Motor unit size in muscular dystrophy, a macro EMG and scanning EMG study

Clustering index analysis on EMG-Torque relation-based representation of complex neuromuscular changes after spinal cord injury

Spinal cord injury (SCI) resulting in complex neuromuscular pathology is not sufficiently well understood. To better quantify neuromuscular changes after SCI, this study uses a clustering index (CI) method for surface electromyography (sEMG) clustering representation to investigate the relation between sEMG and torque in SCI survivors. The sEMG signals were recorded from 13 subjects with SCI and 13 gender-age matched able-bodied subjects during isometric contraction of the biceps brachii muscle at different torque levels using a linear electrode array. Two torque representations, maximum voluntary contraction (MVC%) and absolute torque, were used. CI values were calculated for sEMG. Regression analyses were performed on CI values and torque levels of elbow flexion, revealing a strong linear relationship. The slopes of regressions between SCI survivors and control subjects were compared. The findings indicated that the range of distribution of CI values and slopes was greater in subjects with SCI than in control subjects (p < 0.05). The increase or decrease in slope was also observed at the individual level. This suggests that the CI and its sEMG clustering-torque relation may serve as valuable quantitative indicators for determining neuromuscular lesions after SCI, contributing to the development of effective rehabilitation strategies for improving motor performance.

Critical illness-associated weakness and related motor disorders

Weakness of limb and respiratory muscles that occurs in the course of critical illness has become an increasingly common and serious complication of adult and pediatric intensive care unit patients and a cause of prolonged ventilatory support, morbidity, and prolonged hospitalization. Two motor disorders that occur singly or together, namely critical illness polyneuropathy and critical illness myopathy, cause weakness of limb and of breathing muscles, making it difficult to be weaned from ventilatory support, commencing rehabilitation, and extending the length of stay in the intensive care unit, with higher rates of morbidity and mortality. Recovery can take weeks or months and in severe cases, and may be incomplete or absent. Recent findings suggest an improved prognosis of critical illness myopathy compared to polyneuropathy. Prevention and treatment are therefore very important. Its management requires an integrated team approach commencing with neurologic consultation, creatine kinase (CK) measurement, detailed electrodiagnostic, respiratory and neuroimaging studies, and potentially muscle biopsy to elucidate the etiopathogenesis of the weakness in the peripheral and/or central nervous system, for which there may be a variety of causes. These tenets of care are being applied to new cases and survivors of the coronavirus-2 disease pandemic of 2019. This chapter provides an update to the understanding and approach to critical illness motor disorders.

Ultrasound-guided motor unit scanning electromyography

INTRODUCTION/AIMS

Measuring the spatial dimensions of a single motor unit remains a challenging problem, and current techniques, such as scanning electromyography (EMG), tend to underestimate the true dimensions. In this study we aimed to estimate more accurately the dimensions of a single motor unit by developing a clinically applicable scanning EMG protocol that utilizes ultrasound imaging to visualize and target a transect through the center of a single motor unit.

METHODS

Single motor unit twitches in the tibialis anterior muscles of healthy volunteers were elicited via stimulation of the fibular nerve, visualized with ultrasound, and targeted with an intramuscular EMG electrode. The electrode was moved by hand in small steps through the motor unit territory. Ultrasound video output was synchronized to EMG capture, and the needle position was tracked at each step.

RESULTS

Eight recordings from six participants were collected. The technique was quick and easy to perform (mean time, 6.1 minutes) with reasonable spatial resolution (mean step size, 1.85 mm), yielding motor unit territory sizes between 1.53 and 14.65 mm (mean, 7.15 mm).

DISCUSSION

Ultrasound-guided motor unit scanning EMG is a quick and accurate method for obtaining a targeted motor unit transect. This combination of two readily available clinical tools provides insights into the dimensions and internal structure of the motor unit as a marker for neuromuscular conditions.

Objective and subjective measures of sleep in men with Muscular Dystrophy

Purpose

Despite poor sleep quality being recognised in Duchenne Muscular Dystrophy, reports from milder forms of Muscular Dystrophy (MD), and accompanied associations with quality of life (QoL), pain and fatigue, remain limited however.

Methods

Adult males (n = 15 Beckers MD (BMD), n = 12 Limb-Girdle MD (LGMD), n = 12 Fascioscapulohumeral (FSHD), n = 14 non-MD (CTRL)) completed assessments of body composition (Bio-electrical impedance), sleep (7-day 24-hour tri-axial accelerometer, Pittsburgh Sleep Quality Index (PSQI) and Insomnia Severity Index, QoL (SF36-v2), pain (Visual analogue scale), fatigue (Modified Fatigue Index Scale) and functional assessments (Brookes and Vignos).

