The utility of motor unit number index: A systematic review

Motor Unit Number Index of the Upper Trapezius: A Meta-Analysis and Cross-sectional Study of Its Reliability

Motor unit number index of the upper trapezius (MUNIX-Trapezius) is a candidate biomarker for bulbar lower motor neuron function; however, reliability data is incomplete. To assess MUNIX-Trapezius reliability in controls, we conducted a systematic review, a cross-sectional study (n = 20), and a meta-analysis. We demonstrated a high inter- and intra-rater intraclass correlation (0.86 and 0.94, respectively), indicating that MUNIX-Trapezius is reliable with between-study variability moderated by age and MUNIX technique. With further validation, this measure can serve as a disease monitoring and response biomarker of bulbar function in the therapeutic development for amyotrophic lateral sclerosis.

MUNIX repeatability evaluation method based on FastICA demixing

To enhance the reproducibility of motor unit number index (MUNIX) for evaluating neurological disease progression, this paper proposes a negative entropy-based fast independent component analysis (FastICA) demixing method to assess MUNIX reproducibility in the presence of inter-channel mixing of electromyography (EMG) signals acquired by high-density electrodes. First, composite surface EMG (sEMG) signals were obtained using high-density surface electrodes. Second, the FastICA algorithm based on negative entropy was employed to determine the orthogonal projection matrix that minimizes the negative entropy of the projected signal and effectively separates mixed sEMG signals. Finally, the proposed experimental approach was validated by introducing an interrelationship criterion to quantify independence between adjacent channel EMG signals, measuring MUNIX repeatability using coefficient of variation (CV), and determining motor unit number and size through MUNIX. Results analysis shows that the inclusion of the full (128) channel sEMG information leads to a reduction in CV value by $1.5 \pm 0.1$ and a linear decline in CV value with an increase in the number of channels. The correlation between adjacent channels in participants decreases by $0.12 \pm 0.05$ as the number of channels gradually increases. The results demonstrate a significant reduction in the number of interrelationships between sEMG signals following negative entropy-based FastICA processing, compared to the mixed sEMG signals. Moreover, this decrease in interrelationships becomes more pronounced with an increasing number of channels. Additionally, the CV of MUNIX gradually decreases with an increase in the number of channels, thereby optimizing the issue of abnormal MUNIX repeatability patterns and further enhancing the reproducibility of MUNIX based on high-density surface EMG signals.

A Novel Analysis of CMAP Scans From Perspective of Information Theory: CMAP Distribution Index (CDIX)

OBJECTIVE

The compound muscle action potential (CMAP) scan is a useful technique for examination of neuromuscular disorders. The objective of this study is to develop a novel analysis of CMAP scans from the perspective of information theory.

METHODS

A novel index parameter called CMAP distribution index (CDIX) was developed to characterize CMAP scan based on calculation of the information entropy. The performance of CDIX was evaluated using CMAP scan data from healthy control and spinal cord injury (SCI) subjects, and compared with D50 and MScanFit motor unit number estimation (MUNE).

RESULTS

CDIX was significantly lower for the SCI subjects compared with the healthy control subjects (p < 0.001). A significant correlation ( R2 = 0.58, p < 0.001) was found between CDIX and MScanFit MUNE. Among all tested parameters (maximum CMAP, D50, MScanFit MUNE and CDIX), CDIX achieved the smallest relative width of the overlapping zone (WOZ%) between SCI and healthy control subjects.

CONCLUSION

CDIX can be inferred as a useful index reflecting motor unit loss and muscle fiber reinnervation changes.

Motor unit number index (MUNIX) loss of 50% occurs in half the time of 50% functional loss according to the D50 disease progression model of ALS

