Standardisation of anal sphincter EMG: high and low threshold motor units.

OBJECTIVE: The anal sphincter muscle has a proportion of low threshold motor units (MUs) that are continuously active and other, recruitable high threshold MUs. In standard EMG recordings, motor unit potentials (MUPs) of the later seem to be of higher amplitudes. A quantitative EMG study was performed to assess possible consequences of sampling MUPs at different levels of sphincter activation. METHODS: Fifteen females without uroneurological disorders were studied. After insertion, standard concentric EMG needle was left in the anal sphincter muscle undisturbed for 1 min; then 30 s of the remaining continuous, and 1 min of voluntarily increased EMG activity were recorded on a DAT recorder. MUPs were collected and analysed by 'Multi-MUP' analysis. MUPs analysed during relaxation constituted the 'low threshold MUP pool'. MUPs sampled on activation were checked for those, already sampled during relaxation, (which were discarded), and the remaining MUPs constituted the 'high threshold MUP pool'. Parameters of both MUP pools were compared. RESULTS: High threshold MUPs were found to be significantly larger than low threshold MUPs. CONCLUSIONS: EMG investigator should be aware of the differences of MUPs sampled at various anal sphincter activity levels. For the technique of 'Multi-MUP' analysis sampling at an activity level which provides 3-5 MUPs per detection site would seem practical, providing a standardised approach suitable for comparing normative data with individual findings from most patients.     

Motor unit number estimates based on the automated analysis of F-responses

An automated technique for estimating the number of motor units based on single motor unit action potentials in the F-response is described. The average surface detected motor unit action potential (SMUAP) was calculated from the datapoint-by-datapoint average of a sample of S-MUAPs automatically selected from a population of F-responses. The technique was applied to the thenar muscles of young (n = 18, aged 31 ± 11 years) and older (n = 15, aged 68 ± 3) subjects, Motor unit number estimates based on the automated selection of S-MUAPs from the F-responses compared well with those derived using a computer-assisted manual method for selecting S-MUAPs from the F-response (automated 245 ± 105 vs. manual 241 ± 100, r = 0.93) and were similar to estimates obtained using multiple point stimulation (219 ± 77). The advantages of the automated technique for collecting S-MUAPs from the F-response include the ready tolerance of the technique by subjects, the minimal amount of operator interaction required, and the additional information relating to the conduction velocities and latencies of single motor axons. © 1994 John Wiley & Sons, Inc.     

Size of motor unit potential sample

In quantitative electromyography (EMG), a sample size of 20 motor unit potentials (MUPs) is standard. The effect of increase in the number of MUPs above 20 is not known, although advanced MUP analysis techniques make such samples practical. In the present study, using multi-MUP analysis, pools of 3,720 neuropathic and 2,526 control MUPs were obtained from external anal sphincter muscles. From each pool, 10,000 random samples of 5, 10, 15, 20, 30, 40, 50, and 100 MUPs were obtained by a computer. For each sample size, 95% normative limits for mean values, SDs, and "outliers," and sensitivities were calculated for eight MUP parameters. As the magnitude of MUP samples increased, normative limits narrowed and sensitivities increased (at 5: 20-30%; at 20: 30-55%; at 100: 80-100%) for all statistics of all MUP parameters. Our results demonstrated a substantial increase in sensitivity by increasing the MUP sample to more than 20. This option deserves consideration in an attempt to improve the usefulness of quantitative EMG.     

F-responses: a study of frequency, shape and amplitude characteristics in healthy control subjects.

Characteristics of the surface recorded F-response, including frequency, occurrence of identical responses, shape and amplitude have been investigated in the ulnar nerve of control subjects. During a train of 200 stimuli, F-response frequency varied between 50% and 93% in different subjects with a mean of 79%. The vast majority of responses (96.6%) occurred only once; of those responses which repeated, 89.5% occurred between 2-5 times, 9% between 6-10 times and only 1.5% 11 or more times. F-response shape was variable, the majority containing two or more negative peaks. F-response amplitudes tended to be a relatively small proportion of the compound M-response, with median F%M values ranging from 0.8% to 4%. The data suggest that a large proportion of surface recorded F-responses following supramaximal stimulation are composed of recurrent discharges derived from more than one motor unit.     