Results

FSHD and BMD reported worse sleep than CTRL on the PSQI. FSHD scored worse than CTRL on the Insomnia Severity Index (P<0.05). 25–63% and 50–81% of adults with MD reported poor sleep quality using the Insomnia Severity Index and PSQI, respectively. Accelerometery identified no difference in sleep quality between groups. Associations were identified between sleep measures (PSQI global and insomnia severity) with mental or physical QoL in LGMD, BMD and FSHD. Multiple regression identified associations between sleep impairment and fatigue severity (all MDs), body composition (BMD & LGMD), upper and lower limb function (LGMD, FSHD) and age (FSHD).

Conclusions

25–81% of men with MD, depending on classification, experience sleep impairment, using self-report sleep measures. Whilst BMD and FSHD showed worse sleep outcomes than CTRL, no group difference was observed between LGMD and CTRL, however all groups showed associations with sleep impairment and higher levels of fatigue. These findings, and associations with measures of health and wellbeing, highlight an area for further research which could impact QoL in adults with MD.

Masked least-squares averaging in processing of scanning-EMG recordings with multiple discharges

Removing artifacts from nearby motor units is one of the main objectives when processing scanning-EMG recordings. Methods such as median filtering or masked least-squares smoothing (MLSS) can be used to eliminate artifacts in recordings with just one discharge of the motor unit potential (MUP) at each location. However, more effective artifact removal can be achieved if several discharges per position are recorded. In this case, processing usually involves averaging the discharges available at each position and then applying a median filter in the spatial dimension. The main drawback of this approach is that the median filter tends to distort the signal waveform. In this paper, we present a new algorithm that operates on multiple discharges simultaneously and in the spatial dimension. We refer to this algorithm as the multi-masked least-squares smoothing (MMLSS) algorithm: an extension of the MLSS algorithm for the case of multiple discharges. The algorithm is tested using simulated scanning-EMG signals in different recording conditions, i.e., at different levels of muscle contraction and for different numbers of discharges per position. The results demonstrate that the algorithm eliminates artifacts more effectively than any previously available method and does so without distorting the waveform of the signal. Graphical abstract The raw scanning-EMG signal, which can be composed by several discharges of the MU, is processed by the MMLSS algorithm so as to eliminate the artifact interference. Firstly, artifacts are detected for each discharge from the raw signal, obtaining a multi-discharge validity mask that indicates the samples that have been corrupted by artifacts. Secondly, a least-squares smoothing procedure simultaneously operating in the spatial dimension and among the discharges is applied to the raw signal. This second step is performed using only the not contaminated samples according to the validity mask. The resulting MMLSS-processed scanning-EMG signal is clean of artifact interference.

Recent insights into neuromuscular junction biology in Duchenne muscular dystrophy: Impacts, challenges, and opportunities

Duchenne muscular dystrophy (DMD) is the most common and relentless form of muscular dystrophy. The pleiotropic effects of dystrophin deficiency include remarkable impacts on neuromuscular junction (NMJ) structure and function. Some of these alterations contribute to the severe muscle wasting and weakness that distinguish DMD, while others attempt to compensate for them. Experimental approaches that correct NMJ biology in pre-clinical models of DMD attenuate disease progression and improve functional outcomes, which suggests that targeting the NMJ may be an effective therapeutic strategy for DMD patients. The objectives of this review are to 1) survey the distinctions in NMJ structure, function, and gene expression in the dystrophic context as compared to the healthy condition, and 2) summarize the efforts, opportunities and challenges to correct NMJ biology in DMD. This information will expand our basic understanding of neuromuscular biology and may be useful for designing novel NMJ-targeted drug or behavioural strategies to mitigate the dystrophic pathology and other disorders of the neuromuscular system.

A masked least-squares smoothing procedure for artifact reduction in scanning-EMG recordings

Scanning-EMG is an electrophysiological technique in which the electrical activity of the motor unit is recorded at multiple points along a corridor crossing the motor unit territory. Correct analysis of the scanning-EMG signal requires prior elimination of interference from nearby motor units. Although the traditional processing based on the median filtering is effective in removing such interference, it distorts the physiological waveform of the scanning-EMG signal. In this study, we describe a new scanning-EMG signal processing algorithm that preserves the physiological signal waveform while effectively removing interference from other motor units. To obtain a cleaned-up version of the scanning signal, the masked least-squares smoothing (MLSS) algorithm recalculates and replaces each sample value of the signal using a least-squares smoothing in the spatial dimension, taking into account the information of only those samples that are not contaminated with activity of other motor units. The performance of the new algorithm with simulated scanning-EMG signals is studied and compared with the performance of the median algorithm and tested with real scanning signals. Results show that the MLSS algorithm distorts the waveform of the scanning-EMG signal much less than the median algorithm (approximately 3.5 dB gain), being at the same time very effective at removing interference components.

