Reference values for the motor unit number index and the motor unit size index in five muscles

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.

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.

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.

Changes in neuromuscular function in elders: Novel techniques for assessment of motor unit loss and motor unit remodeling with aging

Functional muscle fiber denervation is a major contributor to the decline in physical function observed with aging and is now a recognized cause of sarcopenia, a muscle disorder characterized by progressive and generalized degenerative loss of skeletal muscle mass, quality, and strength. There is an interrelationship between muscle strength, motor unit (MU) number, and aging, which suggests that a portion of muscle weakness in seniors may be attributable to the loss of functional MUs. During normal aging, there is a time-related progression of MU loss, an adaptive sprouting followed by a maladaptive sprouting, and continuing recession of terminal Schwann cells leading to a reduced capacity for compensatory reinnervation in elders. In amyotrophic lateral sclerosis, increasing age at onset predicts worse survival ALS and it is possible that age-related depletion of the motor neuron pool may worsen motor neuron disease. MUNE methods are used to estimate the number of functional MU, data from MUNIX arguing for motor neuron loss with aging will be reviewed. Recently, a new MRI technique MU-MRI could be used to assess the MU recruitment or explore the activity of a single MU. This review presents published studies on the changes of neuromuscular function with aging, then focusing on these two novel techniques for assessment of MU loss and MU remodeling.

Feasibility and reliability of MScanFit motor unit number estimation in peroneus longus muscle

INTRODUCTION/AIM

Motor unit number estimation (MUNE) methods may be valuable to detect motor involvement earlier than compound muscle action potential (CMAP) amplitude. The most recent MUNE method, MScanFit has been shown to have advantages compared to the previously described methods. However, MScanFit has only been applied in a few lower extremity muscles. We aimed in this study to examine the feasibility and reliability of MScanFit in peroneus longus muscle.

METHODS

Twenty healthy controls (16 males and 4 females, mean age: 36.05 ± 2.58) were examined twice within a 1-2 week interval. Fibular nerve was stimulated at the knee and CMAP scans were recorded from peroneus longus muscle. From this, MScanFit MUNE and size parameters were calculated as well as the CMAP amplitude. The reliability was examined using coefficient of variation (CV) and intraclass correlation coefficient (ICC). MUNE was correlated with CMAP amplitude using linear regression analysis.

RESULTS

The CV between sessions was higher for CMAP amplitude (11.63 ± 1.88 %) than MScanFit MUNE (3.13 ± 0.78%). Among the size parameters, mean unit amplitude (μV) showed the lowest CV (11.46 % ± 1.77). Using ICC, CMAP amplitude exhibited good reliability (0.787) whereas that of MScanFit MUNE was excellent (0.902). Reliability was good for all size parameters. There was no significant correlation between MScanFit MUNE and CMAP amplitude (R= 0.25, p>0.05).

DISCUSSION

MScanFit MUNE is feasible in the peroneus longus muscle with high test-retest reliability in healthy subjects. Studies in patients are needed to examine the sensitivity of this muscle in disease. This article is protected by copyright. All rights reserved.

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.

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.