Capturing disease progression in amyotrophic lateral sclerosis (ALS) is challenging and refinement of progression markers is urgently needed. This study introduces new motor unit number index (MUNIX), motor unit size index (MUSIX) and compound muscle action potential (CMAP) parameters called M50, MUSIX200 and CMAP50. M50 and CMAP50 indicate the time in months from symptom onset an ALS patient needs to lose 50% of MUNIX or CMAP in relation to the mean values of controls. MUSIX200 represents the time in months until doubling of the mean MUSIX of controls. We used MUNIX parameters of Musculi abductor pollicis brevis (APB), abductor digiti minimi (ADM) and tibialis anterior (TA) of 222 ALS patients. Embedded in the D50 disease progression model, disease aggressiveness and accumulation were analyzed separately. M50, CMAP50 and MUSIX200 significantly differed among disease aggressiveness subgroups (p < 0.001) regardless of disease accumulation. ALS patients with a low M50 had a significantly shorter survival compared to high M50 (median 32 versus 74 months). M50 preceded the loss of global function (median of about 14 months). M50, CMAP50 and MUSIX200 characterize the disease course in ALS in a new way and may be applied as early measures of disease progression.

Evaluation method of motor unit number index based on optimal muscle strength combination

Repeatability is an important attribute of motor unit number index (MUNIX) technology. This paper proposes an optimal contraction force combination for MUNIX calculation in an effort to improve the repeatability of this technology. In this study, the surface electromyography (EMG) signals of the biceps brachii muscle of eight healthy subjects were initially recorded with high-density surface electrodes, and the contraction strength was the maximum voluntary contraction force of nine progressive levels. Then, by traversing and comparing the repeatability of MUNIX under various combinations of contraction force, the optimal combination of muscle strength is determined. Finally, calculate MUNIX using the high-density optimal muscle strength weighted average method. The correlation coefficient and the coefficient of variation are utilized to assess repeatability. The results show that when the muscle strength combination is 10, 20, 50 and 70% of the maximum voluntary contraction force, the repeatability of MUNIX is greatest, and the correlation between MUNIX calculated using this combination of muscle strength and conventional methods is high (PCC > 0.99), the repeatability of the MUNIX method improved by 11.5-23.8%. The results indicate that the repeatability of MUNIX differs for various combinations of muscle strength and that MUNIX, which is measured with a smaller number and lower-level contractility, has greater repeatability.

Motor unit number index (MUNIX) in the D50 disease progression model reflects disease accumulation independently of disease aggressiveness in ALS

The neurophysiological technique motor unit number index (MUNIX) is increasingly used in clinical trials to measure loss of motor units. However, the heterogeneous disease course in amyotrophic lateral sclerosis (ALS) obfuscates robust correlations between clinical status and electrophysiological assessments. To address this heterogeneity, MUNIX was applied in the D50 disease progression model by analyzing disease aggressiveness (D50) and accumulation (rD50 phase) in ALS separately. 237 ALS patients, 45 controls and 22 ALS-Mimics received MUNIX of abductor pollicis brevis (APB), abductor digiti minimi (ADM) and tibialis anterior (TA) muscles. MUNIX significantly differed between controls and ALS patients and between ALS-Mimics and controls. Within the ALS cohort, significant differences between Phase I and II revealed in MUNIX, compound muscle action potential (CMAP) and motor unit size index (MUSIX) of APB as well as in MUNIX and CMAP of TA. For the ADM, significant differences occurred later in CMAP and MUNIX between Phase II and III/IV. In contrast, there was no significant association between disease aggressiveness and MUNIX. In application of the D50 disease progression model, MUNIX can demonstrate disease accumulation already in early Phase I and evaluate effects of therapeutic interventions in future therapeutic trials independent of individual disease aggressiveness.

The value of MUNIX as an objective electrophysiological biomarker of disease progression in CIDP

INTRODUCTION/AIMS

Objective outcome measures to monitor treatment response and guide treatment are lacking in chronic inflammatory demyelinating polyneuropathy (CIDP). We aimed to evaluate the motor unit number index (MUNIX) as an outcome measurement in patients with CIDP and investigate the correlation of MUNIX with functional and standard electrodiagnostic tests in a single follow-up study.

METHODS

We evaluated MUNIX of the abductor pollicis brevis (APB), abductor digiti minimi (ADM), and tibialis anterior (TA) muslces bilaterally. Muscle force was assessed by Medical Research Council sumscores (MRCSS). Functional measures used were the Overall Neuropathy Limitation Score (ONLS) and the Rasch-built Overall Disability Scale (R-ODS) at baseline and after six months of treatment. Standard electrophysiology was evaluated by the Nerve Conduction Study Score (NCSS).

