A review of techniques employed to estimate the number of motor units in a muscle

Toward the validation of a new method (MUNIX) for motor unit number assessment

INTRODUCTION

This prospectively designed study analyzed the correlation of a new, non-invasive neurophysiological method (Motor Unit Number Index - MUNIX) with two established Motor Unit Number Estimation (MUNE) methods.

METHODS

MUNIX and incremental stimulation MUNE (IS-MUNE) were done in the abductor digiti minimi muscle (ADM), while MUNIX and spike-triggered averaging MUNE (STA-MUNE) were tested in the trapezius muscle. Twenty healthy subjects and 17 patients with amyotrophic lateral sclerosis (ALS) were examined.

RESULTS

MUNIX and MUNE values correlated significantly (ADM: n=108; Spearman-Rho; r=0.88; p<0.01; trapezius muscle: n=49; Spearman-Rho; r=0.46; p<0.01).

DISCUSSION

MUNIX indeed reflects the number of motor units in a muscle, and may sensibly be recorded from the trapezius muscle. With MUNIX being both much more patient friendly and much more rapid to assess than MUNE, the results support the use of MUNIX when motor unit number assessment is desired.

Biomarkers in rare disorders: the experience with spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by homozygous mutations of the SMN1 gene. Based on clinical severity, three forms of SMA are recognized (type I–III). All patients have at least one (usually 2–4) copies of a highly homologous gene (SMN2) which produces insufficient levels of functional SMN protein, due to alternative splicing of exon7. Recently, evidence has been provided that SMN2 expression can be enhanced by different strategies. The availability of potential candidates to treat SMA has raised a number of issues, including the availability of data on the natural history of the disease, the reliability and sensitivity of outcome measures, the duration of the studies, and the number and clinical homogeneity of participating patients. Equally critical is the availability of reliable biomarkers. So far, different tools have been proposed as biomarkers in SMA, classifiable into two groups: instrumental (the Compound Motor Action Potential, the Motor Unit Number Estimation, and the Dual-energy X-ray absorptiometry) and molecular (SMN gene products dosage, either transcripts or protein). However, none of the biomarkers available so far can be considered the gold standard. Preclinical studies on SMA animal models and double-blind, placebo-controlled studies are crucial to evaluate the appropriateness of biomarkers, on the basis of correlations with clinical outcome.

Motor unit number estimation in the assessment of performance and function in motor neuron disease

Motor unit number estimation (MUNE) is a unique electrophysiologic test used to estimate the number of surviving motor units in a muscle or group of muscles. It is used most frequently to monitor lower motor neuron loss in amyotrophic lateral sclerosis and spinal muscle atrophy. Of particular interest is its use as an endpoint measure in clinical trials for these diseases. This article describes the principles of MUNE and the factors that need to be considered, and reviews several techniques that have been used in clinical trials and in monitoring progression. It then reviews experience with MUNE in clinical trials for amyotrophic lateral sclerosis and spinal muscle atrophy and discusses how MUNE correlates with measures of function.

Amplitude cancellation of motor-unit action potentials in the surface electromyogram can be estimated with spike-triggered averaging

The study presents analytical, simulation, and experimental analyses of amplitude cancellation of motor-unit action potentials (APs) in the interference electromyogram (EMG) and its relation to the size of the spike-triggered average (STA) EMG. The amount of cancellation of motor-unit APs decreases monotonically as a function of the ratio between the root mean square (RMS) of the motor-unit AP and the RMS of the interference EMG signal. The theoretical derivation of this association indicates a method to measure cancellation in individual motor units by STA of the interference and squared EMGs. The theoretical relation was examined in both simulated EMG signals generated by populations of 200 motor units and experimental recordings of 492 and 184 motor-unit APs in the vastus medialis and abductor digiti minimi muscles, respectively. Although the theoretical relation predicted (R2 = 0.95; P < 0.001) the amount of cancellation in the simulated EMGs, the presence of motor-unit synchronization decreased the strength of the association for small APs. The decrease in size of the STA obtained from the squared EMG relative to that extracted from the interference EMG was predicted by the experimental measure of cancellation (R2 = 0.65; P < 0.001, for vastus medialis; R2 = 0.26; P < 0.05, for abductor digiti minimi). The results indicate that cancellation of APs in the interference EMG can be analytically predicted and experimentally measured with STA from the discharge times of the motor units into the surface EMG.

