The technique consists of applying 5-per-second painless transcutaneous electrical stimuli to the median, peroneal, or tibial nerves and recording the evoked potentials (for the upper limb) sequentially as they pass the brachial plexus over the Erb point above the clavicle, over the C2 vertebra, and over the opposite parietal cortex, and (for the lower limb) over the lumbar roots of the cauda equina, the nuclei over the cervical spine, and the opposite parietal cortex. The impulses generated in large touch fibers by 500 or more stimuli and averaged by computer can be traced through the corresponding peripheral nerves, spinal roots, and posterior columns to the cuneate and gracile nuclei in the lower medulla, through the medial lemniscus to the contralateral thalamus, and thence to the sensory cortex of the parietal lobe. Delay between the stimulus site and the Erb point or the lumbar spine indicates peripheral nerve disease; delay from the Erb point (or lumbar spine) to C2 implies an abnormality in the appropriate nerve roots or, more frequently, in the posterior columns; the presence of lesions in the medial lemniscus and thalamoparietal pathway can be inferred from delays of subsequent waves recorded from the parietal cortex (Fig. 2-10).
The normal waveforms are designated by the symbols P (positive) and N (negative), with a number indicating the interval of time in milliseconds from stimulus to recording (e.g., N11, N13, P13, P22, etc.). As shorthand for the polarity and approximate latency, the summated wave that is recorded at the cervicomedullary junction is termed N/P13, and the cortical potential from median nerve stimulation seen in two contiguous waves of opposite polarity is called N19–P22. The corresponding cortical wave after tibial or peroneal nerve stimulation is called N/P37. Each trace is the averaged response to 1,024 stimuli; the superimposed trace represents a repetition to demonstrate waveform consistency.
For purposes of clinical interpretation, the generators of the SEP waves are assumed to be linked in series, so that an interwave prolongation in latency indicates a conduction defect between the generators of the two peaks involved (Chiappa and Ropper). Normal values are shown in Table 2-4. Recordings with pathologically verified lesions at these levels are to be found in the monograph by Chiappa. This test has been most helpful in establishing the existence of lesions in spinal roots, posterior columns, and brainstem in disorders such as the ruptured lumbar and cervical discs, multiple sclerosis, and lumbar and cervical spondylosis when the clinical data are uncertain. The cerebral counterpart also pertains—namely, that obliteration of the cortical waves (assuming that all preceding waves are unaltered) reflects profound damage to the somatosensory pathways in the hemisphere or to the cortex itself. For example, the bilateral absence of cortical somatosensory waves after cardiac arrest is a powerful predictor of a poor clinical outcome; the persistent absence of a cortical potential on one side after stroke usually indicates such profound damage that only a limited clinical recovery is to be expected.
Evoked potential techniques have also been used in the experimental study of olfactory and trigeminal sensation (see Chap. 11).
Magnetic Stimulation of the Motor System