Reinnervation of anal sphincter muscle by low and high-threshold motor neurones.

The external anal sphincter (EAS) has continuously active low-threshold and recruitable high-threshold motor units (MUs), the latter being 'larger'. On performing concentric needle electromyography (EMG) of the EAS, the high-threshold MUs seemed to reveal more neuropathic changes than the low-threshold MUs. To verify this hypothesis, low- and high-threshold motor unit potentials (MUPs) were compared in patients with neuropathic EAS and controls. Fifteen subjects without pelvic disorders and 29 patients with sequela after cauda equina lesions were studied. In patients, only muscles ipsilateral to severe perianal sensory loss were included. MUPs were sampled using multi-MUP analysis during relaxation ('low-threshold'), and on activation ('high-threshold' MUs). MUP parameters of low- and high-threshold MUs from controls and patients were compared, as was the sensitivity and specificity with which MUPs were classified as normal or pathological (using discriminant analysis). MUP changes due to reinnervation, and the sensitivity and specificity in classifying MUPs as normal or pathological were not significantly different between the low- and high-threshold MUPs. Stronger activation of EAS does not improve discrimination between neuropathic and normal MUPs. New EMG techniques for sampling sphincter MUPs at higher activation levels would seem not to yield additional information.     

Usefulness of an increase in size of motor unit potential sample.

OBJECTIVE: It is known that the sensitivity of quantitative electromyographic (EMG) analysis of motor unit potentials (MUPs) improves with an increase in MUP sample size to more than 20. However, no normative data and estimate of sensitivity have been published. METHODS: In the present study sample sizes of 5, 10, 15, 20, 30 and 40 MUPs were obtained from the external anal sphincter (EAS) muscles of 81 controls and 70 patients with cauda equina lesions. For each sample size normative limits and sensitivities for mean values and 'outliers' were calculated for 8 MUP parameters. RESULTS: As the size of the MUP samples increased, normative limits narrowed and sensitivities increased for both statistics of all MUP parameters (sensitivities were 26% at 10, 44% at 20, and 67% at 40 MUPs with mean values and outliers of MUP area, duration and number of turns). CONCLUSIONS: Our results confirmed a substantial increase in the sensitivity of MUP analysis by enlargement of the MUP sample size to more than 20 MUPs. The gain in sensitivity seem to be greater than the increase obtained by examination of contralateral EAS muscle. SIGNIFICANCE: Findings might be useful to clinical neurophysiologists planning strategies for electrodiagnostic evaluation of lower sacral segments.     

Motor unit sound in needle electromyography: assessing normal and neuropathic units

Motor unit action potentials (MUPs) recorded by a monopolar needle electrode in normal and neuropathic muscles were computer-simulated. Five experienced electromyographers acted as examiners and assessed the firing sounds of these MUPs without seeing them on a display monitor. They judged whether the sounds were crisp or close enough to accept for the evaluation of MUP parameters and whether, when judged acceptable, they were neuropathic-polyphasic. The examiners recognized motor unit (MU) sound as crisp or polyphasic when the MUP obtained was 0.15-0.2 mm from the edge of the MU territory. When the intensity of the sound decreased, they were unable to perceive it as crisp. When the intensity exceeded the saturation level of loudspeaker output, the sound was perceived as polyphasic, but the wave form of the MUP was not. When the frequency of the neuropathic MUP was lowered, the examiners were unable to determine whether the MUP was polyphasic. MUPs recognized as acceptable for evaluation can be distinguished by listening to MU sounds. The audio amplifier gain must be appropriately adjusted for each MUP amplitude in order to assess whether an individual MU sound is crisp or polyphasic before MUP parameters are measured on a display monitor.Copyright 2000 John Wiley & Sons, Inc.     

