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ELECTROMYOGRAPHY & NERVE CONDUCTION STUDIES
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Nerve conduction studies and needle electromyography (EMG) provide objective physiologic assessment of peripheral nerves and muscles. These two parts of the examination are performed sequentially, and when a patient is referred to an EMG laboratory, the understanding is that electrodiagnostic evaluation will include both nerve conduction studies and EMG. Special studies are performed in selected patients when clinically indicated.
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NERVE CONDUCTION STUDIES
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General Considerations
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Studies are performed on motor and sensory nerves, but only large myelinated fibers can be evaluated in nerve conduction studies (Figure 2–1). Most studies use surface recording electrodes because of ease and convenience.
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In motor conduction studies, an electrical stimulus is delivered to a skin location known to overlie a peripheral nerve based on anatomical landmarks, and motor responses are recorded from muscles innervated by that nerve (Table 2–1). For example, the median nerve can be stimulated at the wrist and then more proximally at the elbow, with the recording electrode placed over the abductor pollicis brevis muscle in the thenar eminence. The evoked response obtained from the electrical stimulation is called the compound motor action potential (CMAP) (Figures 2–2 and 2–3). By measuring the distance between the two stimulating sites and the difference between latency onset of the resultant CMAPs, the examiner can calculate the motor conduction velocity of that nerve segment.
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Sensory nerve conduction studies directly assess sensory axons by recording a sensory nerve action potential (SNAP) proximal or distal to the site of stimulation (Figure 2–4...