Needle EMG evaluates the electrical activity of muscle, through which neuropathic or myopathic processes can be identified and characterized. The needle electrode examination (NEE) generally consists of three distinct parts: evaluation of spontaneous activity, evaluation of MUP morphology, and evaluation of activation and recruitment.
The first part of the NEE is the evaluation of spontaneous activity—the activity that is present when the muscle is completely relaxed. When a needle electrode is inserted into an electrically active tissue, an initial burst of electrical activity from the depolarizing muscle fibers is to be expected. These bursts generally last no longer than approx 0.5 s (500 ms). With every movement of the needle electrode, a similar burst of electrical activity will be generated. However, when the movement ceases, the electrical activity should end and only electrical silence should normally be identified. This is what is described as "normal spontaneous activity."
A relaxed, normal muscle should be electrically silent; the exception to this rule occurs when the needle comes into close proximity to the end-plate region of the muscle. Intermittent bursts of sharp spike activity, called "end-plate spikes," might be observed. They are generally easy to recognize because they have a sputtering quality, are biphasic, and usually have an initial negativity (see Fig. 2). Another helpful hint is the sound of "end-plate noise"—a very high-frequency hissing sound attributed to the depolarization of the end-plate via the release of individual quanta of acetylcholine. Often end-plate spikes will be seen superimposed on a very irregular baseline (also visible in Fig. 2), the irregular baseline being caused by these individual quanta of acetylcholine. When the needle is near the end-plate, patients generally note an abrupt increase in discomfort. Movement of the needle a short distance away generally relieves the pain, the spontaneous discharges abruptly stop, and the muscle returns to its usual quiet state.
In most pathological states, spontaneous activity increases. In both neuropathic and myopathic diseases, the resting potential of the muscle fibers is elevated (is less negative) because of changes in the ion conductances, and the muscle fibers, thus, are more excitable. The most minor increase in spontaneous activity might be an increase in the length of a needle-induced discharge (i.e., it lasts longer than 500 ms). A number of abnormal spontaneous potentials can be observed with the muscle at rest. These potentials can be divided into two groups: those generated by the depolarization of a single muscle fiber and those generated by the depolarization of a motor neuron. It should also be noted that in some cases (mainly chronic myopathies), spontaneous activity can actually decrease because of the loss of myocytes.
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