Far-field potentials in hypothenar motor unit number estimation

Far-field potential of the compound muscle action potential as a reliable marker in amyotrophic lateral sclerosis

INTRODUCTION/AIMS

Reliable neurophysiological markers in amyotrophic lateral sclerosis (ALS) are of great interest. The compound muscle action potential (CMAP) amplitude has been a conventional marker, although it is greatly influenced by the electrode position. We propose the far-field potential of the CMAP (FFP-CMAP) as a new neurophysiological marker in ALS.

METHODS

Patients with ALS and age-matched healthy controls were enrolled. We used a proximal reference (pref) in addition to the conventional distal reference (dref). Routine CMAP was recorded from the belly-dref lead and FFP-CMAP from the dref-pref lead for the ulnar and tibial nerves. Multiple point stimulation motor unit number estimation (MUNE) was also examined in the ulnar nerve. Inter-rater reproducibility was evaluated by two examiners, and some patients were followed up every 3 mo for 1 y.

RESULTS

We tested 17 patients with ALS and 10 controls. The amplitudes of routine CMAP and FFP-CMAP in the ulnar and tibial nerves, and hypothenar MUNE value in the ulnar nerve were significantly decreased in ALS compared to controls. Ulnar FFP-CMAP achieved the highest inter-rater intraclass correlation coefficient (ICC) value (0.942) when compared with routine CMAP (0.880) and MUNE (0.839). The tibial FFP-CMAP had a higher ICC value (0.986) than the routine CMAP (0.697). In this way, the FFP-CMAP showed high inter-rater reproducibility because its shape was not much influenced by the electrode position. During 1-y follow-up, decline of CMAP, FFP, and MUNE showed significant correlations with the Amyotrophic Lateral Sclerosis Functional Rating Scale - Revised (ALSFRS-R).

DISCUSSION

The FFP-CMAP shows promise as a reliable marker for ALS.

Far-field potentials in the compound muscle action potential

It has long been believed that the compound muscle action potential (CMAP) in motor-nerve conduction studies (MCSs) records the action potential beneath the active electrode over the muscle belly. However, recent studies have revealed the contribution of the reference electrode to the CMAP, most prominent in the tibial nerve, followed by the ulnar nerve. This "reference electrode potential" is recorded when the conventional reference electrode distal to the muscle belly is connected to a proximal reference. It must be a far-field potential (FFP) considering its distribution, although the precise mechanism of its generation has not been clarified. The conventional theory of termination of the action potential at the muscle-tendon junction is insufficient. Regarding the ulnar CMAP, interosseous muscles mostly contribute to the FFPs. New understanding of CMAP based on the FFP theory may provide new insights into the interpretation of MCSs and related techniques, including motor unit number estimation.

A modified multiple point stimulation method for motor unit number estimation of the hypothenar muscles

INTRODUCTION

The goal of this study is to test the hypothesis that single motor unit action potentials (SMUPs) originating from other ulnar nerve-innervated intrinsic hand muscles can inflate the motor unit number estimation (MUNE) of the hypothenar muscles.

METHODS

Using the multiple point stimulation method, SMUPs recorded over the hypothenar muscles from distant origins were characterized through multichannel recordings. The MUNE calculated using only the hypothenar SMUPs was compared with estimations based on the whole ensemble.

RESULTS

Of the 41 studies performed, distant SMUPs represented 17 ± 9.5% (mean ± SD) of the overall sample. MUNE calculated using only hypothenar SMUPs was 423 ± 204, compared with 537 ± 290 if all SMUPs were included (P < 0.05). The extent of increase in MUNE was highly correlated with the proportion of distant SMUPs found (r = 0.89, P < 0.05).

DISCUSSION

Erroneous inclusion of SMUPs from distant muscles can significantly distort the MUNE results. Muscle Nerve 59:337-341, 2019.

Decline of compound muscle action potentials and statistical MUNEs during Wallerian degeneration

AIM OF THE STUDY

In two previous studies, we found that the compound muscle action potential (CMAP) amplitude loss was significantly higher than the loss of estimated motor unit numbers in the course of Wallerian degeneration (WD). In order to overcome some drawbacks of the method previously used, we performed a similar CMAP vs MUNE comparison by using the statistical motor unit number estimation (MUNE) method.

PATIENTS AND METHODS

Initial electrophysiological studies on 6 patients were performed between 22 and 98 hours after the injuries; it was possible to make repeated examinations, four times in 1 nerve, twice in 1 nerve and three times in 4 nerves, before the eventual complete disappearance of the CMAPs.

RESULTS

The transected/intact (T/I) side CMAP ratios declined steeply as WD evolved. They were significantly lower than the relatively stable MUNE ratios 48 hours after the injury.

CONCLUSION

This study, performed with the use of statistical MUNE, strengthens our previous observation by the incremental method that might have some relevance to the pathophysiology of early WD. CMAP amplitude loss that is more than expected from the amount of axonal degeneration may indicate a considerable amount of inactive muscle fibers in the motor units innervated by the nerve fibers, which are undergoing degeneration but still retain their excitability. Although technical sources of error cannot be totally excluded, our findings could more likely be explained by the failing of neuromuscular synapses in an asynchronous order before complete unresponsiveness of the motor unit ensues.