Muscle action potential scans and ultrasound imaging in neurofibromatosis type 2

Exploring the effect of the nerve conduction distance on the MScanFit method ofmotor unit number estimation (MUNE)

OBJECTIVE

MScanFit motor unit number estimation (MUNE) is a sensitive method for detecting motor unit loss and has demonstrated high reproducibility in various settings. In this study, our aim was to assess the outputs of this method when the nerve conduction distance is increased.

METHODS

MScanFit recordings were obtained from the abductor digiti minimi muscle of 20 healthy volunteers. To evaluate the effect of nerve conduction distance, the ulnar nerve was stimulated from the wrist and elbow respectively. Reproducibility of MUNE, compound muscle action potential (CMAP), and other motor unit parameters were assessed using intraclass correlation coefficients (ICCs).

RESULTS

Motor unit numbers obtained from stimulation at the wrist and elbow did not significantly differ and exhibited strong consistency in the ICC test (120.3 ± 23.7 vs. 118.5 ± 27.9, p > 0.05, ICC: 0.88). Similar repeatability values were noted for other parameters. However, the Largest Unit (%) displayed notable variability between the two regions and exhibited a negative correlation with nerve conduction distance.

CONCLUSION

Our findings indicate that MScanFit can consistently calculate motor unit numbers and most of its outputs without substantial influence from nerve conduction distance. Exploring MScanFit's capabilities in various settings could enhance our understanding of its strengths and limitations for extensive use in clinical practice.

Effect of surface electrode recording area on compound muscle action potential scan processing for motor unit number estimation

Introduction

MScanFit is a model-based algorithm for motor unit number estimation (MUNE) from compound muscle action potential (CMAP) scan data. It is a clinically applicable tool because of its quick and automatic implementation. Electrodes with different recording areas were employed to record CMAP scan data in existing studies. However, the effect of electrode recording area on MScanFit MUNE and other CMAP scan parameters has not been studied.

Methods

CMAP scan was performed on the abductor pollicis brevis muscle of both hands on 14 healthy subjects using three different electrodes with recording areas of 10 mm × 10 mm, 11 mm × 14 mm, and 22 mm × 26 mm, respectively. Motor unit number was estimated using MScanFit for each CMAP scan. Two motor unit number index parameters, i.e., D50 and step index (STEPIX), were also derived from the CMAP scan data.

Results

No significant difference in D50, STEPIX, and MScanFit MUNE was observed across three different electrode recording areas, although the amplitude of CMAP decreased significantly when a larger electrode was used. Intraclass correlation coefficients of 0.792 and 0.782 were obtained for MScanFit MUNE and STEPIX, respectively.

Discussion

Compared with CMAP amplitude, D50, STEPIX, and MScanFit MUNE are less sensitive to variation in electrode recording area. However, the repeatability of MScanFit MUNE could be compromised by the inconsistency in the electrode recording area.

Motor unit number estimation by MScanFit in myotonic dystrophies

BACKGROUND

MScanFit is a new motor unit number estimation (MUNE) technique applied in motor neuron diseases and polyneuropathies to assess clinical progression and underlying disease pathology. So far, its value in myopathies, especially myotonic dystrophies (MD), has not been investigated.

METHODS

Motor unit loss and characteristics of patients with genetically confirmed MD type 1 (n = 7) and type 2 (n = 5) were investigated using MScanFit of the abductor pollicis brevis muscle and compared to age-matched healthy controls. MUNE measures were correlated with muscle impairment determined by the MRC sum score and handgrip strength.

RESULTS

MScanFit detected motor unit loss in patients with MD (p = 0.017). There was no significant difference in motor unit loss between MD type 1 and type 2 (p = 0.64). CMAP-discontinuities which, when added up, exceed 50% of the CMAP amplitude were reduced in MD patients (p = 0.0284), but motor unit amplitudes were not significantly different (p = 0.0597). The motor unit loss strongly correlated with the MRC sum score (p = 0.014, Rho = 0.678).

CONCLUSIONS

Our study shows the feasibility of MScanFit in MD and its potential to serve as a surrogate marker for overall muscle impairment. Motor unit analysis indicates that neurogenic alterations in both MD subtypes might be present.

Subclinical motor involvement in nonsystemic vasculitic neuropathy determined by the motor unit number estimation method MScanFit

INTRODUCTION/AIMS

Nonsystemic vasculitic neuropathy (NSVN) is characterized by a predominant lower limb involvement in many patients. Motor unit changes in upper extremity muscles have not been investigated in this subgroup but may be of interest for improving our understanding of the multifocal nature of the disease and counseling of patients about potential future symptoms. In this study we aimed to better understand subclinical motor involvement in the upper extremity muscles of patients with lower limb-predominant NSVN using the new motor unit number estimation (MUNE) method MScanFit.