RESULTS

Twenty patients were included at baseline, and 16 completed the follow-up study. Significant correlations were found between the MUNIX sumscore and both MRCSS and NCSS at baseline, between both the pinch strength and grip and upper limb MUNIX at baseline and follow-up, and between MUNIX of TA and both lower limb MRCSS with lower limb ONLS at baseline and follow-up. Significant correlations also were found between MUNIX sumscore change and MRCSS change, R-ODS change, and ONLS change.

DISCUSSION

MUNIX changes correlated with strength and electrophysiological improvements in CIDP patients. This suggests that MUNIX may represent a useful objective biomarker for patient follow-up.

Assessing inter-rater reproducibility in MScanFit MUNE in a 6-subject, 12-rater "Round Robin" setup

OBJECTIVE

To assess the inter-rater reliability of MScanFit MUNE using a "Round Robin" research design.

METHODS

Twelve raters from different centres examined six healthy study participants over two days. Median, ulnar and common peroneal nerves were stimulated, and compound muscle action potential (CMAP)-scans were recorded from abductor pollicis brevis (APB), abductor digiti minimi (ADM) and anterior tibial (TA) muscles respectively. From this we calculated the Motor Unit Number Estimation (MUNE) and "A50", a motor unit size parameter. As statistical analysis we used the measures Limits of Agreement (LOA) and Coefficient of Variation (COV). Study participants scored their perception of pain from the examinations on a rating scale from 0 (no pain) to 10 (unbearable pain).

RESULTS

Before this study, 41.6% of the raters had performed MScanFit less than five times. The mean MUNE-values were: 99.6 (APB), 131.4 (ADM) and 126.2 (TA), with LOA: 19.5 (APB), 29.8 (ADM) and 20.7 (TA), and COV: 13.4 (APB), 6.3 (ADM) and 5.6 (TA). MUNE-values correlated to CMAP max amplitudes (R²-values were: 0.463 (APB) (p<0.001), 0.421 (ADM) (p<0.001) and 0.645 (TA) (p<0.001)). The average perception of pain was 4.

DISCUSSION

MScanFit indicates a high level of inter-rater reliability, even with only limited rater experience and is overall reasonably well tolerated by patients. These results may indicate MScanFit as a reliable MUNE method with potential as a biomarker in drug trials.

Neuromechanical Biomarkers for Robotic Neurorehabilitation

One of the current challenges for translational rehabilitation research is to develop the strategies to deliver accurate evaluation, prediction, patient selection, and decision-making in the clinical practice. In this regard, the robot-assisted interventions have gained popularity as they can provide the objective and quantifiable assessment of the motor performance by taking the kinematics parameters into the account. Neurophysiological parameters have also been proposed for this purpose due to the novel advances in the non-invasive signal processing techniques. In addition, other parameters linked to the motor learning and brain plasticity occurring during the rehabilitation have been explored, looking for a more holistic rehabilitation approach. However, the majority of the research done in this area is still exploratory. These parameters have shown the capability to become the “biomarkers” that are defined as the quantifiable indicators of the physiological/pathological processes and the responses to the therapeutical interventions. In this view, they could be finally used for enhancing the robot-assisted treatments. While the research on the biomarkers has been growing in the last years, there is a current need for a better comprehension and quantification of the neuromechanical processes involved in the rehabilitation. In particular, there is a lack of operationalization of the potential neuromechanical biomarkers into the clinical algorithms. In this scenario, a new framework called the “Rehabilomics” has been proposed to account for the rehabilitation research that exploits the biomarkers in its design. This study provides an overview of the state-of-the-art of the biomarkers related to the robotic neurorehabilitation, focusing on the translational studies, and underlying the need to create the comprehensive approaches that have the potential to take the research on the biomarkers into the clinical practice. We then summarize some promising biomarkers that are being under investigation in the current literature and provide some examples of their current and/or potential applications in the neurorehabilitation. Finally, we outline the main challenges and future directions in the field, briefly discussing their potential evolution and prospective.