Graphical Abstract

The effect of recording site on extracted features of motor unit action potential

Motor unit action potential (MUAP), which consists of individual muscle fiber action potentials (MFAPs), represents the electrical activity of the motor unit. The values of the MUAP features are changed by denervation and reinnervation in neurogenic involvement as well as muscle fiber loss with increased diameter variability in myopathic diseases. The present study is designed to investigate how increased muscle fiber diameter variability affects MUAP parameters in simulated motor units. In order to detect this variation, simulated MUAPs were calculated both at the innervation zone where the MFAPs are more synchronized, and near the tendon, where they show increased temporal dispersion. Reinnervation in neurogenic state increases MUAP amplitude for the recordings at both the innervation zone and near the tendon. However, MUAP duration and the number of peaks significantly increased in a case of myopathy for recordings near the tendon. Furthermore, of the new features, "number of peaks×spike duration" was found as the strongest indicator of MFAP dispersion in myopathy. MUAPs were also recorded from healthy participants in order to investigate the biological counterpart of the simulation data. MUAPs which were recorded near to tendon revealed significantly prolonged duration and decreased amplitude. Although the number of peaks was increased by moving the needle near to tendon, this was not significant.

Large motor unit territories by scanning electromyography in patients with juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy is a genetically inherited disorder characterized by myoclonic jerks and generalized seizures. It has been proposed that patients with juvenile myoclonic epilepsy have larger motor units (MUs) than normals by MU number estimation and macro electromyography techniques. In this study, an experimental setup for scanning electromyography was built to investigate electrophysiologic cross-sections of the MU territories in 9 patients with juvenile myoclonic epilepsy, 3 patients with spinal muscular atrophy, and 10 healthy volunteers. Scanning electromyography was performed on the biceps brachii muscle. For each MU, three-dimensional maps of the MU territories were plotted. The length of MU cross-section and the maximum amplitude of each MU were measured from these maps and compared among the three groups of subjects. Like spinal muscular atrophy patients, patients with juvenile myoclonic epilepsy had significantly larger MU territories than normal controls.

Quantitative measurements and analysis in electrodiagnostic studies: present and future

Electrodiagnostic studies (nerve conduction and needle electromyography) are powerful methods to study diseases of nerves and muscles. Conduction studies are performed to assess the continuity, anatomic course, excitability, number of axons and their synaptic connections, while needle electromyography examination is performed to assess the ‘electrical stability’ of the muscle fiber membrane, and the study of motor units (architecture, activation and number). We have reviewed the relationship between the quantitative measurements and the generators of the recorded potentials. Based on these relationships, the expected patterns of abnormalities for different pathologies are tabulated. This gives us a better appreciation of the sensitivity, specificity and usefulness of different tests when planning and conducting an electrodiagnostic examination. The limitations of current methods are indicated to suggest the need for future development.

Effects of changes in intracellular action potential on potentials recorded by single-fiber, macro, and belly-tendon electrodes

Some myopathies are accompanied by abnormal calcium homeostasis. Electromyography (EMG) in such patients shows signs of normal or myopathic EMG when detected by a single-fiber electrode and abnormally increased values in macro EMG. As calcium accumulation might be accompanied by changes in intracellular action potential (IAP) and muscle-fiber propagation velocity, we simulated the effects of such changes on motor unit potentials (MUPs) recorded by different kinds of electrodes. We found that: (1) the requirements for what potential can be accepted as a single-fiber action potential (SFAP) are too rigorous; (2) macro MUP amplitude can increase while SFAP amplitude can decrease when there is an increase in the spatial length of IAP spike; and (3) changes in the second phase of a belly-tendon-detected MUP or M wave could be used for noninvasive detection of increased IAP depolarizing (negative) after-potential.

Changes in intracellular action potential profile affect parameters used in turns/amplitude analysis

The influence of changes in the intracellular action potential (IAP) spatial profile on motor unit potentials (MUPs), number of turns per second (NTs), and mean turn amplitude were simulated and analyzed. We show why measurement of NTs was "the best indicator of neurogenic affection" and why the lower diagnostic yield of turns/amplitude analysis in myopathy could be due to changes in IAP shape caused by elevated free calcium concentration. The results explain the complications observed when interference electromyographic signals obtained during high levels of isometric contractions were analyzed. We show that, in contrast to earlier assumptions, the effect of increased IAP spike duration on NTs was stronger than that of a decrease in muscle fiber propagation velocity (MFPV). The decrease in the NTs could occur without a drop-out of MUs and/or a decrease in their firing rates, and without a change in MFPV and synchronous firing.