METHODS

In this single-center, cross-sectional study, 14 patients with biopsy-proven NSVN, with no clinical signs of upper extremity motor involvement, were investigated and compared with 14 age-matched healthy controls. All participants were assessed clinically and by the MUNE method MScanFit to the abductor pollicis brevis muscle.

RESULTS

The number of motor units and peak CMAP amplitudes were significantly reduced in patients with NSVN (P = .003 and P = .004, respectively). Absolute median motor unit amplitudes and CMAP discontinuities were not significantly different (P = .246 and P = .1, respectively). CMAP discontinuities were not significantly correlated with motor unit loss (P = .15, rho = 0.4). The number of motor units did not correlate with clinical scores (P = .77, rho = 0.082).

DISCUSSION

Both MUNE and CMAP amplitudes showed motor involvement in upper extremity muscles in lower limb-predominant NSVN. Overall, there was no evidence of significant reinnervation. Investigations of the abductor pollicis brevis muscle did not show a correlation with overall functional disability of the patients.

A single center report of MScanFit motor unit number estimation in five muscles of healthy subjects

The objective of this study was to estimate the number of motor units in 5 muscles from healthy individuals using the MScanFit program based on compound muscle action potential (CMAP) scan recordings. The examined muscles included first dorsal interosseous (FDI), abductor pollicis brevis (APB), abductor digiti minimi (ADM), second lumbrical (SL), and abductor hallucis (AH). CMAP scans were recorded from a total of 24 healthy participants. Motor unit number estimation (MUNE) values were derived from the MScanFit program. The average MUNE was 136.1 ± 31.1 (mean ± standard deviation) for the FDI, 134.9 ± 37.4 for the APB, 127.3 ± 32.3 for the ADM, 39.6 ± 8.3 for the SL, and 143.9 ± 28.9 for the AH muscles. Findings of the study provide useful information of the MScanFit MUNE for the examined muscles of healthy subjects from a single center.

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.

Motor unit number of the first dorsal interosseous muscle estimated from CMAP scan with different pulse widths and steps

OBJECTIVE

A compound muscle action potential (CMAP) scan has different settings such as stimulus frequency, duration (or pulse width), and the number of stimuli (or steps). This study aims to evaluate the influence of different stimulation protocols on MScanFit, a recently developed approach to motor unit number estimation (MUNE) from CMAP scan.

APPROACH

CMAP scans of the first dorsal interosseous (FDI) muscle were performed using four protocols with different pulse widths (0.1 ms, 0.2 ms) and steps (500, 1000) in twelve neurologically intact subjects. For each CMAP scan, the MUNE was derived using MScanFit.

MAIN RESULTS

Across all subjects, a significantly higher MUNE was obtained using stimulus pulse width of 0.1 ms (500 steps: 107. 7  ±  21.3; 1000 steps: 94.9  ±  22.07) than that using pulse width of 0.2 ms (500 steps: 81.8  ±  9.9; 1000 steps: 77.8  ±  16.1) (p   <  0.001). However, no significant difference in MUNE was observed using 500 and 1000 steps (p   >  0.1). No cross effect of pulse width and steps was found between four different protocols (p   >  0.1).

SIGNIFICANCE

Given these results, a stimulation protocol of 0.1 ms pulse width and 500 steps is recommended for CMAP scan recording of the FDI muscle toward estimating motor unit number using MScanFit.

Peripheral nerve disease secondary to systemic conditions in children

This review is an overview of systemic conditions that can be associated with peripheral nervous system dysfunction. Children may present with neuropathic symptoms for which, unless considered, a causative systemic condition may not be recognized. Similarly, some systemic conditions may be complicated by comorbid peripheral neuropathies, surveillance for which is indicated. The systemic conditions addressed in this review are critical illness polyneuropathy, chronic renal failure, endocrine disorders such as insulin-dependent diabetes mellitus and multiple endocrine neoplasia type 2b, vitamin deficiency states, malignancies and reticuloses, sickle cell disease, neurofibromatosis, connective tissue disorders, bowel dysmotility and enteropathy, and sarcoidosis. In some disorders presymptomatic screening should be undertaken, while in others there is no benefit from early detection of neuropathy. In children with idiopathic peripheral neuropathies, systemic disorders such as celiac disease should be actively excluded. While management is predominantly focused on symptomatic care through pain control and rehabilitation, some neuropathies improve with effective control of the underlying etiology and in a small proportion a more targeted approach is possible. In conclusion, peripheral neuropathies can be associated with a diverse range of medical conditions and unless actively considered may not be recognized and inadequately managed.