Motor unit number index: A potential electrophysiological biomarker for pediatric spinal muscular atrophy

INTRODUCTION/AIMS

In adult spinal muscular atrophy (SMA), the motor unit number index (MUNIX) has been shown to be an useful electrophysiological biomarker. This study evaluated the feasibility and the clinical relevance of using the MUNIX technique for patients with pediatric SMA (Ped-SMA) and correlated MUNIX results with clinical scores.

METHODS

Fourteen patients with type II Ped-SMA (11 females; median age 11 y [interquartile range (IQR), 4.8-17 y]) and 14 controls (nine females; median age 10.75 y [IQR, 6.5-13.4 y]) were enrolled and matched by sex, age, height, weight, and body mass index. Clinical examination included manual muscle testing, dynamometry (grasp and pinch), and motor function measure (MFM). The MUNIX technique was evaluated in the abductor digiti minimi (ADM) and abductor pollicis brevis (APB) on two sides when possible.

RESULTS

In the patients with Ped-SMA, the MUNIX and compound muscle action potential (CMAP) amplitudes were significantly decreased and the motor size unit index (MUSIX) was significantly increased in the ADM and APB when compared to controls. The intraclass correlation coefficient was good for the intrarater variability of the CMAP amplitude, MUNIX, and MUSIX in the ADM (0.95, 0.83, and 0.89, respectively) and the APB (0.98, 0.96, and 0.94, respectively). The total CMAP amplitude correlated with the grasp and pinch scores (P < .05), and the MUNIX measurements correlated with the MFM scores.

DISCUSSION

The MUNIX technique, which accurately estimated lower motor neuron loss and the number of remaining functional motor units, was shown to be a useful electrophysiological biomarker for disease progression and a potential biomarker for treatment response.

Model-Based Analysis of Muscle Strength and EMG-Force Relation with respect to Different Patterns of Motor Unit Loss

This study presents a model-based sensitivity analysis of the strength of voluntary muscle contraction with respect to different patterns of motor unit loss. A motor unit pool model was implemented including simulation of a motor neuron pool, muscle force, and surface electromyogram (EMG) signals. Three different patterns of motor unit loss were simulated, including (1) motor unit loss restricted to the largest ones, (2) motor unit loss restricted to the smallest ones, and (3) motor unit loss without size restriction. The model outputs including muscle force amplitude, variability, and the resultant EMG-force relation were quantified under two different motor neuron firing strategies. It was found that motor unit loss restricted to the largest ones had the most dominant impact on muscle strength and significantly changed the EMG-force relation, while loss restricted to the smallest motor units had a pronounced effect on force variability. These findings provide valuable insight toward our understanding of the neurophysiological mechanisms underlying experimental observations of muscle strength, force control, and EMG-force relation in both normal and pathological conditions.

Quantitative assessment of motor impairment and surgical outcome in Hirayama disease with proximal involvement using motor unit number index

OBJECTIVE

To assess the feasibility of motor unit number index (MUNIX) in quantitatively evaluating Hirayama disease (HD) with proximal involvement and to identify the effectiveness of anterior cervical fusion (ACF) in treating atypical HD with proximal involvement.

METHODS

This study included 28 atypical HD patients with proximal involvement (proximal-distal vs. distal-proximal groups: 5 vs. 23) and 41 healthy controls. All patients underwent pre- and postoperative 1-year MUNIX tests on abductor pollicis brevis (APB), abductor digiti minimi (ADM), biceps brachii (BB) and deltoid (Del). The disabilities of arm, shoulder and hand (DASH) and Medical Research Council (MRC) scales were also performed in these patients before and one year after operation.

RESULTS

Preoperatively, the patients in the distal-proximal group showed reduced compound muscle action potential (CMAP), decreased MUNIX and increased motor unit size index (MUSIX) in bilateral distal muscles and symptomatic-side proximal muscles (P < 0.05), and similar abnormalities were also observed in ADM, BB and Del on the symptomatic side in the proximal-distal groups (P < 0.05). Postoperative follow-up analysis identified increased MUNIX in the symptomatic-side proximal muscles with improved motor function in the proximal-distal groups (P < 0.05), and distal-proximal group patients showed an increase in both CMAP and MUSIX in the symptomatic-side proximal muscles (P < 0.05).

