Site-dependent thenar compound muscle action potential: Comparison between surface and needle recordings

Abstract

BACKGROUND:

Within the thenar eminence, the median nerve innervates three muscles: abductor pollicis brevis (APB), flexor pollicis brevis (FPB), and opponens pollicis (OP). Of these muscles, APB was often considered as the sole contributor to the thenar compound muscle action potential (CMAP).

OBJECTIVE:

To evaluate subcomponents of the thenar CMAP from the median nerve innervated muscles.

METHODS:

Surface and needle CMAPs were recorded in normal human subjects from three recording sites: proximal (site-I), middle (site-II), and distal (site-III) aspects of the thenar eminence when the median nerve was activated at the wrist.

RESULTS:

In the site-I and -II, both the surface and needle CMAPs shared many similar characteristics although the needle CMAPs were larger ( $\sim$ 5 folds) and briefer ( $\sim$ 60%, needle/surface duration). In addition, on the surface recording, the CMAP was larger (by $\sim$ 1.9 mV) when recorded from the site-I comparing to that of the site-II. In the site-III, the surface recordings registered a delayed (by $\sim$ 3.8 ms) CMAP. The muscle fiber action potential (MFAP) study suggested a predominant FPB contribution in the site-III.

CONCLUSION:

The optimal recording site for APB derived thenar CMAP is the site-I and for FPB is the site-III. The CMAPs registered by the needle recordings are more robust than the surface ones.

Keywords

Compound muscle action potential abductor pollicis brevis flexor pollicis brevis opponens pollicis median nerve

1. Introduction

The compound muscle action potential (CMAP) is a product of temporal and spatial summation of the near-field and far-field muscle propagating potentials [1]. In the hand, far-field potentials from interosseous muscles were thought to contribute to the multi-peak configuration of the hypothenar CMAP [2, 3]. Topography of the thenar CMAP, however, showed a single prominent peak and a relative rapid amplitude decay over the thenar surface, suggesting a simple potential source originated from abductor pollicis brevis (APB) [4]. As such, many researchers used the terms thenar and APB CMAP interchangeably [5].

Within the thenar eminence, the median nerve innervates three muscles: APB, flexor pollicis brevis (FPB), and opponens pollicis (OP) [6]. Both APB and FPB are superficially positioned with APB mainly on the proximal/lateral and FPB on the distal/medial aspects of the thenar eminence. OP lies beneath APB and covers the lateral side of the thumb metacarpal bone. In theory, these muscles all contribute to the thenar CMAP. In this study, we have characterized the thenar CMAP from three different recording sites using the surface and needle recordings.

Figure 1.

Surface thenar compound muscle action potential (CMAP) recordings. (A) Schematic drawing of electrode placement on the thenar eminence. Site-I, -II, -III, and -R represented the proximal, middle, distal, and reference recording electrodes, respectively. (B) Three representative CMAPs from the site-I, -II, and -III under identical experimental condition.

2. Materials and methods

Experiments were conducted on 18 normal subjects (11 women and 7 men; age of 25.7 $\pm$ 8.5 years). Of these subjects, seven (2 women and 5 men; age of 31.4 $\pm$ 11.8) volunteered for invasive needle studies. Exclusion criteria included history of neuromuscular disorders or upper extremity trauma injuries. This study was approved by the institutional review board and informed consent was obtained from each subject.

The subjects were seated in a reclined chair with their hands relaxed in a supinated and extended position on the armrest for all recordings. The median nerve was stimulated at the wrist by a cathode taped to the skin 2 cm proximal to the wrist crease between the tendons of flexor carpi radialis and palmaris longus muscles. Stimulus intensity was gradually increased until the supramaximal response was reached.

