Morphology

The source of a spontaneous discharge often can be identified by its distinctive morphology, including the size and shape of the potential (amplitude, duration, number of phases) and its initial deflection ( Figure 14–2 ). By defining its source generator, the type of discharge usually can be identified. The source generators that must be differentiated include (1) the neuromuscular junction (NMJ), (2) a single muscle fiber, (3) the terminal axon twig, (4) a motor neuron/axon, and (5) multiple muscle fibers linked together ( Figure 14–3 , Table 14–1 ).

Spontaneous waveform morphologies.

A: Miniature endplate potential (monophasic negative). B: Muscle fiber action potential, brief spike morphology. Triggered by needle-induced depolarization of a terminal nerve twig (initial negative, diphasic). C: Muscle fiber action potential, brief spike morphology (initial positive, triphasic). D: Muscle fiber action potential, positive wave morphology (initial positive, slow negative). E: Multiple different muscle fiber action potentials linked together. F: Motor unit action potential. Note the longer duration and higher amplitude compared with muscle fiber potentials shown above.

Spontaneous waveform source generators.

Spontaneous activity originates from a variety of source generators. Each generator is associated with a specific morphology.

At the NMJ (i.e., endplate zone), miniature endplate potentials (MEPPs) occur spontaneously. They result from the normal spontaneous exocytosis of individual quanta of acetylcholine traveling across the NMJ, leading to a non-propagated, subthreshold endplate potential. If the EMG needle is near the endplate zone, MEPPs can often be recorded. They have a distinctive small amplitude and monophasic negative morphology ( Figure 14–2A ). These potentials are normal spontaneous discharges and are referred to as endplate noise .

When a muscle fiber depolarizes to threshold, a muscle fiber action potential (MFAP) is created. The MFAP can assume one of two basic morphologies, either a brief spike or a positive wave. The brief spike typically is 1 to 5 ms in duration, biphasic or triphasic, with a low amplitude (typically 10–100 µV). This brief spike morphology is seen most often when a muscle fiber depolarizes spontaneously, for example, in denervation, but it can also occur as the result of an individual terminal axon twig depolarizing and then propagating across the NMJ to create an MFAP. Attention to the initial deflection and to whether the brief spike is biphasic or triphasic often can help distinguish between the two ( Figure 14–4 ). If the depolarization begins under the recording needle electrode, a biphasic potential is seen, wherein an initial negative peak is followed by a short positive phase ( Figure 14–2B ). This signifies that the needle is at the endplate zone, where the depolarization begins, and usually is the result of the EMG needle irritating the terminal nerve twigs near the endplate zone. A nerve twig action potential then leads to an MFAP, known as an endplate spike , which is a normal finding (see section on Endplate Spikes). The reason for the initial negativity is similar to that of the compound muscle action potential (CMAP) in motor nerve conduction studies, wherein the initial deflection is negative when the active recording electrode is properly placed over the motor endplate zone. Otherwise, brief spikes that occur from the spontaneous depolarization of a muscle fiber are associated with an initial positive, usually triphasic morphology. When the depolarization begins at a distance from the needle, there is an initial positive deflection as it moves toward the needle, followed by a negative phase as it moves beneath the needle, and then a final positive deflection as it moves away from the needle ( Figure 14–2C ).

Waveform morphology and site of depolarization.

A: Traveling depolarizing wave will create a biphasic potential if the waveform begins under the recording needle electrode (initial negative peak) and then moves away from the electrode (positive peak). An endplate spike shows this morphology. B: If the waveform begins at a distance from the needle, there is an initial positive deflection as it moves toward the needle, followed by a negative phase as it moves beneath the needle, and then a final positive deflection as it travels away. Fibrillation potentials show this morphology. Endplate spikes are differentiated from fibrillation potentials by the absence of an initial positive deflection because the depolarization begins at the endplate.

In addition to the brief spike, an MFAP can assume a positive wave morphology, with an initial brief positive phase followed by a long negative phase ( Figure 14–2D ). Both positive waves and initial positive, triphasic brief spikes are seen most often as denervating potentials, known as positive sharp waves and fibrillation potentials , respectively. However, it should not be surprising that myotonic discharges, which also originate in muscle fibers, have the same basic morphology as denervating potentials, either positive waves or brief spikes. This point exemplifies the important concept that morphology alone cannot be used to identify a potential. Although the morphology of a potential usually can be used to correctly identify its source generator, additional information regarding its stability and firing characteristics is needed to fully characterize and identify any potential (see later).