CONCLUSIONS

MUNIX may serve as an available supplementary test to quantitatively evaluate the motor dysfunction and treatment outcome in HD with proximal involvement. ACF procedures can effectively treat these atypical HD patients, especially for those whose symptoms started in proximal muscles.

Development of new outcome measures for adult SMA type III and IV: a multimodal longitudinal study

OBJECTIVE

The aim of this study was the comprehensive characterisation of longitudinal clinical, electrophysiological and neuroimaging measures in type III and IV adult spinal muscular atrophy (SMA) with a view to propose objective monitoring markers for future clinical trials.

METHODS

Fourteen type III or IV SMA patients underwent standardised assessments including muscle strength testing, functional evaluation (SMAFRS and MFM), MUNIX (abductor pollicis brevis, APB; abductor digiti minimi, ADM; deltoid; tibialis anterior, TA; trapezius) and quantitative cervical spinal cord MRI to appraise segmental grey and white matter atrophy. Patients underwent a follow-up assessment with the same protocol 24 months later. Longitudinal comparisons were conducted using the Wilcoxon-test for matched data. Responsiveness was estimated using standardized response means (SRM) and a composite score was generated based on the three most significant variables.

RESULTS

Significant functional decline was observed based on SMAFRS (p = 0.019), pinch and knee flexion strength (p = 0.030 and 0.027), MUNIX and MUSIX value in the ADM (p = 0.0006 and 0.043) and in TA muscle (p = 0.025). No significant differences were observed based on cervical MRI measures. A significant reduction was detected in the composite score (p = 0.0005, SRM = -1.52), which was the most responsive variable and required a smaller number of patients than single variables in the estimation of sample size for clinical trials.

CONCLUSIONS

Quantitative strength testing, SMAFRS and MUNIX readily capture disease progression in adult SMA patients. Composite multimodal scores increase predictive value and may reduce sample size requirements in clinical trials.

Clinical and radiological profile of patients with spinal muscular atrophy type 4

BACKGROUND AND PURPOSE

Spinal muscular atrophy (SMA) is the most important cause of motor neuron disease in childhood, and continues to represent the leading genetic cause of infant death. Adulthood-onset SMA (SMA type 4) is rare, with few isolated cases reported. The objective of the present study was to describe a cohort of patients with SMA type 4.

METHODS

A cross-sectional study was conducted to characterize clinical, genetic, radiological and neurophysiological features of patients with adulthood-onset SMA. Correlation analysis of functional assessment with genetic, radiological and neurophysiological data was performed.

RESULTS

Twenty patients with SMA type 4 were identified in a Brazilian cohort of 227 patients with SMA. The most common clinical symptom was limb-girdle muscle weakness, observed in 15 patients (75%). The most frequent neurological findings were absent tendon reflexes in 18 (90%) and fasciculations in nine patients (45%). Sixteen patients (80%) had the homozygous deletion of exon 7 in the SMN1 gene, with 12 patients (60%) showing four copies of the SMN2 gene. The functional scales Hammersmith Functional Motor Scale Expanded, Amyotrophic Lateral Sclerosis Functional Rating Scale Revised, Revised Upper Limb Module and Spinal Muscular Atrophy Functional Rating Scale, as well as the six-minute walk and the Time Up and Go tests showed a correlation with duration of disease. Motor Unit Number Index was correlated both with duration of disease and with performance in functional assessment. Radiological studies exhibited a typical pattern, with involvement of biceps femoris short head and gluteus minimus in all patients.

CONCLUSION

This study represents the largest cohort of patients with SMA type 4 and provides functional, genetic, radiological and neurophysiological features that can be used as potential biomarkers for the new specific genetic therapies for SMA.

Motor unit number index as an individual biomarker: Reference limits of intra-individual variability over time in healthy subjects

OBJECTIVE

Motor unit number index (MUNIX) is proposed to monitor neuromuscular disorders. Our objective is to determine the intra-individual variability over time of the MUNIX.

METHODS

In 11 different hospital centres, MUNIX was assessed twice, at least 3 months apart (range 90-360 days), in tibialis anterior (TA), abductor pollicis brevis (APB), abductor digiti minimi (ADM) and deltoid muscles in 118 healthy subjects. MUNIX sum score 2, 3 and 4 were respectively the sum of the MUNIX of the TA and ADM, of the TA, APB and ADM and of the TA, APB, ADM and deltoid muscles.

