Handgrip maximal voluntary isometric contraction does not correlate with thenar motor unit number estimation

Impact of stimulus duration on motor unit thresholds and alternation in compound muscle action potential scans

OBJECTIVE

To investigate the impact of stimulus duration on motor unit (MU) thresholds and alternation within compound muscle action potential (CMAP) scans.

METHODS

The stimulus duration (0.1, 0.2, 0.6, and 1.0 ms) in thenar CMAP scans and individual MUs of 14 healthy subjects was systematically varied. We quantified variability of individual MU's thresholds by relative spread (RS), MU thresholds by stimulus currents required to elicit target CMAPs of 5% (S5), 50% (S50) and 95% (S95) of the maximum CMAP, and relative range (RR) by 100*[S95-S5]/S50. We further assessed the strength-duration time constant (SDTC). Experimental observations were subsequently simulated to quantify alternation.

RESULTS

RS, unaffected by stimulus duration, was 1.65% averaged over all recordings. RR increased for longer stimulus duration (11.4% per ms, p < 0.001). SDTC shortened with higher target CMAPs (0.007 ms per 10% CMAP, p < 0.001). Experiments and simulations supported that this may underlie the increased RR. A short compared to long stimulus duration recruited relative more MUs at S50 (more alternation) than at the tails (less alternation).

CONCLUSIONS

The stimulus duration significantly affects MU threshold distribution and alternation within CMAP scans.

SIGNIFICANCE

Stimulation settings can be further optimized and their standardization is preferred when using CMAP scans for monitoring neuromuscular diseases.

The presymptomatic phase of amyotrophic lateral sclerosis: are we merely scratching the surface?

Presymptomatic studies in ALS have consistently captured considerable disease burden long before symptom manifestation and contributed important academic insights. With the emergence of genotype-specific therapies, however, there is a pressing need to address practical objectives such as the estimation of age of symptom onset, phenotypic prediction, informing the optimal timing of pharmacological intervention, and identifying a core panel of biomarkers which may detect response to therapy. Existing presymptomatic studies in ALS have adopted striking different study designs, relied on a variety of control groups, used divergent imaging and electrophysiology methods, and focused on different genotypes and demographic groups. We have performed a systematic review of existing presymptomatic studies in ALS to identify common themes, stereotyped shortcomings, and key learning points for future studies. Existing presymptomatic studies in ALS often suffer from sample size limitations, lack of disease controls and rarely follow their cohort until symptom manifestation. As the characterisation of presymptomatic processes in ALS serves a multitude of academic and clinical purposes, the careful review of existing studies offers important lessons for future initiatives.

Improved motor unit number estimate when motor unit alternation is addressed

Motor unit number estimation (MUNE) is important for determining motoneuron survival with age or in conditions such as amyotrophic lateral sclerosis or spinal cord injury. The original incremental method and approaches that were introduced to minimize alternation (e.g., multiple-point stimulation) are most commonly used, but one must accept the limitation that alternation of motor units may still inflate the estimate. Alternation occurs because axon thresholds are probabilistic and overlap for different axons; therefore, different combination of motor units may respond at a given stimulus intensity. Our aims were to quantify motor unit alternation systematically in the thenar muscles of 35 healthy adults by digital subtraction of EMG and force, and to compare MUNE with and without alternation. Alternation was prevalent, with one to nine occurrences in the first 7 to 11 steps in EMG in 34 of 35 muscles. It occurred in the first 3 steps in EMG in 49% of muscles. This alternation resulted in fewer units than steps in EMG (3 to 10 units at step 7 to 11). Accounting for alternation using digital subtraction reduced MUNE by up to 50%, day-to-day, and between-participant variability in MUNE. These results highlight the need to quantify alternation to improve the reliability and precision of motor unit number estimates, which will allow for detection of smaller changes in motoneuron survival with age, various health conditions, and/or due to an intervention. NEW & NOTEWORTHY Motor unit alternation was quantified systematically for the first time, addressing a major limitation of motor unit number estimates. Accounting for alternation decreased motor unit number estimates, and improved the reliability and precision of the motor unit number estimate, which will allow smaller, clinically relevant changes in motoneuron survival to be detected.

Motor unit number index examination in dominant and non-dominant hand muscles

This study investigated the effect of handedness on motor unit number index (MUNIX). Maximal hand strength, compound muscle action potential (CMAP) and voluntary surface electromyography (EMG) signals were measured bilaterally for the first dorsal interosseous (FDI) and thenar muscles in 24 right-handed and 2 left-handed healthy subjects. Mean (±standard error) grip and pinch forces in the dominant hand were 43.99 ± 2.36 kg and 9.36 ± 0.52 kg respectively, significantly larger than those in the non-dominant hand (grip: 41.37 ± 2.29 kg, p < .001; pinch: 8.79 ± 0.46 kg, p < .01). Examination of myoelectric parameters did not show a significant difference among the CMAP area, the MUNIX or motor unit size index (MUSIX) between the two sides in the FDI and thenar muscles. In addition, there was a lack of correlation between the strength and myoelectric parameters in regression analysis. However, strong correlations were observed between dominant and non-dominant hand muscles in both strength and myoelectric measures. Our results indicate that the population of motor units or spinal motor neurons as estimated from MUNIX may not be associated with handedness. Such findings help understand and interpret the MUNIX during its application for clinical or laboratory investigations.

Electrodiagnostics and clinical correlates in acquired polyneuropathies

Peripheral neuropathies result from a variety of inherited and acquired pathologies. They display an assortment of clinical signs and symptoms and present with a broad range of severity. Electrodiagnosis can play a key role in the evaluation of a suspected peripheral neuropathy. A peripheral nerve disorder is first suspected on the basis of history and physical examination findings. Electrodiagnosis is then used to confirm the diagnosis and to characterize the peripheral neuropathy, providing information about its distribution, pathophysiologic process (demyelinating vs axonal), and chronicity.