ECMCneurology

M o t o r U n i t P o t e n t i a l s

By isolating the discharge of single motor units by triggering and delaying the display, parameters of the motor unit potential can be measured. Motor unit parameters vary with the muscle being examined. Amplitude, duration, and number of phases may be measured. Chronic re-innervation is associated with high amplitude, long duration motor unit potentials with a normal number of phases.

As a rule of thumb, motor unit potentials are less than 2 mV in amplitude, and have a duration of 10–5 ms with 3–4 phases.

In chronic partial denervation, where intramuscular sprouting and re-innervation has occurred, amplitudes may be 10– 20 mV and durations 20–30 ms.

In primary muscle disease, motor unit potentials are small and of short duration; typical values would be 0.5 mV and 5–10 ms.

Polyphasicity can be seen in a wide number of situations, but muscle disease and the early stages of re-innervation are the most common.

Recruitment can be characterized by recruitment frequency, which is the frequency of firing of a unit when the next unit is recruited (begins to discharge). This is a function of the number of units capable of firing and is usually between 7–10 HZ for motor units in normal limb muscles and up to 16 HZ for motor units in cranial muscles during mild contraction.28 Recruitment frequencies vary in different muscles and for different types of motor units. Recruitment may also be characterized by the ratio of the rate of firing of the individual motor units to the number that are active. For most normal limb muscles, this ratio averages less than 5 and therefore there will be two or more MUAPs firing if one of them is firing at 10 HZ, three or more at 15 HZ, and four or more at 20 HZ. Therefore, the ratio of the number of units firing to the rate of firing can provide a rough gauge of the loss of motor units in the muscle.

In the presence of lower motor neuron diseases, where the number of motor units in a muscle is decreased from axonal loss, or in disorders characterized by conduction block, recruitment frequency increases and, therefore, MUAPs fire more rapidly before additional motor units are recruited. Conversely, the rate of firing of those MUAPs already firing will be unduly fast for the number of MUAPs that have been activated. Or, less commonly, the first unit begins firing at a higher rate than normal (more than 10 HZ). If the ratio is greater than 5 (for example, 2 units firing at 16 HZ), there is virtually always some decrease in the number of motor units. Thus, the firing rate of MUAPs is an important measure of the loss of axons. This semiquantitative method of determining reduced recruitment provides a more accurate and reproducible estimate of the number of motor units than full interference pattern analysis. However, since there may be selective loss of higher threshold motor units, recruitment analysis should include levels of effort associated with firing rates in the range of 15 HZ.

Recruitment may be defined as normal, reduced (sometimes referred to “reduced numbers” or “discrete firing”), rapid (sometimes referred to as “full recruitment”), or poor activation.

Normal recruitment: the pattern of recruitment is normal for that muscle, with an adequate number of MUAPs being recruited for the frequency of firing present. If maximal effort can be obtained, a full interference pattern is seen, but individual motor unit firing rates of 15 HZ are sufficient for recruitment analysis.
Reduced recruitment: a higher recruitment frequency or a smaller number of MUAPs recruited for firing rates in the range of 15 HZ than expected for that muscle. Reduced recruitment is characteristic of neurogenic disorders in which axonal loss or conduction block is the pathophysiologic mechanism. In patients with severe or endstage myopathic disorders, reduced recruitment may also occur due to the loss of all muscle fibers within a motor unit. This term should not be used to describe the condition of patients in whom relatively few MUAPs fire because of pain, strong muscles, upper motor neuron lesions, or poor cooperation. In these situations, few potentials are fired, although they fire slowly with a normal pattern of recruitment (i.e., poor activation).
Poor activation: a normal recruitment pattern and normal recruitment frequency, but with relatively few motor potentials firing. These potentials fire slowly, but recruitment of additional potentials is normal. This occurs with upper motor neuron disorders, poor cooperation by the patient, pain, excessively strong muscle, or two-joint muscles, such as the gastrocnemius. It is not evidence of lower motor neuron disease.
Rapid recruitment: increased number of motor units relative to the force of contraction. With this type of recruitment, the occurrence of large numbers of MUAPs with normal recruitment frequencies and normal patterns of recruitment occur with minimal patient effort. This must be graded in proportion to the force exerted, because the patterns of firing are entirely normal. It is the only estimate described that requires consideration of the force exerted by the muscle. It is evidence of disease involving the muscle directly

Rise time is the duration of the rapid positive–negative inflection and is a function of the distance of the muscle fibers from the electrode. It is less than 500 us if the electrode is near muscle fibers in the active motor unit. When the needle electrode is more distant to the muscle fibers in the motor unit, the rise time will increase and the amplitude and duration of the motor unit may decrease. As a result, only MUAPs that are near the electrode, with a rise time of 0.5 ms or less, should be analyzed.