The effect of gammahydroxybutyrate on the H-reflex: pilot study

Gammahydroxybutyrate (GHB) promotes cataplexy when given during the day, but decreases the incidence of daytime cataplexy when given at night. To understand the effects of GHB on this form of motor inhibition, we studied its actions on the H- and F-responses. The H-response is attenuated or abolished during cataplexy. GHB markedly inhibited the H-reflex response in normal and narcoleptic subjects, whether or not sleep was induced. It had no effect on the F-response. Prolonged motor inhibition at night by GHB may raise the threshold for inhibition during the day, accounting for the decrease in daytime cataplexy. The failure to affect the F-response suggests that GHB has a presynaptic site of action.     

Standardization of anal sphincter electromyography: uniformity of the muscle

The different parts of the external anal sphincter (EAS) are usually regarded as one muscle with common EMG characteristics. This assumption was addressed by comparing the number of continuously firing motor units (MUs) during relaxation, as well as the parameters of motor unit potentials (MUPs) and interference pattern (IP) in the subcutaneous and the deeper parts of EAS. MUPs and IPs were analyzed in 44 subjects (2008 MUPs and 3014 IPs) without uroneurological or proctological disorders, and the number of continuously active MUs in 34 of these subjects was recorded (221 positions). No significant difference was found in IP and most MUP parameters between the two parts of the EAS muscle, but the number of continuously firing MUs was lower in the deeper part. As far as MUP and IP characteristics are concerned, the whole EAS can be considered as one muscle, but some differences in patterns of activation of MUs may exist in different regions.     

Modeling studies on irregular motor unit potentials.

OBJECTIVE: The aim of these modeling studies on the generation of motor unit potentials (MUPs) was to establish the influence of MU parameters and recording conditions on the shape irregularity of MUPs. The focus was on the dependence of the irregularity of MUPs on fiber density, fiber diameters and the recording distance from the end-plate zone. METHODS: The study was performed using the 'EMG Simulator' software for modeling of MUP and our own software for calculations. RESULTS: The results indicate that the irregularity of a MUP increases with increased recording distance from the end-plate zone and decreases with increased fiber diameter and/or with increased fiber density. The quantitative relationship between these factors has been derived. CONCLUSIONS: The relationships determine the structural conditions in which irregular potential may be generated and therefore may be helpful in the interpretation of atypical MUPs.     

Comparison of quantitative techniques in anal sphincter electromyography.

Data comparing results and utility of different quantitative electromyographic (EMG) techniques are limited. In the present study, we analyzed the EMG signal from the external anal sphincter (EAS) muscle using three techniques of motor unit potential (MUP) analysis, and a technique of interference pattern (IP) analysis. We examined 56 patients with damage to the cauda equina or conus medullaris, and 64 control subjects. Using manual-MUP and multi-MUP analysis about 20 MUPs, using a single-MUP technique about 10 MUPs, and using turn/amplitude (T/A) analysis about 20 IP samples were obtained. The sensitivities of these techniques in distinguishing neuropathic from control muscles were calculated. The single-MUP technique detected 63%, manual-MUP 57%, and multi-MUP analysis 62% of neuropathic muscles, and MUP parameters obtained by each of these differed significantly from the other. The sensitivity of T/A analysis of IP was 29%. Our results confirm the need for separate MUP normative data for each of the MUP analysis techniques, and favor them over the IP analysis technique. The normative data presented for the EAS muscle should improve and promote quantitative EMG in patients.     

