Fasciculation potentials (Fig. 8) result from the spontaneous depolarization of a motor neuron and its associated muscle fibers. The morphology of a fasciculation potential is that of a MUP. Fasciculation potentials are substantially larger than fibrillation potentials, are polyphasic, and fire erratically. Fasciculation potentials will appear only intermittently. In fact, one of the best ways for identifying a fasciculation potential is to simply place the needle into the muscle of interest and for the examiner to remove his/her hand from the needle and simply wait 30 s or longer to see whether a fasciculation potential occurs.
The mechanism underlying a fasciculation potential is that of a reduced threshold for depolarization of the motor neuron. The reduced threshold can be caused by neurogenic disease or can be related to reduced concentration of a serum ion (most notably calcium) leading to intermittent depolarizations of the motor neuron. In addition, potassium channels are important membrane stabilizers; if these channels become dysfunctional, spontaneous depolarization of the motor neuron will occur, potentially leading to fasciculation potentials.
Not surprisingly, the clinical significance of fasciculation potentials is not specific. Although fasciculation potentials commonly occur in severe neurogenic diseases, including motor neuron disease, occasional fasciculation potentials can also occur normally in many individuals. In fact, patients are frequently referred to physicians for evaluation of clinical fasciculations that are often benign—that is, there is no associated neurogenic disease. Fasciculation potentials can be part of the "cramp-fasciculation" syndrome, a disorder of mild nerve hyperexcitability in which patients experience intermittent cramping (usually
more pronounced in the legs than arms) in association with symptomatic muscle twitching (fasciculations). Clinically, such benign fasciculations can be discriminated from the "malignant" fasciculations of amyotrophic lateral sclerosis by their character. Benign fasciculations tend to affect one muscle at a time and cause the muscle to fire frequently; malignant fasciculations tend to affect multiple muscles simultaneously throughout the body. Malignant fas-ciculation potentials tend to be larger, in part because the motor units that are activated with each depolarization have already undergone substantial reinnervation.
These discharges are grouped fasciculations that fire irregularly and may be observed in patients with peripheral nerve hyperexcitability. They most commonly occur in situations of reduced ion concentrations (again, most notably calcium). At one moment a doublet (two fas-ciculation potentials in a row from the same motor unit), then a triplet, and yet later a single fasciculation potential might be identified. These grouped fasciculations have a firing frequency of 0.1 to 10 Hz.
A myokymic discharge (Fig. 9) represents another form of repetitive fasciculation potential, but this time the repetitive depolarization is more "hard wired" than in a doublet or triplet. A disease state within the axon causes a grouped repetitive discharge. There are essentially two firing frequencies of interest in myokymic discharges. The intergroup firing rate is often very slow, with each group appearing once every 2 to 3 s; the intragroup firing rate is substantially higher, with a rate of 50 to 150 Hz. The morphology of these individual waveforms is that of a grouped, repetitive MUP. The MUP itself is usually polyphasic and large suggesting neurogenic injury (see discussion of MUPs in Section 7). The sound of myokymic discharges has been described as "marching soldiers," although the gaps in between the appearance of each group would make a description of "marching soldiers starting and stopping frequently" more accurate. Multiple myokymic discharges can also occur simultaneously, complicating the electrophysiological interpretation.
Myokymic discharges represent the electrical equivalent of clinical myokymia (the bag of worms appearance of muscle). These discharges are most commonly associated with demyelinating lesions that affect the exiting motor axons from the spinal cord or brainstem, and are also observed after radiation injury to nerves. However, these discharges can be observed uncommonly in any neurogenic disease.
Neuromyotonic discharges (Fig. 10) are the most rarely observed of any discharge. They have a characteristic "ping" sound to them, reflecting their very rapid rate of firing (up to 300 Hz), with a rapid decrescendo, lasting less than a second in duration. Visually, a neuromyotonic discharge appears on the oscilloscope as a high-frequency waveform of rapidly decreasing amplitude before it stops, similar to a tornado lying on its side. Unlike a myotonic discharge, a "revving up" is not observed. Although these discharges may seem to have some similarities to a myotonic discharge, there are two obvious differences: first, these discharges are considerably larger than myotonic discharges because they represent the entire motor unit and not just a single muscle fiber. Second, the firing rate is much faster and the duration of the entire discharge is very short. These discharges can be observed very rarely in any neu-rogenic disorder, but are most commonly associated with axonal potassium channelopathies (e.g., Isaac's syndrome). Dysfunction in the potassium channel leads to an elevated resting potential and subsequent instability of the cell membrane, such that recurrent spontaneous depolarizations of the membrane occur.
Cramp discharges are most commonly observed as an involuntary discharge that is produced during a muscle contraction. For example, while asking a patient to contract a muscle with an inserted needle electrode, the patient may suddenly complain that their muscle is very painful and they are getting a cramp. What may be observed on the oscilloscope is a repetitive, rapidly firing (40-60 Hz) single motor unit or group of motor units that stops as the muscle is stretched out. Cramp discharges can be very difficult to distinguish from activity produced by a normal contraction, except that the patient will complain of pain and that, in some situations, a single motor unit will fire very frequently for a short time, before a more normal recruitment pattern is re-established.
Fig. 11. A motor unit potential with five phases.
Fig. 11. A motor unit potential with five phases.
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