AAEM minimonograph #25: Single-fiber electromyography

Single-fiber electromyography (SFEMG) is a selective recording technique in which a needle electrode with a small recording surface in the side is used to identify action potentials from individual muscle fibers. The SFEMG parameters of greatest clinical use are fiber density (FD) and neuromuscular jitter. FD reflects the local organization of muscle fibers within the motor unit; jitter reflects the safety factor of neuromuscular transmission at individual neuromuscular junctions. SFEMG can be of great value in demonstrating or excluding abnormalities in mild or questionable disease of nerve, muscle, or the neuromuscular junction. The neuromuscular jitter may be measured during nerve stimulation, which is particularly useful in uncooperative patients or when it is desirable to control the firing rate precisely, or during voluntary muscle activation, which is less subject to technical artifact. The SFEMG findings may not be specific to a particular diseases, but they frequently increase understanding of the disease process by demonstrating abnormal neuromuscular transmission or rearrangement of muscle fibers within the motor unit, which complements information from more conventional EMG examinations. © 1996 American Association of Electrodiagnostic Medicine. Published by John Wiley & Sons, Inc.     

Concentric macro electromyography

Concentric EMG electrodes can record from a few (10 to 15) muscle fibers of a motor unit (MU). Macro EMG, is able to record from the majority of muscle fibers in the MU. The macro EMG electrode uses a single fiber action potential (SFAP) on one channel to trigger the time locked cannula (macro) response on the other channel. To study the concentric motor unit action potential (MUAP), alongside the macro potential, we built a needle electrode combining concentric and macro recording surfaces. The study of 240 motor units in 10 healthy subjects with the single fiber (SF macro) and concentric macro (conmac) electrodes revealed no significant differences between macro potentials areas and amplitudes obtained with either electrode. The ability to study a small and a large section of the motor unit simultaneously offers insights into the local or global nature of motor unit changes not otherwise available to the electromyographer. It also reveals which concentric parameters correlate best with the macro potential and, can even be of great help with the newer EMG signal decomposition techniques; by identifying each motor unit by its concentric and macro waveform simultaneously, it will allow for the "marking" of these motor units helping to reduce the risk of their misclassification when the concentric MUAP is used alone.     

Macro electromyography, an update

The macro electromyography method was developed in the 1980s.1 Since then, technical modifications have been made, and a number of conditions have been explored.2, 3 This study is a methodological introduction and an update of findings in some nerve-muscle disorders. The spike component of a motor unit potential (MUP) recorded by a concentric or monopolar needle electromyography (EMG) electrode is generated primarily by fibers within 1-2 mm of the needle recording area. Given that a MUP's typical anatomical reach is 5-15 mm in diameter, it follows that conventional EMG is unable to record activity from the entire motor unit. Such information could promote understanding of muscle in health and disease. Macro EMG, with its large recording area, appears to provide this information by recording the activity from most of the fibers in a given motor unit. The value of combining macro EMG with single-fiber EMG and conventional EMG recordings is discussed.     

Needle electromyography: basic concepts and patterns of abnormalities

Needle electromyography (EMG) records electrical signals generated from muscle fibers and interprets the signals to characterize underlying pathologic changes that are occurring in motor units within muscles. Different types of spontaneously firing waveforms and motor unit potential changes occur with different neuromuscular disorders. The performance of reliable EMG studies depends on the technical skills of the physician in inserting, moving, recording with a needle electrode, and analyzing electric signals recorded from muscle. This article reviews the technique of needle EMG and recognition and interpretation of various EMG waveforms. The author presents several demonstrative videos at www.neurologic.theclinics.com.     

Use of single fiber EMG and macro EMG in study of reinnervation

The use of single fiber EMG and macro EMG in studies of reinnervation is discussed. SFEMG gives information about changes in the topography of the motor unit and in function of transmission in terminal nerve, motor endplate and muscle fiber. Dynamics of reinnervation may be studied with this technique. The amount of reinnervation is obtained from macro EMG studies. The capacity for reinnervation is discussed for a few conditions as well as factors that limit the reinnervation process.     

EMG of reinnervated motor units: a simulation study

Using computer simulation techniques, reinnervation of motor units (MUs) was studied by increasing the number of muscle fibers in the MU without changing the MU territory. The fiber density (FD) measured by single fiber EMG electrodes, the amplitude, area and number of turns of concentric needle (CN) EMG motor unit action potentials (MUAPs) and the amplitude of macro EMG MUAPs were most affected by partial reinnervation changes. The values of these features increased during simulated advanced reinnervation, as did the number of CNEMG MUAPs that had increased numbers of phases or turns and the mean CNEMG MUAP duration. The increase in macro EMG MUAP amplitude, FD and CNEMG MUAP area were proportional to the increase in the number of muscle fibers in the MU. When loss of muscle fibers due to so-called MU fractionation was simulated, values of all EMG features fell, but were still increased compared to normal. Two patterns of change in SFEMG and macro EMG values were identified that may distinguish between recordings made from reinnervated low force threshold MUs and those from higher force threshold MUs.     