Motor unit number estimation by decomposition-enhanced spike-triggered averaging: Control data, test-retest reliability, and contractile level effects

Decomposition-enhanced spike-triggered averaging (DE-STA) has been developed as a method for obtaining a motor unit number estimate (MUNE). We describe the method and report control data for the first dorsal interosseous/adductor pollicis and thenar muscles and reliability in the thenar muscles. Seventeen subjects (ages 20-50 years) took part in the study. The maximum M potential was elicited with supramaximal stimulation of the ulnar or median nerve at the wrist. Surface and intramuscularly detected electromyographic signals were then collected simultaneously during mild to moderate contractions. Decomposition algorithms were used to detect and sort the individual motor unit potential (MUP) occurrences of several concurrently active motor units in the needle-detected signals. The MUP occurrences were used as triggering sources to estimate their corresponding surface-detected MUPs (S-MUPs) using STA. The mean S-MUP size was calculated and divided into the maximum M-potential size to derive a MUNE. The MUNE values were consistent with those previously reported with other methods, and thenar MUNEs for the two trials were similar (249 +/- 78 and 246 +/- 90), with high test-retest reliability (r = 0.94, P < 0.05). DE-STA thus appears to be a valid and reliable method to obtain MUNEs.

Motor unit number estimation: new techniques and new uses

MUNE is a unique neurophysiologic tool because it can quantitatively estimate the number of motor neurons innervating a muscle or group of muscles. All other neurophysiologic techniques are influenced by collateral reinnervation and provide only a qualitative estimate of motor unit loss. Further, the S-MUPs obtained with MUNE provide quantitative information about the whole motor unit. Other routine neurophysiologic techniques provide information restricted to a portion of the motor unit. These unique features of MUNE have been applied to neurogenic disorders to yield a better understanding of disease processes. Various modifications are being developed that will provide more data and ease of use. It is anticipated that the availability of MUNE on EMG machines will grow and it use will expand from a research tool to a routine neurophysiologic test.

Motor unit number estimation in infants and children with spinal muscular atrophy

Spinal muscular atrophy (SMA) is a disease of lower motor neurons. Motor unit number estimation (MUNE) is an electrophysiologic method to estimate the number of motor neurons innervating a muscle group. We applied the multiple point stimulation technique to the ulnar nerve--hypothenar muscle group to study lower motor neuron loss in 14 SMA subjects, including those presymptomatic, and varying from newborn through 45 years of age. Preliminary data support the value of MUNE to help understand the time course of motor neuron loss in SMA.

Electrophysiologic endpoint measures in a multicenter ALS drug trial

We report the analysis of a battery of secondary electrophysiologic measurements to assess the progression of amyotrophic lateral sclerosis (ALS) in a two center, six month, double-blind, three arm trial comparing branched chain amino acids to L-threonine with pyridoxal 5-phosphate to placebo. The endpoint measurements were chosen to separately assess the effects of lower motor neuron loss and collateral reinnervation. For tests of inter-center reliability, we found no differences that could not be readily explained by variations in electrophysiologic testing techniques. Since the drug study was negative for the primary endpoint measure (muscle strength), we combined data from both centers and the three treatment arms. For measures of progression, all measures changed in the expected direction during the 6 months of the trial. We conclude that a battery of electrophysiologic measures can be used in a multicenter ALS drug trial to provide information on changes in lower motor neuron numbers and the effects of collateral reinnervation.

Electrodiagnostic studies in clinical trials for motor neuron disease

Motor neuron disease (MND) is a group of neurodegenerative disorders characterized by death of upper motor neurons (UMN) and lower motor neurons (LMN). Clinical study of UMN loss is limited, but electrodiagnostic studies can be used readily to assess the functional state of LMN. Electrodiagnostic studies are regularly used in making the diagnosis of amyotrophic lateral sclerosis (ALS) and other forms of MND. They can also be used to follow disease progression and serve as end point measures in drug trials. In this role, electrodiagnostic studies can provide specific information not readily available from measurement of muscle strength or clinical functional scales. This article emphasizes the primary pathologic and secondary physiologic changes that take place after LMN loss and how they can be assessed by electrodiagnostic studies. It reviews the uses of routine electrodiagnostic studies for the diagnosis of MND but focuses on how special electrodiagnostic studies can be used as end point measures in drug trials.

Validity of electromyograms and tension as a means of motor unit number estimation

The purpose of this study was to validate three different techniques for obtaining motor unit number estimates of the rat medial gastrocnemius muscle. These consisted of two electromyographic techniques using unprocessed and digitally averaged unitary muscle action potentials, and one mechanical technique. We also injected subunit B of cholera toxin into this muscle and counted the number of spinal motor neurons labeled by the toxin. Our results revealed that a motor unit number estimate obtained by using the unprocessed unitary muscle action potential was statistically different from the actual number of motor neurons. The other two motor unit number estimates, however, were not statistically different from the actual motor neuron number. These two methods thus seem more appropriate than the first electromyographic method for obtaining an accurate motor unit number estimate.