The role of different EMG methods in evaluating myopathy

For the diagnosis of myopathy, EMG may have an important role along with blood tests, muscle biopsies and genetic testing. This review evaluates different EMG methods in the diagnosis of myopathy. These include manual analysis of individual motor unit potentials and multi-motor unit potential analysis sampled at weak effort. At high effort, turns-amplitude analyses such as the cloud analysis and the peak ratio analysis have a high diagnostic yield. The EMG can seldom be used to differentiate between different types of myopathy. In the channelopathies, myotonia, exercise test and cooling of the muscle are helpful. Macro-EMG, single-fibre EMG and muscle fibre conduction velocity analysis have a limited role in myopathy, but provide information about the changes seen. Analysis of the firing rate of motor units, power spectrum analysis, as well as multichannel surface EMG may have diagnostic potential in the future. EMG is of great importance in the diagnosing of patients with myopathy, preferably a needle electrode and quantitative analyses should be used. A combination of a method at weak effort as well as a method at stronger effort seems optimal.

Causes of the increased fiber density in muscular dystrophies studied with single fiber EMG during electrical stimulation

The impulse transmission in motor end-plates and along muscle fibers was studied with single fiber EMG in 13 patients with muscular dystrophy during intramuscular nerve stimulation and direct muscle fiber stimulation. For comparison, three patients with spinal muscular atrophy (SMA) and five normal subjects were studied. At nerve stimulation, 25% of the recordings in the patients with muscular dystrophy showed "increased" jitter (> 55 microsec), 48% "normal" (10-55 microsec), and 27% "abnormally low" jitter (< 5 microsec). At direct muscle fiber stimulation, the jitter was "abnormally low" in 91% of the recordings; 9% of the recordings had normal jitter. In SMA, no abnormally low jitter values were found. Increased jitter is a sign of impaired impulse transmission in immature intramuscular nerves and motor end-plates. "Abnormally low jitter" suggests the presence of branched muscle fibers and ephaptic transmission between hyperexcitable fibers.

Electrodiagnostic studies in the management and prognosis of neuromuscular disorders

Prognosis remains a neglected aspect of modern medical care and research, behind diagnosis and treatment. The very term "electrodiagnosis" implies as much. Despite this, much has been published regarding the use and benefit of electrodiagnostic techniques in assessing prognosis and assisting in management of patients after the diagnosis has been established. This information is often hidden or otherwise not emphasized. This review summarizes the literature regarding the use of such techniques for prognosis and management of disorders of lower motor neurons, peripheral nerves, neuromuscular transmission, and muscle.

Quantitative electromyographic examination in myogenic disorders of 6 horses

Electromyographic needle examination (EMG), including the semiautomatic quantitative analysis of motor unit action potential (MUAP), is an important diagnostic tool for myopathy in humans. The diagnostic possibilities of this technique have not been fully explored in horses; however, recent studies have shown that MUAP analysis can be performed in conscious horses. To determine the diagnostic possibilities of EMG in horses, we compared the EMG results of the subclavian muscle, the triceps, and the lateral vastus muscle in 6 equine patients thought to have myogenic disorders with those in 7 normal control horses. The EMG results were compared with the results of the histopathologic examination of the lateral vastus muscle in patients and controls. Histopathologic examination showed muscle disease in 3 patients. In the patient group, several types of abnormal spontaneous activities were observed (mainly fibrillation potentials and positive sharp waves), and the MUAPs of the patient group had a markedly shorter duration and lower amplitude than those of the control group. In the subclavian muscle, triceps, and lateral vastus muscle of affected horses, the MUAP duration was 5.0 +/- 0.4 (mean +/- SD), 3.9 +/- 0.3, and 4.7 +/- 1.1 milliseconds, respectively. The MUAP amplitude was 217 +/- 55, 150 +/- 74, and 180 +/- 54 microV; the number of phases was 2.4 +/- 0.2, 2.5 +/- 0.3, and 2.3 +/- 0.1; and the number of turns was 2.6 +/- 0.2, 2.4 +/- 0.2, and 2.8 +/- 0.5, respectively. In conclusion, it appears that the EMG may be a more sensitive method than other techniques for examining muscle biopsies for diagnosis of early-stage myopathy in horses.

