Basic Principles and Technique

Needle electromyography is generally performed in conjunction with nerve conduction studies, which may be undertaken to determine the functional integrity of the peripheral nerves. For motor conduction studies, the nerve is stimulated at two or more points along its course while the electrical response is recorded of one of the muscles

Figure 24-21 A myotonic discharge evoked by electrode movemeifFrom Aminoff MJ: Electromyography in Clinical Practice, 3rd ed. New York, Churchill Livingstone, 1998.)

supplied by it. Electrical stimuli are preferred and must be of sufficient intensity to excite all of the fibers in the nerve. The muscle response is recorded by surface or subcutaneous needle electrodes, with the active electrode being placed over the endplate region and the reference electrode over the muscle tendon. The response recorded in this way is called the compound muscle action potential (CMAP), or M wave, and represents the sum of the electrical activity of all of the activated muscle fibers within the pickup region of the recording electrode. The shape, size, and latency of the response obtained by stimulating the nerve at different sites is compared. By measuring the distance between stimulation sites and the difference in latency of the responses elicited by stimulation at these sites, the conduction velocity can be determined for the fastest conducting fibers along the intervening segments of the nerve ( ...Fig, 24-22 ). In the arms, the normal range of maximal motor conduction velocity is between 50 and 70 m/sec, while in the legs the corresponding velocities are between 40 and 60 m/sec.

Sensory nerve conduction studies involve stimulating a sensory nerve either orthodromically or antidromically and recording the response at another point along the course of that same nerve. The calculated conduction velocity is the same, but the response is larger with antidromic stimulation. Responses can also be recorded from a purely sensory nerve after stimulation of the parent nerve trunk from which it originates, or vice versa.

In patients with focal conduction block involving an individual nerve, the CMAP elicited by stimulating the nerve above the site of the lesion is reduced in amplitude compared with more distal stimulation and, in severe cases, is completely lost. Sensory nerve action potentials are also small or unrecordable when the lesion is located between stimulation and recording sites. With focal conduction slowing (but not block), the size of the CMAP is reduced as the distance increases between the stimulating and recording electrodes. Motor and sensory conduction velocities are slowed across the region of the nerve encompassing the lesion, and sensory nerve action potentials may be markedly attenuated or unrecordable because of dispersion. Axon-loss lesions are characterized by an attenuated muscle response, by reduced or absent sensory nerve action potentials, and by electromyographic signs of denervation in the affected muscle. Motor or sensory conduction velocity is normal or reduced only minimally, when it can be recorded.

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