The surface CMAP recordings were obtained by using disposable disk electrodes (length 8 mm, width 15 mm). Specifically, electrode II was placed on the middle of the thenar eminence (site-II), whereas the centers of electrode I (site-I) and III (site-III) were placed $\sim$ 15 mm proximal and distal to the center of the site-II, respectively (Fig. 1A). For reference electrode placement, we found the distal thumb and hypothenar yielded very similar responses. The hypothenar was used because of the approximate equal separation between recording and reference electrodes. The needle recordings were performed by inserting disposable monopolar EMG needle electrodes (26 Gauge $\times$ 37 mm) in the recording sites. The muscle fiber action potential (MFAP) study was performed by applying electrical current to the muscle fibers through a monopolar EMG needle electrode (cathode) and recording from another monopolar EMG needle electrode (active recording electrode) inserted 2–3 cm away from the stimulus site (Fig. 4A). Both the surface and needle recordings were repeated for at least three times, only the trial showed highly consistent waveforms was used for data analyses. We did not use the concentric EMG needle [7], as the waveform polarity became unpredictable with the concentric EMG needle recording.

All waveforms were recorded by a Cadwell Cascade (Cadwell Laboratories, Inc., Kennewick, WA, USA). The waveform measurement was performed as following: 1) Amplitude: from the onset to the peak; 2) Latency: from the offset of stimulus to the peak; 3) Duration: between the onset and the first baseline crossing of an upward waveform.

The differences between the needle and surface recordings were analyzed by independent $t$ -tests; whereas the differences between recording sites were analyzed by paired $t$ -tests. All statistical analyses were performed with SPSS software (v24.0). Differences were considered significant when $P<$ 0.05.

3. Results

3.1 Surface thenar CMAPs from the three recording sites

In all the subjects, a biphasic “peak-trough” thenar CMAP configuration was shown in the site-I and -II by the surface recordings (Fig. 1B). The peak latencies measured from the site-I and -II were similar (latency I-II $=$ 0.29 $\pm$ 0.63 ms, $P=$ 0.065). However, a larger thenar CMAP was often recorded from the site-I (amplitude I-II $=$ 1.93 $\pm$ 2.39 mV, $P=$ 0.003). Recordings from the site-III yielded a delayed “trough-peak” waveform configuration: An initial small downward potential that was mirror-imaged to the CMAP peak of the site-I or -II, followed by an upward potential that had the peak latency longer than that of the site-I (latency I-III $=$ 3.77 $\pm$ 0.59 ms, $P<$ 0.001) (Fig. 1B).

Figure 2.

Simultaneous needle and surface electrode recordings. (A) Needle electrode was inserted in the site-II, whereas surface electrodes were placed in the site-I and -III. (B) Semi-log plotting of the stimulus-amplitude curves for the needle and surface recordings from (A).

3.2 Simultaneous needle and surface thenar CMAP recordings

When monopolar EMG needle replaced surface electrode in the site-II, a robust (needle: 39.7 $\pm$ 3.09 mV vs. surface: 8.06 $\pm$ 1.79 mV in amplitude, $\sim$ 5 folds, $P<$ 0.001) but brief (needle: 2.28 $\pm$ 0.23 ms vs. surface: 3.67 $\pm$ 0.51 ms in duration, $\sim$ 60%, $P<$ 0.001) supramaximal response was obtained (Fig. 2A). By a stepwise increase of stimulus intensity, the stimulus-amplitude curves were established (Fig. 2B). On the semi-log plotting, the needle and surface recordings showed near parallel stimulus-amplitude responses. Similarly, stimuli required to elicit the threshold and supramaximal responses were about the same for both types of recordings.

3.3 Needle vs. surface thenar CMAP from the three recording sites

Next, we performed the needle recordings from all three sites and compared to the corresponding surface recordings (Fig. 3). Overall, the needle potentials showed similar pattern in all sites, although a “double-peak” potential (Fig. 3, inlet) could be registered in $\sim$ 19% of recordings from the site-III. The “double-peak” configuration suggested the possibility of two separate propagating muscles.

Figure 3.

The needle and surface recordings of the same individual from all the three recording sites. The inlet showed “double peak” waveform.