Discriminant analysis of various concentric needle EMG and macro-EMG parameters in detecting myopathic abnormality

OBJECTIVES

The aim of the study was to evaluate the effectiveness of various concentric needle electromyography (EMG) motor unit action potentials (cnMUPs) and macro-EMG motor unit potentials (mMUPs) parameters for differentiation between myopathic motor unit action potentials (MUPs) and normal MUPs.

METHODS

We have analyzed 112 cnMUPs and 84 mMUPs recorded from 7 patients with myopathy and 256 cnMUPs, 256 mMUPs from 14 healthy subjects. Biceps brachii muscle was investigated. Evaluated variables were duration, amplitude, area, number of phases, area/amplitude ratio, size index and area/number of phases ratio for cnMUPs, area and amplitude for mMUPs. Univariate statistical analysis and discriminant analysis for each parameter were performed.

RESULTS

The variable 'area ' gave rather good discrimination than duration, amplitude, number of phases, area/amplitude ratio, and size index. As demonstrated by discriminant analysis, area/phase ratio is more useful than area alone if myopathic MUPs had to be discriminated from normal MUPs. Discriminant efficiency of mMUP parameters were lower than all cnMUP parameters except number of phases.

CONCLUSIONS

The new parameter area/number of phases ratio seemed to be promising, since it produced a better yield in detecting of myopathic abnormality than other investigated parameters in discriminant analysis. Discriminating ability of macro-EMG was lower than that of cnEMG.

Scanning electromyography

A special electromyography (EMG) method, scanning EMG, was introduced by Stålberg and Antoni in 1980 to study the electrophysiological cross sections and sizes of motor units. Scanning EMG gives a new approach for the evaluation of the electrical properties of motor units, providing new data on the normal anatomical distribution of muscle fibers and its changes in different pathologies of the muscle. The description of scanning EMG recordings required the introduction of new parameters (lengths of motor unit cross sections, fractions of motor units, and silent areas), in addition to those used with conventional EMG recordings, and the traditional parameters (duration, amplitude, etc.) acquired new and more accurate explanations. Normal scanning EMG recordings are available for biceps brachii, anterior tibial, and masseter muscles. The findings in normal muscles agree with the nonrandom distribution of muscle fibers in motor units and confirm the suggestion that muscle fibers within motor units tend to be arranged in clusters. In muscular dystrophies, the sizes of motor unit territories do not differ significantly from the normal values. However, the configuration of motor units changes considerably. Abrupt changes in amplitude and duration, segments of short and long duration, increased numbers of fractions, and silent areas have been revealed, showing that dystrophic motor units are definitely fragmented. Scanning EMG supports the assumption that there is clustering of muscle fibers within the dystrophic motor unit, with local grouping of muscle fibers. In neurogenic lesions, the length of motor units is normal or only slightly increased. Reinnervated motor units are restricted to the fascicles in which they are originally found. Reinnervation does not result in an increase in the number of fractions, but the amplitude of the potentials, the length of polyphasic sections, and the duration increase. The increase in the number and length of polyphasic sections can differentiate normal motor units from abnormal ones. However, other features (amplitude, duration, number of fractions, and presence of silent areas) are also necessary to distinguish neurogenic processes from myogenic ones.

Macroelectromyography: a review of the technique and its value in the investigation of neuromuscular disorders

The background for the macroelectromyography technique, which was developed by Erik Stålberg to measure the size of motor unit potentials in human muscles, is reviewed. The method employs a modified single-fiber electrode with a large nonselective recording surface capable of recording the activity from all the fibers of a motor unit. The findings in normal subjects and its application in the study of motor unit recruitment are described. The value of the technique in the investigation, diagnosis, and monitoring of various neuromuscular diseases, including primary myopathies and neurogenic conditions, is discussed.

Concentric-needle versus macro EMG. II. Detection of neuromuscular disorders

OBJECTIVE

Little is known about the relation and sensitivity of macro-EMG (MA-EMG) compared with concentric-needle EMG (CN-EMG) in the detection of neuromuscular disorders.

METHODS

CN-EMGs and MA-EMGs were recorded from the right brachial biceps muscle of 40 healthy subjects, aged 17-83 years, 20 patients with neurogenic disorders, aged 25-75 years, and 20 patients with myopathy, aged 18-76 years. Motor unit action potentials (MUAPs) were examined.

RESULTS

In patients with neurogenic disorders CN-MUAP duration, CN-MUAP amplitude, percent polyphasia, MA-MUAP amplitude, MA-MUAP area and fibre density were significantly increased. In patients with myopathy, only fibre density was significantly increased. In patients with neurogenic disorders, the sensitivity of CN-EMG was 80%, and that of MA-EMG 85%. In myopathies, the sensitivity was 50% for each technique. Pooling the results of both EMG techniques, the sensitivity increased to 90% in patients with neurogenic disorders, and to 65% in myogenic disease.

