulus artifact to the onset of the compound muscle action potential (CMAP), the distal (or terminal) latency; and from the stimulus to the peak of the CMAP, peak motor latency (see Fig. 2-11). A stimulus is then applied to the nerve at a second site more proximally, and a conduction time can be measured over a longer segment of nerve. When the distance (in millimeters) between the two sites of stimulation is divided by the difference in the distal latencies (in milliseconds), one obtains a conduction velocity (in meters per second). This method isolates the conduction time across a segment of a peripheral nerve by eliminating the transmission time across the neuromuscular junction and the duration of muscle depolarization. Motor nerve conduction velocity represents the maximal velocity of propagation of the action potentials in the largest-diameter and fastest- conducting nerve fibers. These velocities in normal subjects vary from a minimum of 40 or 45 m/s to a maximum of 65 to 75 m/s, depending upon which nerve is studied (e.g., slower in the legs than in the arms; see Table 2-5). Values are lower in infants, reaching the adult range by the age of 2 to 4 years, and declining again slightly with advancing age. Conduction velocity also is diminished with exposure to cold, a potentially important factor if these recordings are taken when the patient’s skin is cool; consequently, measurement of skin temperature is routinely done prior to performing the nerve conduction tests.
Normal values have been established for distal latencies from the usual sites of stimulation on various mixed nerves to the appropriate muscles. Stimulating the median nerve at the wrist, for example (see electrode 1 and segment A in Fig. 2-11), has a latency for motor conduction through the carpal tunnel to the median-innervated thenar muscles of less than approximately 4.5 ms in healthy adults. Similar normal values have been compiled for orthodromic and antidromic sensory conduction velocities and for distal latencies in all the main peripheral nerves (see Table 2-5).
The main effect of disease processes that preferentially injure axons, as mentioned above, is a reduction in the CMAP amplitude (Fig. 2-12B). However, some processes affect the fastest-conducting, large-diameter fibers and also usually reduce the conduction velocity because the remaining thinner fibers conduct more slowly. In most neuropathies, all the axons are affected either by a fairly uniform “dying-back” phenomenon or by wallerian degeneration as described in Chap. 43, and nerve conduction velocities are then less affected. This is true, for example, in typical alcoholic-nutritional, carcinomatous, uremic, diabetic, and other metabolic neuropathies, in which conduction velocities range from the low-normal range to mildly slowed. By contrast, demyelinating neuropathies (see Chap. 46) show marked slowing of conduction and, in the case of the acquired demyelinating diseases, there is also dispersion of the motor action potential and a characteristic conduction block (Fig. 2-12C).
The sensory nerve action potential (SNAP) is far lower in amplitude than the CMAP. It directly represents the action potentials in a group of sensory nerve axons. When one attempts to measure sensory nerve action potentials, the summation of electrical activity provided by many motor units discharging at the muscle is not available and electronic amplification is required. In contrast to motor conduction measurements, the nerve is typically stimulated at one site and recordings are performed at two distal sites (therefore antidromically for sensory conduction) in order to obtain both the amplitude (at the more proximal site) and conduction velocity by the subtraction method (Fig. 2-13). Sensory potentials, measured in microvolts, are sometimes very small or absent and sensory conduction measurements may then be difficult to determine. Table 2-5 gives the range of normal values for sensory nerve action potential amplitudes and velocities.