RESULTS

The repeatability of the MUNIX was better for sum scores than for single muscle recordings. The variability of the MUNIX was independent of sex, age, interval between measurements and was lower for experienced than non-experienced operators. The 95th percentile of the coefficient of variability of the MUNIX sum score 2, 3 and 4 were respectively 22%, 18% and 15% for experienced operators.

CONCLUSIONS

The MUNIX technique must be performed by experienced operators on several muscles to reduce its variability and improve its reliability.

SIGNIFICANCE

A variation of the MUNIX sum score ≥20% can be interpreted as a significant change of muscle innervation.

The evolving role of surface electromyography in amyotrophic lateral sclerosis: A systematic review

OBJECTIVE

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease that leads to inexorable motor decline and a median survival of three years from symptom onset. Surface EMG represents a major technological advance that has been harnessed in the development of novel neurophysiological biomarkers. We have systematically reviewed the current application of surface EMG techniques in ALS.

METHODS

We searched PubMed to identify 42 studies focusing on surface EMG and its associated analytical methods in the diagnosis, prognosis and monitoring of ALS patients.

RESULTS

A wide variety of analytical techniques were identified, involving motor unit decomposition from high-density grids, motor unit number estimation and measurements of neuronal hyperexcitability or neuromuscular architecture. Some studies have proposed specific diagnostic and prognostic criteria however clinical calibration in large ALS cohorts is currently lacking. The most validated method to monitor disease is the motor unit number index (MUNIX), which has been implemented as an outcome measure in two ALS clinical trials.

CONCLUSION

Surface EMG offers significant practical and analytical flexibility compared to invasive techniques. To capitalise on this fully, emphasis must be placed upon the multi-disciplinary collaboration of clinicians, bioengineers, mathematicians and biostatisticians.

SIGNIFICANCE

Surface EMG techniques can enrich effective biomarker development in ALS.

Why individuals with cerebral palsy are at higher risk for respiratory complications from COVID-19

Respiratory dysfunction is a leading cause of morbidity and mortality in individuals with cerebral palsy (CP). In children and adults with CP, movement and physical function is always affected. Yet, many clinicians overlook potential for impaired movement and function of the diaphragm muscle (DIAm) in individuals with CP. Since individuals with pre-existing respiratory disorders are at greater risk for respiratory complications if they contract COVID-19, understanding potential risks to individuals with CP is important. In this review we present research on respiratory function and DIAm force generation in children with CP. We compare this clinical work to basic science research investigating phrenic motor neuron and DIAm motor unit dysfunction in an animal model with CP symptoms, the spa mouse. Finally, we integrate the clinical and basic science work in respiratory function in CP, discussing potential for individuals with CP to have severe respiratory symptoms from COVID-19.

Motor unit number index (MUNIX) in myopathic disorders: Clinical correlations and potential pitfalls

Our aim was to study motor unit number index (MUNIX) in myopathic disorders. We studied 11 patients with myopathy, and healthy controls. We obtained MUNIX, compound muscle action potential (CMAP), motor unit size index (MUSIX) and alpha (α, power exponent from MUNIX equation) measurements from three different muscles. MUNIX and CMAP were significantly lower in one muscle. This MUNIX decrease may not be related to motor neuron loss, but rather to muscle fiber atrophy. MUSIX and α did not change and may be useful in determining whether the MUNIX decrease is indeed due to motor unit loss.

Lipid Biomarkers for Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal degenerative disease primarily characterized by the selective loss of upper and lower motor neurons. To date, there is still an unmet need for robust and practical biomarkers that could estimate the risk of the disease and its progression. Based on metabolic modifications observed at the level of the whole body, different classes of lipids have been proposed as potential biomarkers. This review summarizes investigations carried out over the last decade that focused on changes in three major lipid species, namely cholesterol, triglycerides and fatty acids. Despite some contradictory findings, it is becoming increasingly accepted that dyslipidemia, and related aberrant energy homeostasis, must be considered as essential components of the pathological process. Therefore, it is tempting to envisage dietary interventions as a means to counterbalance the metabolic disturbances and ameliorate the patient's quality of life.