Needle electromyography (EMG) complements nerve conduction studies and serves a critical role in the evaluation of both muscle and nerve diseases. With experience, needle EMG can be rapidly performed and interpreted. However, to the uninitiated, the procedure can seem very complicated and the interpretation of the waveforms arbitrary. In this chapter, we provide a background for a general understanding of how needle EMG is performed and interpreted, but leave its detailed application to specific disorders for discussion in other chapters of this text.
There are several different types of needle electrodes. The monopolar needle electrode provides referential recordings. The needle is Teflon covered, except for the tip, which is exposed and where electrical activity is recorded; these signals are compared with those recorded from the surface via a separated electrode placed on the skin at a distance from the muscle. Although monopolar needle electrodes continue to be widely used, concentric needle electrodes have become the favored choice among most electromyographers. With this type of electrode, a barrel that surrounds the active needle electrode serves as the reference, allowing the recorded signal from the active needle electrode to be compared with that of the
From: The Clinical Neurophysiology Primer Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ
barrel, without requiring a separate surface electrode (see Fig. 1). An insulating material separates these two surfaces. Because the placement of the reference electrode is practically adjacent to the active site, motor unit potentials (MUPs) using a concentric needle tend to be smaller in duration and amplitude than those same MUPs measured with a monopolar electrode.
Waveforms representing MUPs are visually displayed on the oscilloscope in real time, and both qualitative and quantitative assessments can be made of MUPs. Qualitative visual analysis of the MUPs on the oscilloscope allows the real-time assessment of the duration, amplitude, and number of phases of each waveform, along with the recruitment ratio (discussed in more detail in Section 7 below). MUP morphology determination can be also be confirmed by a more accurate, but usually more time consuming, quantitative assessment of waveforms, which are analyzed by available software. Standard EMG is generally performed in a qualitative fashion, because an experienced electromyographer can often assess abnormalities rapidly and accurately; quantitation of MUP morphology is usually reserved for subtle processes (e.g., a mild myopathy) and research purposes.
2.3. The Loudspeaker/Amplifier
Listening to the electrical activity can tell the electromyographer a great deal regarding the condition of the muscle. Musical concepts apply in this regard: the volume (loud vs soft) of a given element tells you about its amplitude, the pitch (high vs low) tells you about the duration of the potential, and the timbre (quality of the sound) tells you about its phases. Other electrical discharges, such as myotonia, neuromyotonia, and complex repetitive discharges (CRDs) have characteristic sounds that are helpful in determining the type of disease present. Similar to the visual interpretation of waveforms, the auditory interpretation gradually develops with time and experience. These issues are discussed in further detail below, in the section on the evaluation of voluntary activity.
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