In vivo evidence for reduced ion channel expression in motor axons of patients with amyotrophic lateral sclerosis

KEY POINTS

The progressive loss of motor units in amyotrophic lateral sclerosis (ALS) is initially compensated for by the reinnervation of denervated muscle fibres by surviving motor axons. A disruption in protein homeostasis is thought to play a critical role in the pathogenesis of ALS. The changes in surviving motor neurons were studied by comparing the nerve excitability properties of moderately and severely affected single motor axons from patients with ALS with those from single motor axons in control subjects. A mathematical model indicated that approximately 99% of the differences between the ALS and control units could be explained by a non-selective reduction in the expression of all ion channels. These changes in ALS patients are best explained by a failure in the supply of ion channel and other membrane proteins from the diseased motor neuron.

ABSTRACT

Amyotrophic lateral sclerosis (ALS) is characterised by a progressive loss of motor units and the reinnervation of denervated muscle fibres by surviving motor axons. This reinnervation preserves muscle function until symptom onset, when some 60-80% of motor units have been lost. We have studied the changes in surviving motor neurons by comparing the nerve excitability properties of 31 single motor axons from patients with ALS with those from 21 single motor axons in control subjects. ALS motor axons were classified as coming from moderately or severely affected muscles according to the compound muscle action potential amplitude of the parent muscle. Compared with control units, thresholds were increased, and there was reduced inward and outward rectification and greater superexcitability following a conditioning impulse. These abnormalities were greater in axons from severely affected muscles, and were correlated with loss of fine motor skills. A mathematical model indicated that 99.1% of the differences between the moderately affected ALS and control units could be explained by a reduction in the expression of all ion channels. For the severely affected units, modelling required, in addition, an increase in the current leak through and under the myelin sheath. This might be expected if the anchoring proteins responsible for the paranodal seal were reduced. We conclude that changes in axonal excitability identified in ALS patients are best explained by a failure in the supply of ion channel and other membrane proteins from the diseased motor neuron, a conclusion consistent with recent animal and in vitro human data.

CMAP Scan MUNE (MScan) - A Novel Motor Unit Number Estimation (MUNE) Method

Like other methods for motor unit number estimation (MUNE), compound muscle action potential (CMAP) scan MUNE (MScan) is a non-invasive electrophysiologic method to estimate the number of functioning motor units in a muscle. MUNE is an important tool for the assessment of neuropathies and neuronopathies. Unlike most MUNE methods in use, MScan assesses all the motor units in a muscle, by fitting a model to a detailed stimulus-response curve, or CMAP scan. It thereby avoids the bias inherent in all MUNE methods based on extrapolating from a small sample of units. Like 'Bayesian MUNE,' MScan analysis works by fitting a model, made up of motor units with different amplitudes, thresholds, and threshold variabilities, but the fitting method is quite different, and completed within five minutes, rather than several hours. The MScan off-line analysis works in two stages: first, a preliminary model is generated based on the slope and variance of the points in the scan, and second, this model is then refined by adjusting all the parameters to improve the fit between the original scan and scans generated by the model. This new method has been tested for reproducibility and recording time on 22 amyotrophic lateral sclerosis (ALS) patients and 20 healthy controls, with each test repeated twice by two blinded physicians. MScan showed excellent intra- and inter-rater reproducibility with ICC values of >0.98 and a coefficient of variation averaging 12.3 ± 1.6%. There was no difference in the intra-rater reproducibility between the two observers. Average recording time was 6.27 ± 0.27 min. This protocol describes how to record a CMAP scan and how to use the MScan software to derive an estimate of the number and sizes of the functioning motor units. MScan is a fast, convenient, and reproducible method, which may be helpful in diagnoses and monitoring disease progression in neuromuscular disorders.

Neuropathies in the setting of Neurofibromatosis tumor syndromes: Complexities and opportunities

The term 'Neurofibromatosis' (NF) comprises a group of rare diseases with related clinical presentations but distinct genetic conditions. All currently known types - NF1, NF2 and Schwannomatosis - predispose afflicted individuals to the development of glial cell-derived (gliogenic) tumors. Furthermore, the occurrence of neuropathic symptoms, which add to the overall neurologic disability of patients, has been described in all disease entities. We show that neuropathic symptoms are a common and clinically important, yet infrequently studied feature in the NF spectrum. However, the clinical relevance and respective underlying pathogenesis, varies greatly among the different NF types. In this review, we summarize and interpret the latest basic research findings, as well as clinical observations, in respect of Neurofibromatosis-associated neuropathies.