The next major category of spontaneous discharges is that which arises from motor neurons or their axons. Any discharge that occurs as a result of the spontaneous depolarization of a motor neuron or its axon (prior to its terminal branches) leads to a potential with the morphology of a motor unit ( Figure 14–2F ) known as a motor unit action potential (MUAP). Spontaneous discharges generated by the motor neuron or its axon include fasciculation potentials, doublets, triplets, and multiplets, myokymic discharges, neuromyotonic discharges, and cramp potentials, all of which lie along the spectrum of abnormal spontaneous MUAPs. They can be differentiated from each other, however, by their stability and firing characteristics (described in the following subsections). If the motor unit is normal, the MUAP morphology will be normal: typically two to four phases, 5 to 15 ms in duration, and variable amplitude depending on the needle position. If the motor unit is pathologic, the number of phases, duration, and amplitude of the MUAP may be abnormal. Differentiating a MUAP from a single MFAP usually is straightforward and typically can be done quite simply by analyzing its duration and amplitude.

The last distinctive waveform that must be recognized is that of time-linked individual muscle fibers, such as occurs in complex repetitive discharges. One might ask how this waveform differs from an MUAP, which also represents many muscle fibers linked together. The difference is that the muscle fibers in a motor unit fire more or less synchronously and, in almost every situation, summate to create a larger potential 5 to 15 ms in duration. In contrast, the multiple muscle fibers in a complex repetitive discharge fire consecutively and usually are discernible as individual spikes that are time linked together ( Figure 14–2E ).

Stability

Assessment of the stability of a waveform can be very informative. Nearly all spontaneous potentials are relatively stable in their morphology. If the morphology of the potential changes, note should be made of whether it waxes and wanes, wanes (decrements), or changes abruptly. MFAPs that wax and wane in amplitude are characteristically seen in myotonic discharges. Marked decrementing of an MUAP amplitude occurs in neuromyotonic discharges. Complex repetitive discharges typically are perfectly stable, but if additional loops or circuits drop in or out, the morphology may change abruptly in distinct or quantal jumps.

Firing Characteristics

After assessing the potential’s morphology and stability, the electromyographer should look at the potential’s firing characteristics, including the discharge pattern and firing rate. Note whether the pattern is regular or irregular. If it is regular, is it perfectly regular? Fibrillation potentials and positive sharp waves are more or less regular, but complex repetitive discharges are perfectly regular. If it is irregular, is it sputtering (e.g., endplate spikes), waxing/waning (e.g., myotonic discharges), or waning (e.g., neuromyotonic discharges)? Is there a bursting pattern (relative electrical silence between groups of discharges)? Such a pattern is characteristic of doublets and triplets as seen in tetany, and myokymic discharges. Note if the firing rate is very slow (<4–5 Hz). This is important because a slow firing rate signifies that the discharges cannot be voluntary. Voluntary activation of a motor unit has a firing frequency of at least 4 to 5 Hz. Any potential that fires more slowly than 4 to 5 Hz cannot be under voluntary control. Conversely, extremely high firing rates are characteristic of neuromyotonic discharges, which can fire as fast as 150 to 250 Hz.

Table 14–1 summarizes the morphology, stability, and firing characteristics of the common spontaneous potentials seen during the needle EMG.

Endplate Noise

These are low-amplitude, monophasic negative potentials that fire irregularly at 20 to 40 Hz and have a characteristic “seashell” sound on EMG ( Figure 14–6 ). Physiologically, they represent MEPPs. They are recognized by their characteristic shape and sound and by their frequent association with endplate spikes (described in the next subsection).

Endplate noise.

Small, high-frequency, predominantly monophasic negative potentials, which are recognized by their characteristic shape and “seashell” sound on EMG.

Endplate Spikes (“Nerve Potentials”)

Endplate spikes are MFAPs that fire irregularly up to a frequency of 50 Hz ( Figure 14–7 ) and usually are seen along with endplate noise. They are biphasic, with an initial negative deflection, reflecting that the needle is at the site where the action potential is generated. They have a cracking, buzzing, or sputtering sound on EMG. The key features that differentiate endplate spikes from fibrillation potentials, which are also brief spikes, are their initial negative deflection and their highly irregular firing rate.

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