3.4 MFAP from FPB and APB

To clarify the origin of the CMAP from the site-III, electric current was applied to the site-III by the needle electrode and the MFAPs were recorded by the needle electrodes inserted in the areas of the site-II corresponding to APB (Fig. 4A, electrode II) and FPB (Fig. 4A, electrode IIa) concurrently. Overall, FPB showed smaller ( $P=$ 0.02) MFAP thresholds (0.29 $\pm$ 0.13 mA) comparing to those of APB (2.16 $\pm$ 1.65 mA) (Fig. 4B).

Figure 4.

Muscle fiber action potential (MFAP) of flexor pollicis brevis (FPB) and abductor pollicis brevis (ABP). (A) MFAPs were elicited by applying electric current to the needle electrode inserted in the site-III (cathode) and recorded from the needle electrodes inserted in the site-II (montage: II-R; APB) and -IIa (montage: IIa-R; FPB). (B) Simultaneous MFAP recordings of APB (II-R) and FPB (IIa-R) at different stimulus (mA) intensities.

4. Discussion

The findings of this study support that APB is the primary contributor to the thenar CMAP when recorded from the proximal and middle portions of the thenar eminence [5]. This is based on the waveform similarities between the two recording sites; the lack of FPB in the site-I makes it an improbable source of the CMAP proximally, thus the waveform similarity between the two sites argues for a major APB contribution to the CMAP in the middle of the thenar. The deep situated OP is unlikely to be a major CMAP component for either site due to its far field nature in the volume conduction [4].

The early downward deflection on the surface recording from the site-III can be explained by a far-field APB “initial potential” when the electrode sits away from the motor point [8, 9]; the following upward peak on the surface recording is presumably derived from FPB owing to its lower MFAP thresholds. This is supported by the delayed (by $\sim$ 3.8 ms) CMAP peak distally, which implies a CMAP origin differs from APB. The latency delay is unlikely a result of the muscle propagation along APB, as this was not observed in the proximal recordings. APB tendon/muscle within the distal site may contribute to the “double-peak” CMAP configuration (Fig. 3, inlet), which is presumably recorded when the needle is inserted in juxtaposition between APB and FPB.

The needle recording suggested that the APB motor point was closer to the site-II as indicated by absence of initial positive potential (Fig. 3) [10]. However, a larger thenar CMAP was often recorded from the site-I by the surface electrodes (Fig. 1B). Previous study had noted that maximal CMAP amplitude was not always recorded over the estimated motor point of APB [11]. This is consistent with the anatomy of APB: A larger muscle mass (more excitable membrane area) is in the proximal one third of APB, close to its origin at flexor retinaculum [6]. In addition, less temporal dispersion from the intrusion of FPB potential may spare the site-I from the phase cancellation of the superimposed peaks [1]. Thus, the optimal recording site for APB derived CMAP appears to be in the proximal aspect of the thenar eminence.

The CMAP from the needle recording was more robust ( $\sim$ 5 folds in amplitude) than the surface recordings. In addition, the needle recording of specific muscle is presumably less influenced by the far-field potentials from neighbored muscles innervated by the median nerve, as the needle electrode should be more sensitive to the propagating fibers in a small radius nearby, whereas surface electrodes take into consideration of distant active fibers from the source muscles.

Although motor conduction study of the median nerve recording from the thenar muscle is a recommended electrodiagnostic study for carpal tunnel syndrome, its diagnostic sensitivity ( $<$ 0.69) is lower than the sensory conduction study ( $<$ 0.85) [12, 13]. Among the possible confounding factors, the summating potential of multi-source nature is prone to increase variabilities between individuals, hence affecting the sensitivity. As such, both needle and site-specific surface recordings may potentially improve the diagnostic sensitivity over the traditional “Belly-tendon” surface recording technique by targeting to specific muscle within the thenar eminence.

Footnotes

Conflict of interest

The authors declare that there is no conflict of interest.

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