Fasciculation potentials A fasciculation is the spontaneous or involuntary contraction of the muscle fibers of a single motor unit or part of a unit. Fasciculation potentials are evidence of motor nerve fiber irritability and are generally markers of denervation and reinnervation of muscle. Such contractions cause a visible dimpling or twitching under the skin, and infrequently they are of sufficient force to move a small distal joint. They occur irregularly and may be infrequent so that prolonged inspection of the skin overlying a muscle may be necessary to detect them. The accompanying electrical form of any individual fasciculation potential is relatively constant. Typically, a fasciculation potential will have three to five phases (“polyphasic” as described later, in contrast to normal biphasic muscle activity), a duration of 5 to 15 ms (far longer than fibrillation potentials and more dispersed than a typical CMAP), and an amplitude of several millivolts (see Fig. 2-18B). The combination of fibrillations and fasciculations indicates active denervation combined with more chronic reinnervation of muscle.
The precise mechanism of fasciculation is still contested. Forster and colleagues challenged the original belief that the discharge originated in anterior horn cells by demonstrating that in disease of the anterior horn cell, fasciculations persisted after nerve block. These observations favored a distal axonal site of generation. It seems that several regions of the axon are capable of spontaneous impulse generation, depending on the underlying disease. Most of the diseases that produce fasciculations involve the anterior horn cell or the motor root, but more distal sites in the motor axon may become spontaneously active in cases of nerve compression.
Occasional fasciculation potentials, particularly in the calves, hands, and periocular or paranasal muscles, occur in many normal persons. They can be almost constant for days or weeks on end, or even for years in some individuals, without weakness or wasting; therefore, they need not be taken as evidence of disease (“benign fasciculations”). Certain quantitative features of fasciculations, such as brief duration and a consistent pattern and location of firing on the needle EMG examination, favor benign over pathologic discharges. Shivering induced by low temperature and twitchings associated with low serum calcium levels are other forms of fasciculatory activity. The main diseases that cause fasciculations are discussed in Chap. 45 and other related sections of the book.
Other types of spontaneous and elicited electrical activity (See also Chaps. 45 and 46) These various phenomena can be classified according to their generating source. The muscle fibers themselves are the source of fibrillations, positive sharp waves, and complex repetitive discharges (CRDs). The motor axons produce fasciculation potentials, myokymic discharges, neuromyotonia, and cramp syndromes; and the CNS is the source of complex ensembles of continuous motor activity such as occur in the stiff man syndrome, all described just below.
The common phenomenon of complex repetitive discharges, referred to in the past as bizarre high-frequency discharges, consists of repetitive spontaneous potentials created by numerous single muscle fibers that fire in near synchrony; there is often an erratic configuration and abrupt starting and stopping of the discharges. They are seen in some myopathies, in hypothyroidism, and in certain denervating disorders, and are a mark of chronicity (lesions >6 months old). High-frequency coupling of action potentials into doublets, triplets, or higher multiples of single units, indicating instability in repolarization of the nerve fiber to a muscle, occurs in tetany and in the early stages of myokymia.
Myokymia is a persistent quivering and rippling of muscles at rest. The EMG picture is distinctive. The spontaneously firing MUPs are called myokymic potentials, or discharges and consist of groups of repetitive discharging units, each firing at its own rate, quasirhythmically, usually several times per second, followed by a briefer period of silence. The small motor unit discharges may occur singly or as doublets, triplets, or multiplets.