Duration and Amplitude:

The duration of the MUAP is the time from the initial deflection away from baseline to the final return to baseline . It varies with the muscle, muscle temperature, and the patient's age. The duration is the parameter which most accurately reflects the area of the motor unit. The amplitude of the potential is the maximal peak-to-peak amplitude of the main spike of the potential and varies with the size and density of the muscle fibers in the region of the recording electrode and with their synchrony of firing. The amplitude typically consists of the action potentials of a few muscle fibers within the motor unit that are closest to the recording tip of the electrode. It also differs with the muscle, muscle temperature, and the patient's age. Decreased muscle temperature produces higher amplitude and longer duration MUAPs.

The polarity of all potentials recorded on needle EMG depends on recording the potential with the active (G1 amplifier input) electrode. In most cases, where the potential is recorded by the active central recording wire at the tip of the electrode, MUAPs will appear predominantly negative. However, if an MUAP is recorded with the shaft of a standard concentric electrode or with the reference of a monopolar electrode, it will be displayed as an inverted triphasic potential (apparently negative–positive–negative).

Phases:

The number of phases of an MUAP can be defined as the number of times the potential crosses the baseline plus one. The configuration of an MUAP may be monophasic, biphasic, or triphasic, or it may have multiple phases. The configuration depends on the synchrony of firing of the muscle fibers in the region of the electrode. Usually, only a small proportion of MUAPs have more than four phases; those that do are called polyphasic potentials. The percentage varies with the muscle being tested and the age of the patient, but it is usually no more than 15% of MUAPs in most muscles. A late spike, distinct from the main potential, that is time locked to the main potential is called a satellite potential . The satellite potential is generated by a muscle fiber in a motor unit that has a long nerve terminal, narrow diameter, or distant endplate region. If an MUAP is recorded from damaged muscle fibers or from the end of the muscle fibers it may have the configuration of a positive wave with low amplitude, long, late negativity phases.

Normal Spontaneous Activity (Endplate Activity).

Normal muscle fibers show no spontaneous electric activity outside of the endplate region. In the endplate region, miniature endplate potentials (MEPPs) occur randomly due to spontaneous release of individual quanta of acetylcholine. These MEPPs may be recorded with needle electrodes as monophasic negative waves that have amplitudes less than 10 μV and durations of 1–3 ms or less. Individual potentials occur irregularly but usually cannot be distinguished. This activity is usually seen as an irregular baseline called endplate noise and has a typical “seashell sound”

The action potentials of some individual muscle fibers may be recorded in the endplate region as brief spike discharges called endplate spikes.Endplate spikes are caused by mechanical activation of a nerve terminal with secondary discharge of a muscle fiber. They have a rapid irregular firing pattern, often with interspike intervals of less than 50 ms. Although usually initially negative, endplate spikes may be triphasic or, if the needle electrode has damaged the muscle fibers, may also be recorded as rapid, irregularly firing positive waves. Endplate spikes sound like “sputtering fat in a frying pan” or “slowly ripping Velcro.”

Endplate activity is normal, occurs in every individual, and has no clinical significance. However, since recording from the endplate region is usually uncomfortable, identification of an endplate should prompt repositioning of the needle electrode. It is particularly important that endplate spikes not be mistaken for fibrillation potentials or short-duration MUAPs, both of which may have the same size and shape but fire in a different pattern

Normal Voluntary Activity (Normal MUAPs).

All voluntary muscle activity is mediated by lower motor neurons and the muscle fibers they innervate (motor units) and is recorded electrically as MUAPs. All MUAPs under voluntary control fire in a semirhythmic pattern and at a relatively constant frequency, although this frequency continuously changes as the voluntary activation increases or decreases. MUAPs are characterized by their firing pattern, firing rates (recruitment), and by their configuration or appearance.

The MUAP is the sum of the potentials of the individual muscle fibers innervated by a single anterior horn cell that are near the recording electrode (primarily those within 0.5 mm). These fibers generally discharge in near synchrony. A number of factors will change the appearance of an MUAP. Loss of muscle fibers in a myopathy will make it smaller, while addition of muscle fibers by reinnervation of denervated muscle fibers will make it larger. Loss of synchrony results in a more complex configuration due to several possible mechanisms. Collateral sprouts of nerve terminals that reinnervate denervated fibers may have different lengths of nerve terminals that innervate the fiber and different rates of conduction may occur along the nerve terminals and may produce dispersion of the endplate zones along different muscle fibers. Additionally, changes in muscle fiber size with fiber splitting, atrophy, fiber regeneration or differences in the conduction velocity along the muscle fiber may have a significant effect.

Firing Rate and Recruitment of MUAPs.