CONCLUSIONS

MA-EMG has a similar sensitivity in the detection of neuromuscular disorders as CN-EMG. Particularly when myopathy is suspected, both techniques should be applied if one is unrevealing.

Concentric needle EMG versus macro EMG I. Relation in healthy subjects

OBJECTIVES

The relation between motor unit action potentials (MUAPs) recorded via a macro needle electrode (MA-MUAP, MA-EMG) and MUAPs recorded via a concentric needle electrode (CN-MUAP, CN-EMG) is under debate. In particular it is not known to what degree CN-MUAP variables reflect the electrical properties of a motor unit.

METHODS

CN-EMGs and MA-EMGs of the right brachial biceps muscle were recorded from 40 healthy subjects (23 women and 17 men) aged 17-83 years and CN-MUAP and MA-MUAP variables were cross-correlated.

RESULTS

CN-MUAP duration was positively and significantly correlated with CN-MUAP area (r=0.52), rate of polyphasia (r=0.45), MA-MUAP amplitude (r=0.47) and MA-MUAP area (r=0.45). CN-MUAP amplitude was positively and significantly correlated with the rate of polyphasia (r=0.39) and the fibre density (r=0.45).

CONCLUSIONS

CN-MUAP duration appropriately reflects the motor unit's electrical activity and may substitute MA-MUAP area and amplitude.

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.

A morphological analysis of the macro motor unit potential

The technique of macro EMG is used to investigate the motor unit architecture in a number of pathological conditions. Amplitude and area are the most commonly used criteria, but these parameters alone are not sufficient to assess the complexity of the macro MUP morphology. In an attempt to examine the morphology of the macro MUP in more detail, additional measures were investigated including, (i) average power, (ii) duration, and (iii) number of phases. Macro MUP duration was defined as the time parameter that contains a particular fraction (90%) of the total power of the potential. The above mentioned parameters were evaluated for normal subjects and for patients suffering with motor neuron disease (MND), spinal muscular atrophy (SMA), and Becker's muscular dystrophy (BMD). It is shown that high amplitude and average power macro MUPs give shorter macro MUP duration than macro MUPs with normal amplitude. In contrast, in low amplitude macro MUPs there is a tendency towards a higher duration measure, as compared with the duration of the normal amplitude macro MUPs. Also, t-test results for the duration measure gave a significant difference between the NOR-MND, and no significant difference between the NOR-BMD and NOR-SMA groups at P<0. 05. Significant difference between the NOR and the three disease groups investigated was obtained for the parameters log amplitude, log area, and log average power. The number of phases was not significantly different between the NOR and the rest of the groups. In conclusion, the average power and duration parameters can possibly be used as additional discriminators to detect abnormalities of the macro motor unit potential in both needle and surface EMG but further investigation is necessary.

The role of electromyography in neurology

A review is given of the role of electromyography (EMG) for diagnosis, pathophysiological description and monitoring of patients with disorders of the peripheral nervous system. Various EMG methods are presented and their principal differences are discussed. The usefulness of these methods varies depending on the pathology to be studied. With modern quantitative methods for analysis, EMG has become more sensitive and accurate and is therefore an important part in the evaluation of the neurologic patient. EMG results are usually combined with findings from other neurophysiological investigations (neurography, evoked potentials), histochemistry, biochemistry and most importantly with the clinical signs to give as complete a picture of the condition as possible. The usefulness of EMG depends on a number of factors other than the quality of the investigation as such. These aspects are discussed briefly.

Electromyography in myopathy

Electromyography (EMG) is the most common procedure for screening patients with myopathies and remains the most important technique for assessing the course of the disease over time. Fibrillation potentials, positive sharp waves, myotonic or complex repetitive discharge, as well as polyphasic potentials are non specific and can occur in both myopathic and neurogenic lesions. The most sensitive and specific parameter for myopathy in conventional EMG is the decreased duration of motor unit potentials (MUP), but this can also be seen in disorders of the terminal motor fibers or the neuromuscular junction. More advanced techniques such as single fiber EMG, macro EMG, scanning EMG and turns/amplitude analysis have opened additional possibilities for analysis of the motor unit and the interference pattern, by which both the sensitivity to early changes and specificity for myopathic alterations is increased. The importance of combining different techniques to improve diagnostic yield and specificity is stressed.

Quantitative motor unit potential analysis

A review of quantitative methods for electromyography is given. Background information about motor unit anatomy, physiology, and pathology is provided to explain some of the presented electrophysiological phenomena. Different aspects of quantitation, such as motor unit action potential parameters, automatic analysis methods, reference values, and findings in abnormal conditions, are discussed.

The spectrum of concentric macro EMG correlations. Part II. Patients with diseases of muscle and nerve

In the second part of this study we investigate the correlations between the concentric and conmac action potentials in motor units of individuals with diseases of muscle and nerve. We studied 86 myopathic and 86 neurogenic motor units and compared their concentric and conmac action potentials. In the patients with myopathy, we found that the concentric motor unit action potential's (MUAP) area correlates strongly with the conmac potential, even better than in normals, while its amplitude correlates less. In the neurogenic group, we found that both the concentric MUAP's area and amplitude correlated very well with their conmac counterpart, more so than in normals. Thus, in pathology, as in normals, measuring the concentric MUAP's area in addition to its amplitude adds to the diagnostic sensitivity of motor unit potential measurements. These findings are discussed in light of the known dynamic and architectural motor unit changes which take place in the myopathic and neurogenic motor unit.

A study of motor unit structure by means of scanning EMG

We used the scanning EMG technique to investigate the structure of human quadriceps muscle motor units. A group of healthy volunteers and 2 groups of patients with proven neurogenic or myogenic neuromuscular pathology have been studied. In total, 86 scans were obtained. An estimate of the motor unit territory (S) separates the 2 patient groups: the majority of territories of myogenic patients are smaller than 4 mm, whereas almost all motor unit territories of neurogenic patients are larger. However, the sizes of the pathologic units only occasionally exceed the upper and lower limits of normal units (2 to 8 mm). The myogenic scans show, on average, a much larger temporal dispersion (T) between the MUAPs within a motor unit than the normal scans. These findings are in accordance with observations with the same technique by others. For the neuropathic scans, there is a significant positive correlation between S and T, which is completely absent in myopathic scans. These observations are confronted with current morphometrical knowledge on motor unit physiology and anatomy.

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.

Myopathy in Marinesco-Sjögren syndrome: an electrophysiological study

Electrophysiological studies were performed in 7 patients with Marinesco-Sjögren syndrome in order to search for neuromuscular involvement in this multiorgan disorder. In 6 patients muscle biopsies were also obtained. Light microscopic examinations of the biopsies showed extensive myopathic changes, and in two patients ragged red fibers were found. Electron microscopy showed subsarcolemmal accumulation of abnormal mitochondria in all. Concentric needle EMG revealed unequivocal myopathic changes, more extensive in the anterior tibial than in the biceps brachii muscle. Motor and sensory conduction velocities in the peripheral nerves were normal. There were remarkably high amplitudes of sensory responses. Macro EMG studies in the biceps brachii muscle in four patients showed increased amplitude and area of the macro MUPs. This may be due to abnormal membrane function. Both electrophysiological and morphological findings confirm myopathic features of Marinesco-Sjögren syndrome.

Invited review: motor unit estimation: methods, results, and present status

The renewed interest in motor unit estimation (counting) has coincided with the introduction of computer-based methodology and with the application of the technique to proximal as well as distal muscles. The advantages and disadvantages of the different methods are considered, together with the assumptions inherent in this type of examination. In normal subjects, the extensor digitorum brevis (EDB) muscle has approximately 200 motor units while each of the intrinsic muscles of the hand has about 100 units; larger muscles in the limbs contain greater numbers of units. Beyond the age of 60 years, there is a decline in the number of functioning motor units in both proximal and distal muscles. In denervating disorders, motor unit estimation is useful for diagnosis and assessment; abnormal values may often be observed in muscles judged clinically to be unaffected. Serial studies have enabled the rate of motor unit loss to be determined in ALS and in spinal muscular atrophy. Depletion of motor units has also been found following upper motoneuron lesions caused by injury to the spinal cord or by cerebral hemorrhage; trans-synaptic dysfunction has been presumed responsible. Rather surprisingly, reduced numbers of motor units have been observed in a variety of myopathic disorders; of these, the most consistent abnormalities have been reported in myotonic muscular dystrophy.

Invited review: electrodiagnostic assessment and monitoring of motor unit changes in disease

The motor unit characteristics change dynamically with disease. This is the basis for the use of neurophysiologic methods for diagnostic purposes and for long-term monitoring. A great variety of nerve conduction and EMG parameters are available to study the motor unit. They reflect different aspects of motor unit function. Therefore, the electromyographer must choose his technique and optimal parameters depending on the situation, type of condition, whether the purpose is diagnosis or monitoring, and so on. In this presentation the relationship between EMG parameters and physiologic and morphologic counterparts in some pathologic conditions will be discussed.

Nerve-muscle involvement in a large family with mitochondrial cytopathy: electrophysiological studies

Thirteen patients with mitochondrial cytopathy were investigated. They represent different generations, ages, stages, and severities of the disease. All were assumed to have the same metabolic defect. The disease is a multisystem disorder with a metabolic defect located at complex 1 in the respiratory chain. Clinically, the disorder gives symptoms such as hearing loss, retinal pigmental degeneration, ataxia, cardiomyopathy, muscular fatiguability and neuropathy. The patients were investigated with nerve conduction studies, concentric needle EMG, SFEMG, and macro EMG examinations. Neurophysiologic studies revealed signs of myopathy in both the younger members and in those with slight muscular symptoms. In the more advanced stages, neuropathic changes of the axonal type were seen as well. Macro EMG was interpreted as indicating muscle fiber membrane abnormalities in the early stages. Single fiber EMG studies indicate that this metabolic defect does not disturb neuromuscular transmission.

Principal component analysis of the features of concentric needle EMG motor unit action potentials

Motor unit action potentials (MUAPs) were recorded from the biceps muscle of normal subjects and of patients with nerve or muscle diseases. Principal component analysis of the MUAP amplitude, area, area/amplitude ratio, duration, and the number of turns and phases produced three components that among them contained 90% of the variance of the data set. Thus the dimensionality of data was reduced from six to three. The first component reflected changes in the size of the MU, whereas the second reflected variations in the arrival time at the recording electrode of the action potentials of muscle fibers in the motor unit. The third factor reflected local loss of muscle fibers within the MU territory. Patterns of variations in the three components were different in patients with neuropathy and myopathy.

Estimation of the number of motor units based on macro-EMG

The technique of the macro-EMG was used to estimate the number of motor units in the tibialis anterior muscles of healthy subjects in a wide range of ages, and of patients with myasthenia gravis and patients with amyotrophic lateral sclerosis or spinal muscular atrophy. The results obtained suggest a decrease in the number of motor units in the tibialis anterior muscle with increasing age in normal subjects. In myasthenic patients the motor unit count was within the normal range for their age group. Patients with motor neuron disorders on the average had a very low number of motor units.

Afferent projections to human tibialis anterior motor units active at various levels of muscle contraction

The synaptic efficacy of muscle and cutaneous afferents on single tibialis anterior motoneurones in man was derived from changes in the firing probability of single, voluntarily activated, motor units in response to electrical stimulation of peripheral nerves or skin. The motor units were recorded with a Macro EMG electrode. The Macro motor unit potential (Macro MUP) recorded with this electrode reflects the electrical activity of all of the muscle fibres in a single motor unit. The amplitude of the Macro MUP is positively correlated with the recruitment threshold of the unit. Motor units with different Macro MUP amplitudes were examined at approximately the same level of voluntary contraction (less than 20% of maximum). The synaptic efficacy of muscle and cutaneous afferents was similar for units with small and with large Macro MUP amplitudes. Single motor units were examined at several different levels of muscle contraction. There was no consistent change in the facilitation from muscle afferents but there was less facilitation from cutaneous afferents during stronger contractions. This was not simply a consequence of the units faster firing rate. It is concluded that, with increasing voluntary drive to tibialis anterior motoneurones in man, there is a reduction in transmission in the pathways from cutaneous afferents to tibialis anterior motoneurones. There is no evidence that low and high threshold units (judging from their Macro MUP amplitudes) have different afferent connections.

Electrophysiological study of size and fibre distribution of motor units in the human masseter and temporal muscles

Probably related to their unique function, human jaw muscles have a specialized muscle-fibre composition different from that of limb and trunk muscles. In jaw muscle from healthy young adults, there were large groups of densely-packed fibres of the same histochemical type, a finding which indicates re-innervation when seen in limb muscles. The number of muscle fibres per motor unit, the local distribution of muscle fibres within motor units and the neuromuscular transmission in human masseter and temporal muscles were examined by single-fibre EMG and macro EMG. One hundred and forty-seven low-threshold motor units in four healthy subjects were recorded. The first dorsal interosseus muscle of the hand was similarly measured. In the jaw and hand muscles the low-threshold motor units were slightly smaller, than in large arm and leg muscles. The temporal muscle differed from the masseter in having significantly-higher fibre density, which may be explained by the higher frequency of small type-II fibres in the temporal causing closer packing of the type-I, low-threshold motor units. The fibre concentration within motor units was similar to that of normal limb muscles without any sign of grouping of fibres from the same motor unit. Normal motor end-plate transmission was another argument against re-innervation.