Motor unit number estimation (MUNE): how may it contribute to the diagnosis of ALS?

Anconeus motor unit number estimates using decomposition-based quantitative electromyography

INTRODUCTION

Motor unit number estimates (MUNEs) provide important information in health, aging, and disease, and can be determined using decomposition-enhanced spike-triggered averaging (DE-STA). Discrimination of surface-detected motor unit potentials (S-MUPs) has been limited to contractile forces of ∽30% maximum voluntary contraction (MVC), which is insufficient to recruit a representative sample of the entire MU pool in most muscles. Unique features of the anconeus may permit MUNEs at high muscle activation levels.

METHODS

In 10 men (25 ± 3 years), anconeus MUNEs were performed using DE-STA at 10%, 30%, and 50% root-mean-square of MVC (RMS(MVC)).

RESULTS

The mean compound muscle action potential of the anconeus was ∽6 mV, and average S-MUP amplitudes were ∽100 μV, 145 μV, and 235 μV at 10%, 30%, and 50% RMS(MVC), resulting in low average MUNEs of 58, 38, and 25, respectively.

CONCLUSIONS

Elbow extensor force-EMG relationships suggest full recruitment of the anconeus MU pool at 50% RMS(MVC), thus providing a representative sample for MUNE.

Differential motor neuron impairment and axonal regeneration in sporadic and familiar amyotrophic lateral sclerosis with SOD-1 mutations: lessons from neurophysiology

Amyotrophic Lateral Sclerosis (ALS) is a degenerative disorder of the motor system. About 10% of cases are familial and 20% of these families have point mutations in the Cu/Zn superoxide dismutase 1 (SOD-1) gene. SOD-1 catalyses the superoxide radical (O⁻²) into hydrogen peroxide and molecular oxygen. The clinical neurophysiology in ALS plays a fundamental role in differential diagnosis between the familial and sporadic forms and in the assessment of its severity and progression. Sixty ALS patients (34 males; 26 females) were enrolled in the study and examined basally (T0) and every 4 months (T1, T2, and T3). Fifteen of these patients are SOD-1 symptomatic mutation carriers (nine males, six females). We used Macro-EMG and Motor Unit Number Estimation (MUNE) in order to evaluate the neuronal loss and the re-innervation process at the onset of disease and during follow-up period. Results and Discussion: SOD-1 mutation carriers have a higher number of motor units at the moment of diagnosis when compared with the sporadic form, despite a more dramatic drop in later stages. Moreover, in familiar SOD-1 ALS there is not a specific time interval in which the axonal regeneration can balance the neuronal damage. Taken together, these results strengthen the idea of a different pathogenetic mechanism at the base of sALS and fALS.

Designing clinical trials in amyotrophic lateral sclerosis

Clinical trials in amyotrophic lateral sclerosis have significantly evolved over the last decade. New outcome measures have been developed that have reduced the sample size requirement as compared with survival studies. There has been increasing recognition that dose-ranging studies are crucial to full evaluation of experimental agents. While the requirements of late stage trials have not changed, many new designs have been suggested for earlier phase development. While no design achieves the perfect balance of sensitivity and efficiency, clinical trialists continue to work toward the goals of smaller and shorter trials so that more compounds can be studied concurrently.

Proposed modification to data analysis for statistical motor unit number estimate

Motor unit number estimation (MUNE) is an important electrophysiological technique for quantitative measurement of motor neuron loss. Although commonly used, there is no consensus concerning the optimal procedure for statistical MUNE, particularly regarding several operator-dependent variables. To assess the variables, we analyzed 500 sequential, submaximal compound muscle action potential (CMAP) responses at three or four stimulus intensities in 10 controls and 10 patients with amyotrophic lateral sclerosis (ALS). In both controls and ALS patients, we found that posttest filtering data based on 20% or 25% windows or 2, 2.5, or 3 SD excludes <5% of data. Windows of 10% or 15% excluded <5% of data in controls but not in ALS patients. Excluding data based on +/-2 SD, the coefficient of variation for final MUNE was 12% in controls and 6% in ALS patients. Group sizes of 30 or 50 and sample sizes of 300 to 500 sequential CMAP responses per run yielded the lowest coefficient of variation. We propose that statistical MUNE data should be analyzed based on excluding data >2 SD from the mean, because this is operator independent, includes the majority of data, effectively excludes clearly outlying data, such as fasciculations or movement artifact, and has a reasonable coefficient of variation.

Statistical motor unit number estimation: from theory to practice

Statistical motor unit number estimation (MUNE) is one of several experimental techniques used to estimate the number of lower motor neurons innervating a given muscle. All are fairly reproducible and have been applied successfully in monitoring neurogenic disease progression. Quantitating the number of lower motor neurons is important, since the compound muscle action potential (CMAP) and strength may not change as rapidly over time due to the confounding effect of reinnervation. MUNE techniques differ in the way they obtain samples of surface-recorded motor unit potentials (SMUP). Statistical MUNE is based on Poisson statistics, uses surface stimulation, and is useful in testing distal, superficial nerves. This review focuses on the theory behind the development of the technique, critiques the publications resulting from applying the technique in control and disease subjects, and discusses the future developments needed for clinical utility.

Electrodiagnostic approach to the patient with suspected motor neuron disease

The diagnosis of amyotrophic lateral sclerosis (ALS) per se may be challenging since there is no single diagnostic test for ALS (with the exception of finding a mutation in the SOD1 gene). Additionally, the disease may begin focally and resemble a variety of other neurologic disorders that share clinical features with ALS. This latter point emphasizes an important imperative for the clinician--the need to consider a broad range of peripheral and central nervous system disorders in the process of differential diagnosis of ALS, especially when the disease is in its early stages. The authors review the diagnostic criteria for ALS and discuss which features to consider in determining the degree of certainty or level of confidence in the diagnosis. The authors then enumerate the important differential diagnostic possibilities that emerge from a careful consideration of the clinical features and comment on neuroimaging studies and laboratory tests employed in the diagnostic process. Next, the authors turn their attention to the important role played by electrophysiologic studies in the diagnostic evaluation of the patient with suspected ALS. The authors then return to a focused consideration of selected disorders in the differential diagnosis of ALS and conclude with a summary of their diagnostic approach for this disease.