Motor unit size estimation: confrontation of surface EMG with macro EMG

Surface EMG (SEMG) is little used for diagnostic purposes in clinical neurophysiology, mainly because it provides little direct information on individual motor units (MUs). One of the techniques to estimate the MU size is intra-muscular Macro EMG. The present study compares SEMG with Macro EMG. Fifty-eight channel SEMG was recorded simultaneously with Macro EMG. Individual MUPs were obtained by single fiber triggered averaging. All recordings were made from the biceps brachii of healthy subjects during voluntary contraction at low force. High positive correlations were found between all Macro and Surface motor unit potential (MUP) parameters: area, peak-to-peak amplitude, negative peak amplitude and positive peak amplitude. The MUPs recorded with SEMG were dependent on the distance between the MU and the skin surface. Normalizing the SEMG parameters for MU location did not improve the correlation coefficient between the parameters of both techniques. The two measurement techniques had almost the same relative range in MUP parameters in any individual subject compared to the others, especially after normalizing the surface MUP parameters for MU location. MUPs recorded with this type of SEMG provide useful information about the MU size.     

Simulation studies on the motor unit potentials with satellite components in amyotrophic lateral sclerosis and spinal muscle atrophy

Introduction: We compared motor unit potentials (MUPs) with satellite components recorded in two anterior horn disorders: amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA, types II and III). Methods: We analyzed MUPs recorded from biceps brachii muscle, including 209 associated with ALS (12 patients) and 127 with SMA (5 patients). Simulations were applied to determine the origin of satellites in these processes. Results: MUP parameters differ in ALS and SMA. Simulations indicate that the satellite potential in ALS often originated from a single fiber, whereas in SMA it originated from a group of fibers of smaller diameters than the surrounding ones. Conclusions: These results suggest that, except for neurogenic factors, the variability of muscle fiber diameters also leads to the formation of MUPs with satellites. This variability seems to be responsible for the differences in the shape of the main and satellite MUP components in ALS and SMA. Muscle Nerve, 2012     

Can the size principle be detected in conventional emg recordings?

According to Henneman's size principle, small motor units are recruited before large ones. It is commonly believed that this can be detected in routine conventional EMG recordings even among the earliest recruited motor units. That is, the MUP amplitude, area, and thickness should increase with recruitment order. We studied the first four motor unit potentials (MUPs) recruited within the pickup area of the electrodes. Data were obtained from 179 different sites in monopolar recordings and in 153 concentric recordings from 5 health subjects. In the pooled material, amplitude, area, and thickness increased slightly between consecutively recruited MUPs. However, at individual recording sites the size of consecutively recruited MUPs varied considerably. At some recording sites the first recruited MUP had the largest amplitude and the later MUPs has successively smaller amplitudes. We conclude that, at individual recording sites, the size principle cannot be detected in low threshold motor units with monopolar or concentric EMG electrodes. The reason for this is the small uptake area of these electrodes in relation to the motor unit territory. © 1995 John Wiley & Sons, Inc.     

Quantitative motor unit potential analysis in the diaphragm: a normative study

Although quantitative motor unit potential (MUP) analysis has an established role in other skeletal muscles, it has not been performed in the diaphragm. The aim of the present study was to test whether such studies are possible and to establish normative data. Twenty-nine healthy volunteers (15 men), aged 21-65 years (median, 33 years), were studied using standard concentric needle electrodes, and equipment with the facility for template-operated multi-MUP analysis. Needle electrodes were inserted into the right medial recess of the seventh to ninth intercostal spaces. During MUP sampling, subjects were asked to hold their breath in partial inspiration for 5 s. At least 20 MUPs were obtained in 28 subjects. Diaphragmatic MUPs were confirmed to be much smaller than those of limb muscles. We found diaphragmatic quantitative MUP analysis to be possible in healthy volunteers. However, further studies in patient groups are needed to establish the feasibility and clinical value of such studies.     

