It was long ago discovered that muscle would contract when a pulse of electric current was applied to the skin, near the point of entrance of the muscular nerve (the motor point). The electrical pulse required is brief, less than a millisecond, and is most effectively induced by rapidly alternating (faradic) current. If there has been muscle denervation, an electrical pulse of several milliseconds induced by a constant electrical (galvanic) stimulus is required to produce the same response. For decades, this was the standard electrical method for evaluating denervation of muscle. Although still valid, it was replaced by nerve conduction studies and by the needle electrode examination. The latter test, based on the sherringtonian concept of the “motor unit” described in Chap. 3, is accomplished by the insertion into muscle of needle electrodes to measure spontaneous and voluntarily evoked muscle fiber activity. The terms electromyography and electromyogram were used originally to describe the needle electrode examination but are now a common shorthand designation for the entire electrodiagnostic evaluation, including the nerve conduction studies.
The main laboratory technique for the study of peripheral nerve function involves the transcutaneous stimulation of motor or sensory nerves and recording of the elicited action potentials in the muscle (CMAP) and the sensory nerve action potential (SNAP). The results of these motor and sensory nerve conduction studies, expressed as amplitudes, conduction velocities, and distal latencies, yield certain quantitative information and additional qualitative observations regarding the waveform of electrical neural and muscular impulses.
Hodes and coworkers, in 1948, were the first to describe nerve conduction studies in patients, and the techniques used currently are not much changed. An accessible nerve is stimulated through the skin by surface electrodes, using a stimulus that is large enough to recruit (cause a discharge in) all the available nerve fibers. The resulting action potential is recorded by electrodes on the skin (1) over the muscle distally in the case of motor fibers stimulated in a mixed or motor nerve (CMAP), (2) over the nerve more distally, using antidromic techniques for sensory nerve conduction studies (this has technical advantages over orthodromic techniques), and (3) over the nerve more proximally for mixed (sensory and motor) nerve conduction studies. These techniques are the ones used most often in clinical work. An alternative but much more demanding experimental technique uses “near-nerve” needle electrodes to record action potentials as they course through the nerve. The main characteristics of the conventional nerve conduction studies are described below.
The peak amplitude of the evoked muscle action potential to a maximal stimulus (CMAP) yields valuable information about peripheral nerve function. The amplitude, usually in the order millivolts, reflects the summated electrical potential generated by the depolarization of a muscle innervated by the motor nerve (Fig. 2-11). In instances of disease, the amplitude is a semiquantitative measure of the number of remaining normal nerve fibers and of the innervated volume of muscle. It is usually possible to obtain a reliable motor conduction study as long as some functioning nerve fibers remain intact, although the compound muscle potential recorded may be very low. The latency to the CMAP waveform is the basis for calculations of motor nerve conduction velocity. Reduction in motor amplitudes is a specific and sensitive indicator of axonal loss. Demyelinative lesions affecting the large, fast-conducting fibers also reduce the peak summated amplitude of the CMAP but are the result of differential arrival times of the electrical potentials from each axon at the muscle. Table 2-5 shows the range of normal amplitudes for the CMAPs that are elicited by stimulation of the main motor nerves.