The standard concentric needle cannula cannot replace the Macro EMG electrode

ObjectiveTo establish the usefulness of the single use and affordable standard concentric EMG electrode as a substitute for the expensive standard macro electrode.MethodsMacro EMG performed with macro electrode is compared with recordings from the uninsulated cannula of a standard EMG electrode at two different recording depths in the tibialis anterior muscle. This was performed both in muscles with signs of collateral reinnervation and without.ResultsThe amplitude of the motor units recorded with the uninsulated concentric needle cannula were lower for the deeply recorded motor units compared to motor unit potential (MUP) amplitudes recorded with the standard macro electrode. The deeply recorded concentric needle (CN) cannula recorded MUPs amplitudes were also lower than superficially recorded CN cannula MUPs. The standard Macro EMG signals show no difference between deeply and superficially recorded motor units.ConclusionThe uninsulated cannula of the concentric needle electrode cannot replace the standard Macro EMG electrode due to technical reasons, probably from different effects of shunting of the bare cannula in deep vs. superficially recorded motor units.SignificanceThe standard CN electrode could not be used as substitute for the standard Macro EMG needle.     

The spectrum of concentric macro EMG correlations. Part I. Normal subjects.

Using a concentric macro electrode, both the concentric and macro action potentials of a motor unit were analyzed for duration, area, and amplitude. Eighty-six different motor units were analyzed from the tibialis anterior muscle in 10 normal subjects. The aim of this work was to compare concentric and macro action potential measurements of the same motor unit. The study revealed significant correlations between concentric and macro samples, with the concentric potential's area correlating better with the macro potential than its amplitude. This shows that the 10 to 15 muscle fibers studied by the concentric electrode serve as a good sample of the motor unit as a whole. We briefly review the technique used in concentric macro EMG, as well as describe the methods used for collecting and comparing both concentric and macro action potentials.     

Simulation of the normal concentric needle electromyogram by using a muscle model.

OBJECTIVES: To study the correlation between anatomical parameters and EMG signals by means of simulations. METHODS: A mathematical model of the electrical activity from muscle fibres and motor units has been developed. The electrical fields around the muscle fibres are simulated using a line source model. The model permits the simulation of single muscle fibre action potentials obtained by SFEMG, concentric and Macro EMG electrodes. By using appropriate anatomical parameters EMG recordings with these electrodes can be simulated. The model is flexible and permits a number of anatomical parameters to be changed such as; number of muscle fibres in a motor unit, fibre diameter distribution, and motor end-plate geometry. Some physiological parameters can be optionally varied; firing rate, threshold for recruitment, jitter. RESULTS: In this study, simulations of CNEMG are performed and the influence of a number of parameters on the CNEMG signal is studied. It is shown that the model produces motor unit potentials reasonably well resembling those from live recordings. More important is however the relative change in MUP parameters when certain conditions are changed; number of muscle fibres in a motor unit, recording position, muscle fibre diameters and some special effects of the recording conditions. CONCLUSIONS: The simulated muscle and corresponding EMG recording can be used both as a research tool and for teaching.     

Electromyographic and morphological functional compensation in late poliomyelitis

Patients with prior poliomyelitis may experience muscle function deterioration decades after onset of disease. The present study is aimed at describing electromyographic and morphometric evidence of muscular compensation and of on-going muscular instability. Ten subjects 42-62 years of age with onset of polio 25-52 years earlier were studied with macro EMG, single-fiber EMG (SFEMG), muscle strength measurement, and morphometrical analysis of muscle biopsies from the vastus lateralis muscle. SFEMG revealed increased fiber density (FD) and large macro-MUP potentials indicating pronounced reinnervation as compensation to loss of motor neurons. From electrophysiological data of motor unit size, morphometric measures of fiber size, and muscle strength data, the minimal degree of motor neuron loss was estimated to be greater than 70%.     

Neuromuscular transmission as a function of motor unit size in patients with prior poliomyelitis.