Motor unit number estimation by spatial-temporal summation of single motor unit potentials

Current techniques for motor unit number estimation (MUNE) rely on the amplitude of the compound muscle action potential (CMAP) evoked by supramaximal stimulation and mean amplitude of single motor unit potentials (SMUPs). The phase cancellation during summation is not considered. We developed a technique to address this issue. Slow and fast types of motor unit potentials were collected from 5 normal subjects from their abductor pollicis brevis muscles by low-level voluntary contractions, and near-threshold nerve stimulation, respectively. Two of each type of SMUPs were used as templates for reconstructing the best fitted CMAP using a feed-forward neural network. The total number of SMUPs simulated from the four templates during the reconstruction served as MUNE. The mean MUNE was 222 +/- 98. The technique is simple and noninvasive, and may be applied in the future for MUNE in patients.

Motor unit number estimation: sample size considerations

A computer model of the motor unit number estimation procedure was developed to evaluate the sampling error associated with estimates of the number of motor units in muscles. Two different distributions were used to model the motor unit amplitude distribution and were chosen in such a manner that they qualitatively matched the distributions observed under both normal and neurogenic conditions. As expected, the results indicated that estimation error decreases as a function of sample size. However, the relationship between these two variables was nonlinear in the sense that successive increases in sample size lead to progressively smaller decreases in estimation error. The results also indicated that the shape of the motor unit amplitude distribution plays an important role. Specifically, estimates obtained using the distribution modeling normal muscle were generally higher than the actual number of motor units in the muscle, which was not the case for the distribution modeling neurogenic muscle. In addition, the neurogenic distribution was associated with much smaller estimation error, suggesting that motor unit number estimation is well suited to the analysis of neurogenic disease processes.

Thenar motor unit number estimates using the multiple point stimulation technique: reproducibility studies in ALS patients and normal subjects

Thenar motor unit number estimate (MUNE) reproducibility was assessed in 20 patients with amyotrophic lateral sclerosis (ALS) and 16 normal subjects using the multiple point stimulation (MPS) technique. The MUNE was calculated by dividing the thenar compound muscle action potential negative-peak (n-p) area by the mean n-p area of 10 lowest threshold, all-or-nothing, surface-recorded motor unit action potentials. Two trials (test-retest) were performed by the same examiner either on separate days or on the same day with new electrode placements. The mean test MUNE was 43.4 (SD: 35.9, range: 6-145) for ALS patients and 219.4 (SD: 80.8, range: 122-368) for normal subjects. Test-retest MUNE differences were not significant for ALS patients or normal subjects. The test-retest correlation coefficient (r) was 0.99 for ALS patients and 0.85 for normal subjects. The mean difference between test-retest values was 10% for ALS patients and 17% for normal subjects. Test-retest reproducibility of the thenar MUNE using the MPS technique is high in both ALS patients and normal subjects. The reliability of the MPS technique in estimating motor unit numbers may make it a useful outcome measure in following the course of patients with progressive lower motor neuron disease, especially those enrolled in experimental drug trials.

A new approach to motor unit estimation with surface EMG triggered averaging technique

A new method for estimating the number of motor units using a surface EMG triggered averaging technique is described. This method provides an estimation of mean motor unit potential (MUP) amplitude at different levels of contraction, which can be utilized to estimate the number of motor units in a given muscle. Motor unit count estimated in abductor pollicis brevis (APB) muscle of 11 normal healthy subjects ranged from 131 to 371 with a mean of 246 +/- 68. In our preliminary study of patients with lower motor neuron lesions, there was a significant reduction in the number of motor units. We believe our new noninvasive method of motor unit counting is a relatively simple and reproducible physiological technique.

Sources of error in the spike-triggered averaging method of motor unit number estimation (MUNE)

Motor unit number estimation (MUNE) is an electrophysical technique to estimate the number of motor units innervating a muscle or muscle group. MUNE may be useful as a measure of progression of lower motor neuron loss in amyotrophic lateral sclerosis (ALS). Several methods of MUNE have been developed. The spike-triggered averaging method can be readily performed on EMG machines with signal averaging capabilities and is suitable for estimating the number of motor neurons innervating proximal muscles. We have used MUNE as a measure of disease state in a drug efficacy trial for ALS. From our experience with this method we have identified sources of error which can affect MUNE accuracy. We have investigated these sources and report their effect on MUNE.