Macro EMG follow-up study in post-poliomyelitis patients

We investigated the muscle strength and motor unit (MU) territory of five patients with postpolio syndrome (PPS), six stable patients with prior poliomyelitis, and five healthy volunteers. The MU territory was assessed by measuring amplitudes of motor unit potentials (MUPs) recorded by the macro EMG technique. The investigations were repeated after 11–20 months (mean 15.6). The macro MUP amplitudes in both patient groups were markedly increased (P = 0.02). However, no statistical difference was found between the two groups in the initial amplitude values. Macro MUP amplitudes obtained on repeated examinations did not differ significantly from the initial macro MUP amplitudes in any of the three groups. In three individual PPS patients, a decline in muscle strength on the follow-up study was documented, providing the diagnosis of post-poliomyelitis muscular atrophy (PPMA). The three PPMA patients had the highest initial macro MUPs. Two of them showed a decrease in macro MUP amplitudes on follow-up. These findings suggest that a later breakdown of oversized MUs may play a role in the pathogenesis of PPMA.     

Turn and phase counts of individual motor unit potentials: correlation and reliability.

Different turn algorithms are used for quantitative motor unit potential (MUP) analysis. To compare their retest reliability, 420 myopathic and neuropathic MUPs were recorded twice and the turn count of the first registration was correlated with that of the second. Reliability was best for the algorithm according to Willison as compared to the conventionally used algorithms based on amplitude criteria for 2 or 3 successive relative extrema. As demonstrated by discriminant analysis, an amplitude limit of 25 microV yielded more useful turn counts than a limit of 50 microV if myopathic MUPs had to be discriminated from normal MUPs. For this discrimination the turn count was superior to the phase count which did not further improve the discriminant model. This was different for the discrimination between normal and neuropathic MUPs. In this case, both parameters measured partly independent features of the MUP and had to be considered together.     

Range of variations in motor unit potentials during reinnervation after traumatic nerve lesions in humans

Successive stages in the reinnervation of denervated muscle fibers following a complete or partial motor nerve lesion in humans were analyzed with the concentric needle electrode during voluntary contraction. Motor unit potential (MUP) patterns were displayed by coherent EMG, in which the potential triggers an oscilloscope sweep and a digital delay line enables the early part of the MUP to be displayed. After a complete nerve lesion, MUPs showing the response of as few as one to three muscle fibers can be observed. MUPs become highly polyphasic as more muscle fibers are incorporated. Marked latency jitter and intermittent blocking of components as well as the occurrence of linked potentials result in complex patterns that may be confused with a “myopathic” pattern of brief small spikes if conventional observation of free-running sweeps is used alone. After a partial nerve lesion, the healthy motor axons achieve extensive collateral reinnervation of the denervated muscle fibers whereby linked potentials of various latencies are added after the original MUP. Adequate understanding of these characteristic features can help clarify some current issues in electromyography. The mechanisms by which long-latency linked potentials and desynchronized components are produced during remodeling of motor units in the course of regeneration involve a number of factors such as slow conduction along regenerated axon branches, ectopic motor end-plates, insecure neuromuscular transmission, and muscle fiber atrophy.     

F-response frequency in motor neuron disease and cervical spondylosis.

During a train of 200 stimuli, F-response frequency, frequency of identical responses, and F-response shape were studied in the ulnar nerve of 17 patients with motor neuron disease (MND) and 16 patients with cervical spondylosis (CS). In MND patients, F-response frequency varied between 5% and 96% with a median of 39% which was significantly lower than controls (p less than 0.001), and showed a significant rank correlation with the M-response amplitude (r = 0.62, p less than 0.004). Identical responses occurred more frequently than in controls (median 30.8%, p less than 0.001). F-response frequency was normal in CS patients (median 76.3%, range 35% to 97%), but the frequency of identical responses (median 6.3%) was higher than normal (p less than 0.01). Reduced F-response frequency in MND was thought to reflect loss of lower motor neurons, while the presence of spasticity was probably the major factor underlying the increased frequency of identical responses in both disorders. F-response shape tended to be simpler in MND and rather more complex in CS patients than controls.