We studied neuromuscular transmission in 16 patients with prior poliomyelitis by measuring single fiber electromyographic (SFEMG) jitter. This was compared with 3 indirect methods of assessing reinnervation: SFEMG fiber density, macro EMG, and the presence of fiber type grouping on muscle biopsy. In patients with acute poliomyelitis before the age of 10, there was a positive correlation between the extent of neuromuscular transmission impairment, demonstrated by increased SFEMG jitter, and the enlargement of the motor unit, as indicated by increased fiber density, increased macro EMG signals, and fiber type grouping on muscle biopsy. However, there was no correlation between any of these parameters and the presence or absence of new symptoms of weakness. These findings suggest that impaired neuromuscular transmission is most common in patients with prior poliomyelitis whose motor units have been maximally enlarged by axonal sprouting, but is independent of the presence or absence of new symptoms of weakness.     

Macro EMG in neuromuscular diseases]

The aim of this study is the introducing of macro-emg method as electrophysiological test used in diagnosis of neuromuscular diseases. The macro motor unit potentials (macro MUPs) obtained by recording macroelectrode (modified single-fibre electrode) represents temporal and spatial summation of individual single fiber action potentials belonging to whole motor unit territory--so the uptake area is larger for macroelectrode than for the concentric electrode, commonly used in emg routine work, when central main complex is generated only from less than 15 muscle fibers [10, 12, 13]. Additional information obtained by macro-emg method is spatial organisation of muscle fibers within the motor unit, so-called fiber density (F.D) In our study macro-emg examinations were performed in 20 healthy subjects, aged 21-55, without signs and symptoms of neuromuscular disorders. Macro MUPs were recorded using special programme for macro-emg and performed on electromyograph Counterpoint. 37 muscles (20 BB and 17 RF) were examined, and median values of amplitude, area of macro MUPs and F.D. in healthy subjects of different age were analyzed. Mean values of median for amplitude and area of macro MUPs in BB and RF muscles show respectively--148 microV, 382 microV x ms, and 319 microV, 763 microV x ms. Parameters of macro MUPs obtained in healthy subjects were compared to results obtained in 10 patients with myopathy and lower motor neuron lesion. Our results have confirmed the value of macro-emg method for investigating of the pathophysiological changes in motor units in neurogenic disorders, in myopathy the study should be continued.     

Macro EMG, a new recording technique.

A new electromyographic technique for the study of the motor unit is described. A modified single fibre EMG electrode is used. The electrical activity obtained by the electrode shaft during voluntary muscle contraction is averaged after triggering from a single muscle fibre action potential and the contribution from one motor unit is thus extracted. This "Macro EMG" signal gives information about the whole motor unit in contrast to the regional or focal electrical activity measured by conventional or single fibre EMG recordings, respectively.     

Open-biopsy electromyography. Direct correlation of a pattern of excessively recruited, pathologically small motor unit potentials with histologic evidence of neuropathy.

Open-biopsy electromyography (EMG) of two muscles of a 29-year-old man with slowly progressive proximal weakness demonstrated a striking pattern of excessively recruited, pathologically small motor unit potentials. This pattern is usually equated with myopathy. Histologic study of tissue enclosing the recording sites, however, yielded evidence of neurogenic disease alone. In muscle, this included isolated and small groups of atrophic type I, IIA, and IIB fibers, and in intramuscular nerve a loss of myelinated fibers with connective tissue and Schwann cell proliferation. The EMG pattern is considered to reflect a reduced number of activated muscle fibers within motor units due to random neurogenic involvement of terminal axons.     

The distribution and propagation pattern of motor unit action potentials studied by multi-channel surface EMG

We developed the multi-channel surface EMG system using a matrix-type of surface electrode and with the aid of digital signal processing. The subjects were 14 normals (4-50 years) and 2 patients with Duchenne muscular dystrophy (7 and 8 years). The biceps brachii and the tibialis anterior muscles were investigated. The location of the motor end-plates and the measurement of muscle fiber conduction velocity were evaluated by the time shift of bipolar EMG arrays along muscle fibers, or by the distribution map of averaged motor unit action potentials (MUAPs). The lateral extension of a motor unit could be also estimated from the changes of averaged MUAP's amplitudes in the distribution map. Moreover in the biceps of 2 patients with Duchenne dystrophy, the mean muscle fiber conduction velocities were reduced compared to normal subjects, and characteristic propagation patterns of action potentials were obtained. In the 2-dimensional or 3-dimensional distribution map of integrated monopolar EMGs, the high density area agreed with the motor end-plate band.     

Acoustic myography: A noninvasive monitor of motor unit fatigue

Acoustic myography is the recording of sounds produced by contracting muscle. These sounds become louder with increasing force of contraction. We have compared muscle sounds with surface EMG to monitor the dissociation of electrical from mechanical events (presumably, the loss of excitation–contraction coupling) which occur with motor unit fatigue. Acoustic signals were amplified using a standard phonocardiograph, recorded on FM magnetic tape, and digitally analyzed. Muscles were examined at rest, with intermittent contractions, and with sustained contractions. We found that with fatigue, the acoustic amplitude decayed, but the surface EMG amplitude did not. With decreased effort, however, the acoustic and the surface EMG amplitudes declined simultaneously. By simultaneously recording acoustic signals and needle EMG, individual motor units were resolved acoustically in two muscles with decreased numbers of motor units and increased motor unit size. Fasciculations also produced acoustic signals, although no acoustic signal has yet been found that correlates with fibrillations. Analysis of acoustic signals from muscle provides a noninvasive method for monitoring motor unit fatigue in vivo. It may also be useful in distinguishing muscle fatigue from decreased volition.     

Simulation of concentric needle EMG motor unit action potentials

Computer simulations of motor unit action potentials (MUAPs) as measured by a concentric needle (CN) electromyography (EMG) electrode in normal motor units (MUs) indicated that the MUAP amplitude is determined mainly by the proximity of the electrode to the closest muscle fiber. The area and duration of the simulated MUAPs were affected by all muscle fibers in front of the active recording surface but mainly by those that were less than 2 and 2.5 mm, respectively, from the active recording surface. The MUAP area was also affected by the proximity of the electrode to the closest muscle fiber. The number of phases of the simulated MUAPs increased when the dispersion of the arrival times of individual muscle fiber APs at the electrode was increased. Increased temporal dispersion of APs decreased the MUAP amplitude and area slightly but did not affect the MUAP duration. It is inferred that different features of the CN MUAP are determined by the distribution of muscle fibers within different portions of the MU territory and thus provide complementary information about the MU architecture.     

Comparison between concentric needle EMG and macro EMG in patients with a history of polio.

OBJECTIVES: Acute poliomyelitis causes degeneration of anterior horn cells, followed by denervation. Reinnervation and muscle fibre hypertrophy are mechanisms that compensate this loss of neurones. Concentric needle EMG (CNEMG) and macro EMG are two methods to assess the magnitude of initial involvement and the compensatory reinnervation. The aim of this study is to explore the difference between CNEMG and macro EMG describing the status of the motor unit in patients previously affected by polio. METHODS: Macro and concentric needle EMG investigations were performed in 261 muscles in 121 patients with a remote history of polio. RESULTS: CNEMG was abnormal in 211 muscles, macro EMG was abnormal in 246 muscles. The macro amplitude was 3-4 times 'more abnormal' than CNEMG amplitude relative to the reference values. CNEMG duration was less abnormal and showed only weak correlation with macro amplitudes. The most likely explanation for the difference in magnitude of deviation from reference values for CNEMG and macro EMG, is a more pronounced 'phase cancellation' between single fibre action potentials in CNEMG. This is supported by simulation studies reported here. CONCLUSIONS: In conclusion macro EMG better reflects the size of the motor unit than the CNEMG. For detection of concomitant disorders, CNEMG is the method of choice.     

The motor unit firing rate and the power spectrum of EMG in humans

The EMG power spectrum is influenced by many factors such as the conduction velocity of the muscle fiber, the action potential of the motor unit, the number of motor units firing near the electrode, and the recording conditions. Model studies of the relation between motor unit firing rate and power spectrum of EMG have produced conflicting results. To examine this relation in vivo the brachial biceps muscle was examined in 14 controls at a force of 10% of maximum. The motor unit firing intervals were obtained from 164 motor units, sampled with a single fiber electrode. The EMG was sampled at 10 sites in each muscle with a concentric electrode and the power spectrum was obtained using fast Fourier transformation. The mean power frequency of the interference pattern as well as the relative power at 1400 Hz both decreased with increasing motor unit firing intervals between subjects. The study thus indicates that the amount of high frequencies in the power spectrum is greater in a subject with a high firing rate of the motor units than in a subject with a low firing rate.     

Location of needle electrode recording sites in the human masseter muscle by magnetic resonance imaging

A stereotactic method was developed for locating needle electrode recording sites within the human masseter muscle. The method combines a single motor unit (SMU) electromyographic (EMG) technique, magnetic resonance imaging, the 3-dimensional reconstruction of orofacial tissues, and a common reference systems. SMU EMG activity can be recorded from different sites in the masseter muscle, and the location of these sites displayed graphically in 3 dimensions. The technique should be a useful adjunct in future studies of the internal architecture and electrophysiological properties of the human masseter muscle.