Recording characteristics of the surface EMG electrodes

Routine motor nerve conduction studies are conducted using surface EMG electrodes. Most techniques of estimating the number of motor units (MUs) are based on surface EMG recordings. Therefore, it is important to assess the uptake area of these electrodes. We recorded surface EMG motor unit action potentials (SMUAPs) from the biceps muscle of normal subjects. The SMUAP amplitude fell from 42 microV for the superficially located MUs (i.e., within 10 mm of skin surface) to 11 microV for the deep MUs (i.e., more than 20 mm from the skin surface). We infer that the pickup radius of the surface electrode is less than 20 mm. The implications of the limited uptake area of the surface electrodes to the analysis of compound muscle action potentials, estimation of the number of MUs, and the surface EMG recordings are discussed.

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 (S-MUAP) 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.

AAEM case report #5: Amyotrophic lateral sclerosis

A 40-year-old man presented with a gradual onset of gait unsteadiness and weakness in the arms. The stretch reflexes were normal in the upper extremities but hyperactive in the lower extremities with bilateral Babinski signs. A myelogram revealed a partial obstruction at C-5-6. Two prior electromyograms, 7 and 5 months prior to admission, reportedly showed positive waves only in two peroneal supplied muscles. Repeat electromyographic testing demonstrated normal nerve conduction velocities and needle electrode abnormalities in upper and lower extremities as well as thoracic paraspinal muscles allowing a diagnosis of amyotrophic lateral sclerosis (ALS). The importance of electromyographic testing in clinically nonaffected areas is stressed as well as its role in patients presenting with upper motor neuron signs. It is the task of the clinical electromyographer to consider other entities in the differential diagnosis, such as a multifocal motor neuropathy with conduction blocks and design the tests accordingly. The role of electromyography in the prediction of the course of ALS by assessing the degree of reinnervation is discussed. This will become increasingly important in the design of treatment trials.

Motor unit estimation: reproducibility of the spike-triggered averaging technique in normal and ALS subjects

Reproducibility of the spike-triggered averaging technique of motor unit estimation (MUE) was assessed in biceps-brachialis muscle in 10 normal subjects and 15 subjects with amyotrophic lateral sclerosis (ALS). MUE was calculated by dividing the compound muscle action potential by the mean amplitude of 15 surface motor unit potentials (S-MUPs) of low recruitment threshold. Averaged MUE values in normal subjects were higher than in ALS subjects, with few values overlapping. Differences between test and retest MUE values were not significant for either subject group. The relative differences between test-retest values were 45.3% for normal subjects and 32.6% for ALS subjects. Correlation coefficients between test and retest values were low (r = 0.07) for normal subjects when influential outlying points were removed, and higher (r = 0.65) for ALS subjects when individuals with MUE values within the normal range were removed. The higher correlation of test-retest MUE values in ALS subjects compared to normal subjects may be due to a greater probability of resampling among the smaller number of motor units in ALS subjects. In summary, the reproducibility and technical aspects of the spike-triggered averaging technique are similar to those reported for other MUE techniques.

The estimated numbers and relative sizes of thenar motor units as selected by multiple point stimulation in young and older adults

Multiple point stimulation (MPS) is described as a method of estimating the numbers of motor units in the median innervated thenar muscles of young and older control subjects. Stimulation at multiple sites along the course of the median nerve was employed to collect a sample of the lowest threshold, all-or-nothing surface-recorded motor unit action potentials (S-MUAPs). The average, negative peak area, and peak-to-peak amplitude of the sample of S-MUAPs was determined and divided into the corresponding value for the maximal compound muscle action potential to derive the motor unit estimate (MUE). In 37 trials from 17 younger subjects (20-40 years), the mean MUE was 288 +/- 95 SD based on negative peak area and, in 33 trials from 20 older subjects, mean values were 139 +/- 68. In 23 young and older subjects, MPS was performed on at least two occasions and the MUEs were found to be highly correlated (r = 0.88).

A pilot trial of dextromethorphan in amyotrophic lateral sclerosis

Assuming the presence of glutamate-induced neurotoxicity in amyotrophic lateral sclerosis 14 patients were treated with dextromethorphan, an N-methyl-D-aspartate receptor antagonist. The patients were treated with 150 mg dextromethorphan or placebo daily for 12 weeks in a double-blind crossover trial, with a wash out period of 4 weeks between the two treatment periods. Thereafter the surviving patients were treated with 300 mg dextromethorphan daily for up to 6 months in an open trial. No positive effects on clinical or neurophysiological parameters (relative number of axons, and compound muscle action potentials in the abductor digiti minimi muscle) were observed either in the double-blind trial or in the open trial.