Clinical EMG judges the number of motor units present in a muscle. The number of motor units in a muscle may be considered in two ways. The first is the total number of motor units that could be fired if the anterior horn cell pool received adequate central nervous system input. This refers to the actual number of motor units within an individual muscle. The second is the actual number of motor units that are activated when a patient attempts a voluntary contraction. Both of these are used to assess the presence or absence of disease involving the lower motor neuron, although the second is quite variable and changes with the patient's cooperation, the strength of the muscle, pain, and the presence or absence of disease of the upper motor neuron.

Judgment of the number of motor units within a muscle can be performed by assessing MUAP recruitment—defined as the initiation of the firing of additional motor units as the rate of discharge of the active MUAP increases. Recruitment can be assessed by comparing the rate of firing of single units with the total number of motor units that are firing. In most normal muscles, motor units initiate firing rates at 5–8 HZ and gradually increase up to 20–40 HZ as the effort exerted by the patient increases. The rate of firing is used as a gauge of the intensity of excitation of the anterior horn cell by the central nervous system. As the firing rates increase, additional motor units begin to fire (are recruited). Slow firing is a term referring to individual MUAPs that fire at rates slower than 10 HZ and rapid firing refers to individual MUAPs firing faster than 12 HZ. When possible, the rate of firing of the motor unit initially activated is measured at the time the second unit begins to fire. In most muscles, this occurs at 8–10 HZ. Normally, recruitment of additional MUAPs occurs at low levels of effort and at slow rates of firing

Although recruitment analysis is reasonably reproducible and clinically reliable, it is usually a subjective judgment made by EDX physicians on the basis of experience. It requires taking into account differences in recruitment in different areas of individual muscles and the even greater differences among different muscles. Automated methods for formally quantitating the recruitment pattern have been developed. In automated studies, individual MUAPs were isolated in human muscles under voluntary control in an experimental setting. The interpotential interval (the inverse of frequency of firing) was determined for a population of normal subjects and for patients with amyotrophic lateral sclerosis (ALS). The normal onset frequency in the biceps muscle ranged from 6–8 HZ, with the recruitment frequency of the second motor unit at 7–12 HZ. In patients with ALS, the onset frequency was from 8–20 HZ, with recruitment frequencies of 12–50 HZ. These studies provided quantitative measures of motor unit number estimate. Formal quantitative measures can provide evidence of the reliability of the clinical methods; however, they are so time-consuming and complex that they have not been applied clinically. Further studies and technical developments may eventually allow recruitment analysis to provide more accurate estimates of the number of motor units in a muscle.

MUAP Configuration.

An MUAP is also characterized by its appearance, including duration, amplitude, number of turns, area, and rate of rise of the fast component (rise time) . Each of these characteristics has multiple determinants, including technical, physiologic, and pathologic factors. Technical factors that have a major influence on the appearance of MUAPs include the type of needle electrode used to record the potentials, the area of exposed surface of the active leads of the electrode, the characteristics of the metal recording surfaces, and the electric characteristics of the cables, preamplifier, and amplifier. The appearance of an MUAP from one motor unit also varies with electrode position, since only a small proportion of the fibers in a motor unit are near the electrode and those at a distance contribute little to the recorded MUAP. Thus, no single MUAP characterizes a motor unit, but rather the characteristics of multiple MUAPs recorded from different sites allow for the optimal assessment of the morphology of the motor unit . The appearance of MUAPs also changes with several normal physiologic variables, including the subject's age, the muscle being studied, the location of the needle in the muscle, the manner of activation of the potentials (minimal voluntary contraction, maximal voluntary contraction, reflex activation, or electric stimulation), and the temperature of the muscle

If these technical and physiologic factors are controlled, the normal anatomical and histological features of the motor unit and any pathologic changes that may affect these features will determine the characteristics of the MUAPs. The anatomical and histological features include innervation ratio (number of muscle fibers in the motor unit), fiber density (number of muscle fibers per given cross-sectional area), the distance of the needle tip from the muscle fibers and from the endplate region, and the direction of the axis of the muscle fiber. The characteristics of the action potentials generated by individual muscle fibers depend on muscle fiber membrane resistance and capacitance (which may be affected by the amount of connective tissue, blood vessels, and fat between the electrode and discharges muscle fibers), intracellular and extracellular ionic concentrations, muscle fiber diameter, and conduction velocity. The synchrony of firing of the muscle fibers in a motor unit depends on the length, diameter, and conduction velocity of the nerve terminals, the diameter of the muscle fibers, and the relative location of the endplates on the muscle fibers. The firing characteristics of the motor unit depend on the amount of overlap with other motor units, the number of motor units in the muscle (or per given area), the differential response to sources of activation (monosynaptic, local spinal cord, higher centers), and the rates and patterns of discharge of the anterior horn cell.

Stability: