Estimation of contractile parameters of successive twitches in unfused tetanic contractions of single motor units - A proof-of-concept study using ultrafast ultrasound imaging in vivo

During a voluntary contraction, motor units (MUs) fire a train of action potentials, causing summation of the twitch forces, resulting in fused or unfused tetanus. Twitches have been important in studying whole-muscle contractile properties and differentiation between MU types. However, there are still knowledge gaps concerning the voluntary force generation mechanisms. Current methods rely on the spike-triggered averaging technique, which cannot track changes in successive twitches' properties in response to individual neural firings. This study proposes a method that estimates successive twitches contractile parameters of single MUs during low force voluntary isometric contractions in human biceps brachii. We used a previously developed ultrafast ultrasound imaging method to estimate unfused tetanic activity signals of single MUs. A twitch decomposition model was used to decompose unfused tetanic activity signals into individual twitches. This study found that the contractile parameters varied within and across MUs. There was an association between the inter-spike interval and the contraction time (r = 0.49,p < 0.001) and the half-relaxation time (r = 0.58,p < 0.001), respectively. The method shows the proof-of-concept to study MU contractile properties of individual twitches in vivo, which can provide further insights into the force generation mechanisms of voluntary contractions and response to individual neural discharges.

Reinnervation as measured by the motor unit size index is associated with preservation of muscle strength in amyotrophic lateral sclerosis, but not all muscles reinnervate

INTRODUCTION/AIMS

The motor unit size index (MUSIX) may provide insight into reinnervation patterns in diseases such as amyotrophic lateral sclerosis (ALS). However, it is not known whether MUSIX detects clinically relevant changes in reinnervation, or if all muscles manifest changes in MUSIX in response to reinnervation after motor unit loss.

METHODS

Fifty-seven patients with ALS were assessed at 3-month intervals for 12 months in four centers. Muscles examined were abductor pollicis brevis, abductor digiti minimi, biceps brachii, and tibialis anterior. Results were split into two groups: muscles with increases in MUSIX and those without increases. Longitudinal changes in MUSIX, motor unit number index (MUNIX), compound muscle action potential amplitude, and Medical Research Council strength score were investigated.

RESULTS

One hundred thirty-three muscles were examined. Fifty-nine percent of the muscles exhibited an increase in MUSIX during the study. Muscles with MUSIX increases lost more motor units (58% decline in MUNIX at 12 months, P < .001) than muscles that did not increase MUSIX (34.6% decline in MUNIX at 12 months, P < .001). However, longitudinal changes in muscle strength were similar. When motor unit loss was similar, the absence of a MUSIX increase was associated with a significantly greater loss of muscle strength (P = .002).

DISCUSSION

MUSIX increases are associated with greater motor unit loss but relative preservation of muscle strength. Thus, MUSIX appears to be measuring a clinically relevant response that can provide a quantitative outcome measure of reinnervation in clinical trials. Furthermore, MUSIX suggests that reinnervation may play a major role in determining the progression of weakness.

Identification of single motor units in skeletal muscle under low force isometric voluntary contractions using ultrafast ultrasound

The central nervous system (CNS) controls skeletal muscles by the recruitment of motor units (MUs). Understanding MU function is critical in the diagnosis of neuromuscular diseases, exercise physiology and sports, and rehabilitation medicine. Recording and analyzing the MUs’ electrical depolarization is the basis for state-of-the-art methods. Ultrafast ultrasound is a method that has the potential to study MUs because of the electrical depolarizations and consequent mechanical twitches. In this study, we evaluate if single MUs and their mechanical twitches can be identified using ultrafast ultrasound imaging of voluntary contractions. We compared decomposed spatio-temporal components of ultrasound image sequences against the gold standard needle electromyography. We found that 31% of the MUs could be successfully located and their firing pattern extracted. This method allows new non-invasive opportunities to study mechanical properties of MUs and the CNS control in neuromuscular physiology.

Concentric Needle Jitter in 97 Myasthenia Gravis Patients

Objectives: To estimate the jitter parameters (single-fiber electromyography) in myasthenia gravis patients mostly by electrical activation in Frontalis, Orbicularis Oculi, and Extensor Digitorum muscles using a concentric needle electrode. Methods: Between 2009 and 2019, a total of 97 myasthenia gravis patients, 52 male, and mean age 54 years were included. Results: Any abnormal jitter parameter in individual muscles was 90.5% (Frontalis), 88.5% (Orbicularis Oculi), and 86.6% (Extensor Digitorum). Any jitter parameter combining Orbicularis Oculi and Frontalis muscle was abnormal in 100% for the ocular, and in 92.9% for the generalized myasthenia gravis. The most abnormal muscle was Orbicularis Oculi for the generalized, and Frontalis for the ocular myasthenia gravis. The decrement was abnormal in 78.4%, 85.9% for the generalized, and 25% for the ocular myasthenia gravis. The mean jitter ranged from 14.2 to 86 μs (mean 33.3 μs) for the ocular myasthenia gravis and from 14.4 to 220.4 μs (mean 66.3 μs) for the generalized myasthenia gravis. The antibody titers tested positive in 86.6%, 91.8% for the generalized, and 50% for the ocular myasthenia gravis. Thymectomy was done in 48.5%, thymoma was found in 19.6%, and myasthenic crisis occurred by 21.6%. Conclusion: The jitter parameters achieved a 100% abnormality in ocular myasthenia gravis if both the Orbicularis Oculi and Frontalis muscles were tested. There was a high jitter abnormality in generalized myasthenia gravis cases with one muscle tested, with about a 2% increase in sensitivity when a second is added. Concentric needle electrode jitter had high sensitivity similar to the single fiber electrode (93.8%), followed by antibody titers (86.6%), and abnormal decrement (78.4%).

Appropriate window width for the "clustering index method" in the tibialis anterior muscle

We previously reported a new quantitative analysis of single-channel surface electromyography (EMG), the "clustering index method" (CI method), in the tibialis anterior muscle, which achieved sufficiently good sensitivity to detect neurogenic or myogenic abnormalities. The window width is a fundamental parameter of the CI method, and was arbitrarily set at 15 ms in that study. In this study, we searched for the most appropriate window width using expanded patient data. The data from our previous study were reanalyzed, and new patients were enrolled. Window width in the CI method was changed from 5 to 27.5 ms with a step of 2.5 ms. For each window width, Z-score values of individual subjects were calculated and the diagnostic yield was investigated. We enrolled 67 controls, 29 subjects with neurogenic disorders, and 39 with myogenic disorders. When the window width was set at 22.5 ms, the highest sensitivity was achieved both for neurogenic (97%) and myogenic (72%) disorders, with a specificity of 97%. Seven of 10 patients with inclusion body myositis were also abnormal. Reliable results were obtained by collecting 15 epochs per subject. There are two conflicting effects that appear to be best balanced at a window width of 22.5 ms: a wider width decreases the chance that a motor unit potential (MUP) is divided into two adjacent windows, and a narrower width reduces the possibility that an MUP firing at a low-frequency is counted twice by the differential sequences. CI is promising as a non-invasive method to diagnose neuromuscular disorders.

Updated size index valid for both neurogenic and myogenic changes

BACKGROUND

Size index (SI) is a motor unit potential (MUP) parameter in concentric needle electromyography calculated from amplitude and area/amplitude, which can sensitively discriminate between control and neurogenic MUPs. In this study, we investigated the application of SI to myogenic MUPs based on expanded data.

METHODS

MUPs were collected from the biceps brachii (BB) and tibialis anterior (TA) muscles. Muscles showing unequivocal neurogenic or myogenic changes by visual inspection were selected for patients. In addition to the original SI, a revised SI (rSI) was defined using the logarithmic scale for area/amplitude. The coefficient for area/amplitude was varied and that achieving the best sensitivity both for BB and TA was selected.

RESULTS

Analyzed were 1619, 340, and 498 MUPs from the BB of 26, 10, and 14 subjects (control, neurogenic, and myogenic), respectively, and 1245, 536, and 473 MUPs from the TA of 23, 18, and 13 subjects (control, neurogenic, and myogenic), respectively. For neurogenic MUPs, the original SI and the newly defined rSIn were similarly sensitive (82.1% and 81.8% sensitivity for SI and rSIn, respectively, for BB, and 68.1% and 69.6% for TA), and were more sensitive than area (72.6% for BB and 57.6% for TA), the most sensitive parameter among conventional ones. For myogenic MUPs, the sensitivity of rSIm was 9.0% for BB and 24.5% for TA, which was not significantly different from duration (7.4% for BB and 21.8% for TA), the most sensitive parameter among conventional ones.

CONCLUSIONS

SI, rSIn, and rSIm are promising as new MUP parameters.

Standards of instrumentation of EMG

Standardization of Electromyography (EMG) instrumentation is of particular importance to ensure high quality recordings. This consensus report on "Standards of Instrumentation of EMG" is an update and extension of the earlier IFCN Guidelines published in 1999. First, a panel of experts in different fields from different geographical distributions was invited to submit a section on their particular interest and expertise. Then, the merged document was circulated for comments and edits until a consensus emerged. The first sections in this document cover technical aspects such as instrumentation, EMG hardware and software including amplifiers and filters, digital signal analysis and instrumentation settings. Other sections cover the topics such as temporary storage, trigger and delay line, averaging, electrode types, stimulation techniques for optimal and standardised EMG examinations, and the artefacts electromyographers may face and safety rules they should follow. Finally, storage of data and databases, report generators and external communication are summarized.

Guidelines for single fiber EMG

This document is the consensus of international experts on the current status of Single Fiber EMG (SFEMG) and the measurement of neuromuscular jitter with concentric needle electrodes (CNE - CN-jitter). The panel of authors was chosen based on their particular interests and previous publications within a specific area of SFEMG or CN-jitter. Each member of the panel was asked to submit a section on their particular area of interest and these submissions were circulated among the panel members for edits and comments. This process continued until a consensus was reached. Donald Sanders and Erik Stålberg then edited the final document.

Between genetics and biology. Is ENMG useful in peripheral neuropathy diagnosis and management?

Neurography and EMG are complementary techniques used in the diagnosis and monitoring of neuropathies. Both assess function of the peripheral nervous system and provide clinically useful information regarding the functional status of peripheral nerves. This information is not readily obtainable using biochemical, genetic or imaging techniques. I will discuss the role of these techniques in the diagnosis and management of neuropathies and some limitations of these techniques. These methods are routinely used in an EMG lab. These are most useful when used in conjunction with clinical examination to answer a well-defined clinical question. Reference values are required for interpretation of the data.

Tracking motor neuron loss in a set of six muscles in amyotrophic lateral sclerosis using the Motor Unit Number Index (MUNIX): a 15-month longitudinal multicentre trial

BACKGROUND

Motor Unit Number Index (MUNIX) is a novel neurophysiological measure that provides an index of the number of functional lower motor neurons in a given muscle. So far its performance across centres in patients with amyotrophic lateral sclerosis (ALS) has not been investigated.

OBJECTIVE

To perform longitudinal MUNIX recordings in a set of muscles in a multicentre setting in order to evaluate its value as a marker of disease progression.

METHODS

Three centres applied MUNIX in 51 ALS patients over 15 months. Six different muscles (abductor pollicis brevis, abductor digiti minimi, biceps brachii, tibialis anterior, extensor dig. brevis, abductor hallucis) were measured every 3 months on the less affected side. The decline between MUNIX and ALSFRS-R was compared.

RESULTS

31 participants reached month 12. For all participants, ALSFRS-R declined at a rate of 2.3%/month. Using the total score of all muscles, MUNIX declined significantly faster by 3.2%/month (p ≤ 0.02). MUNIX in individual muscles declined between 2.4% and 4.2%, which differed from ASLFRS-R decline starting from month 3 (p ≤ 0.05 to 0.002). Subgroups with bulbar, lower and upper limb onset showed different decline rates of ALSFRS-R between 1.9% and 2.8%/month, while MUNIX total scores showed similar decline rates over all subgroups. Mean intraclass correlation coefficient for MUNIX intra-rater reliability was 0.89 and for inter-rater reliability 0.80.

CONCLUSION

MUNIX is a reliable electrophysiological biomarker to track lower motor neuron loss in ALS.

Jitter analysis with concentric needle electrodes

Because of certain restrictions in medical praxis, reusable materials are only allowed in some countries. This also applies to electrodes for electromyography; the special single-fiber electromyography electrode must be replaced. This article gives some details of the possibilities of using an alternative-a small concentric needle electrode. Practical hints, reference values, and the application in diagnostic work for myasthenia gravis are summarized.

Stimulated jitter with concentric needle in 42 myasthenia gravis patients

Objective: To estimate jitter parameters in myasthenia gravis in stimulated frontalis and extensor digitorum muscles using the concentric needle electrode. Methods: Forty-two confirmed myasthenia gravis patients, being 22 males (aged 45.6±17.2 years-old) were studied. Jitter was expressed as the mean consecutive difference (MCD). Results: MCD in extensor digitorum was 61.6 µs (abnormal in 85.7%) and in frontalis 57.3 µs (abnormal in 88.1%). Outliers represented 90.5% for extensor digitorum and 88.1% for frontalis. At least one jitter parameter was abnormal in 90.5% of the combined studies. Acetylcholine receptor antibody was abnormal in 85.7% of the cases. Conclusions: Stimulated jitter recordings measured from muscles using concentric needle electrode can be used for myasthenia gravis diagnosis with high sensitivity. Extensive normative studies are still lacking and, therefore, borderline findings should be judged with great caution.

Peripheral neuropathy caused by severe hypothermia

OBJECTIVE

To report follow-up data in the evaluation of peripheral neuropathy in a 29-year old female after accidental deep hypothermia (13.7°C) in 1999.

METHODS

Nerve conduction studies (NCS) and electromyography (EMG) were performed 20 days after the accident and again after 5 months and 1, 3, 5 and 11 years. Macro EMG was performed after 3, 5 and 11 years. To evaluate small fiber function, RR-interval, sympathetic skin response, quantitative sensory testing and skin biopsy for quantification of intra-epidermal nerve fiber density were performed in 2009.

RESULTS

In the intensive care unit sensory and motor responses were absent except for the tibial nerves, and EMG showed profuse denervation. Improvement of amplitudes and conduction velocities was seen during the first 5 years. Muscular atrophy of hand muscles persisted. Large fibers were involved more extensively than small fibers.

CONCLUSIONS

A severe axonal sensorimotor polyneuropathy developed in the intensive care unit following severe hypothermia. The mechanism was most likely cold injury to peripheral nerves.

SIGNIFICANCE

The clinical picture and the laboratory findings indicate that even multi-organ dysfunction and, of specific interest in this study, a severe axonal degeneration may come to a good restitution after long time.

Evaluation of spinal and bulbar muscular atrophy by the clustering index method

INTRODUCTION

A reliable electrophysiological marker for clinical trials is increasingly needed in spinal and bulbar muscular atrophy (SBMA). We previously developed a quantitative analysis method for surface electromyography (SEMG), the clustering index (CI) method. Our purpose was to test the utility of the CI method for evaluating lower motor neuron involvement in SBMA patients.

METHODS

Subjects included 29 SBMA patients and 27 healthy controls. The recording electrode was placed over the abductor digiti minimi (ADM) muscle with a proximal reference. The Z-score, based on the CI method, was compared with compound muscle action potential (CMAP) amplitude and motor unit number estimation (MUNE), with regard to sensitivity.

RESULTS

The Z-scores of the CI method, CMAP amplitude, and MUNE were abnormal in 100%, 72%, and 93% of the patients, respectively. Interrater reliability of the CI method was sufficiently high.

CONCLUSION

The CI method is promising as a non-invasive electrophysiological marker in SBMA.

Single-fiber conduction velocity test allows earlier detection of abnormalities in diabetes

INTRODUCTION

The purpose of this study was to determine whether single-fiber conduction velocity (SF-CV) of a small number of axons increases sensitivity for identification of motor nerve conduction alterations in patients with diabetes.

METHODS

Twenty-one consecutive diabetic patients in good metabolic control were studied. For each patient, conventional (C-CV) and SF-CV results were correlated with the presence of neuropathic symptoms.

RESULTS

Nine of 21 patients reported symptoms suggestive of mild nerve impairment. Three patients had abnormal sural nerve CV, 1 of whom also had abnormal motor nerve conduction. Eighteen patients had normal findings on conventional tests, 3 of whom had slowing of SF-CV.

CONCLUSIONS

SF-CV is able to detect mild myelin damage with higher sensitivity than conventional tests. The use of SF-CV may be a helpful tool in the early identification of diabetic polyneuropathy, and it may be useful for tailoring an approach to diabetic polyneuropathy.

Macro electromyography and motor unit number index in the tibialis anterior muscle: differences and similarities in characterizing motor unit properties in prior polio

Our objective was to establish the usefulness of the noninvasive method of the motor unit number index (MUNIX) in a large muscle and to study how macro electromyography (EMG) and MUNIX complement each other in describing the motor units (MUs) in prior polio. MUNIX and macro EMG were performed in 48 tibialis anterior muscles in 33 prior polio patients. In addition, the reproducibility of MUNIX was investigated. It is shown that MUNIX can be used to characterize MUs with high reproducibility, even in a large muscle. As judged by MUNIX values, the patients had a 25% reduction of motor neurons, whereas the macro EMG indicated a loss of 60% of the neurons. Macro EMG showed more pronounced changes compared with control material than the MUNIX. One of the reasons for this finding may be the difference in MU populations studied with the two methods.

Polyneuropathy in type 1 and type 2 diabetes: comparison of nerve conduction studies, thermal perception thresholds and intraepidermal nerve fibre densities

BACKGROUND

To evaluate possible differences in distal polyneuropathy (PN) characteristics and degree of abnormalities for various small and large fibre parameters in diabetes type 1 (DM1) and type 2 (DM2).

METHODS

Sixty-six DM1 and 57 DM2 patients with or without PN symptoms were included. Nerve conduction studies (NCS), quantitative sensory testing (QST) and quantification of intraepidermal nerve fibres (IENFs) were performed. Z-scores were calculated from reference materials.

RESULTS

In both groups, 42% had abnormal NCS classification, 42% (DM1) and 39% (DM2) abnormal QST, as well as 40% (DM1) and 32% (DM2) abnormal IENF density. Seventy percent (DM1) and 65% (DM2) had one of the three tests abnormal (differences not significant). Correlations were found between most Z-score parameters and disease duration and HbA1c in DM1, but fewer in DM2. In multivariate analysis, some NCS and QST Z-scores were more abnormal in DM2. Symptom scoring correlated better with NCS and QST parameters in DM1.

CONCLUSIONS

The differences could be referred to disease duration, glycaemic control and possibly patient age. The various parameters from NCS, QST and IENF analysis contribute differently in the assessment of polyneuropathy.

The difficulty in confirming clinical diagnosis of myasthenia gravis in a seronegative patient: a possible neurophysiological approach

In seronegative myasthenia gravis repetitive nerve stimulation and single-fibre EMG have a crucial diagnostic value but they may be negative, particularly in repetitive nerve stimulation studies. We report the case of a 43-year-old patient with generalized seronegative myasthenia gravis with negative 3 Hz repetitive nerve stimulation at Erb's point and voluntary single-fibre EMG in the orbicularis oculi. We also performed 6 and 12 Hz repetitive nerve stimulation at Erb and stimulated single-fibre EMG in the extensor digitorum communis and our findings were pathological. Our data suggest that, for individual patients with an atypical picture characterised by dissociation between a severe clinical pattern and no definite neurophysiological findings on conventional tests, repetitive nerve stimulation with a stimulation rate higher than 3 Hz and/or stimulated single-fibre EMG with an increasing stimulation rate may be helpful.

Acetylcholinesterase inhibitors in MG: to be or not to be?

Myasthenia gravis (MG) is an autoimmune disorder usually caused by antibodies against either the acetylcholine receptor (AChR) or muscle-specific tyrosine kinase (MuSK) at the neuromuscular junction. Neuromuscular transmission failure results in muscle fatigue and weakness that can be treated symptomatically with acetylcholinesterase inhibitors (AChEIs). Long-term treatment with nonselective AChEIs may have considerable drawbacks; thus, this medication is ideally tapered when strength improves. Patients with AChR antibodies respond beneficially to treatment, whereas patients with MuSK antibodies generally do not. Recently, the selective AChEI EN101, which specifically targets the isoform of "read-through" AChE (AChE-R), has been developed and may be of importance for symptomatic relief in AChR-antibody seropositive MG. This article is a review of the mechanisms, therapeutic effects, and drawbacks, with both old and new AChEIs in MG.

Quantitative sensory testing in patients with polyneuropathy and healthy individuals

AIMS

Elderly individuals and patients with polyneuropathy often feel heat pain or burning sensation on quantitative sensory testing (QST) of warm perception distally in the lower limbs. We therefore studied heat pain threshold (HPT), warm perception threshold (WPT) and the difference between heat pain and warm perception thresholds in 48 patients with symptoms and signs of polyneuropathy matched according to age and gender with 48 healthy persons.

METHODS

QST (using method of limits) was performed on the distal calf and the dorsal foot.

RESULTS

Particularly in the neuropathy group several individuals (58%) had an unpleasant feeling, often burning, when the thresholds according to the WPT algorithm were recorded. Difference between heat pain and warm perception thresholds in the lower calf of the patients was 3.9 +/- 3.5 and 5.8 +/- 3.4 degrees C in the controls (P = 0.012), and on the foot 3.8 +/- 2.8 vs 5.3 +/- 3.6 degrees C (P = 0.02).

CONCLUSIONS

When performing QST it is important to assess also quality features of warm perception, such as burning and heat pain sensation.