Motor unit number in a small facial muscle, dilator naris

Abstract

A loss of functioning motor units underlies many neuromuscular disorders. The facial nerve innervates the muscles of facial expression, including nasal muscles, which also play an important role in the regulation of airflow resistance. It is difficult to accurately assess motor unit number in the facial muscles, because the muscles are difficult to activate in isolation. Here, we apply the manual McComas method to estimate the number of motor units in a nasal dilator muscle. EMG of the dilator naris was recorded during graded stimulation of the zygomatic branch of the facial nerve in 26 subjects (12 males and 14 females), aged 20-41 years. Each subject was studied twice, on separate days, to estimate method reproducibility. As a check on our use of the McComas method, we also estimated motor unit number in the first dorsal interosseus muscle (FDI) of six subjects, as the muscle is also small and has been studied with the McComas method. Reproducibility was evaluated with a rigorous statistical approach, the Bland-Altman procedure. We estimate that dilator naris is composed of 75 ± 15.6 (SD) motor units, compared to 144 ± 35.5 in FDI. The coefficient of variation for test-retest reproducibility of dilator naris motor unit estimates was 29.6 %, similar to separate-day reproducibility reported for other muscles. Recording and stimulation were done with surface electrodes, and the recordings were of high quality and reproducible. This simple technique could be applied clinically to track motor neuron loss and to monitor facial nerve integrity.

Keywords: EMG; Facial nerve; MUNE; Motor units; Nasal muscles.

Fig. 1

Estimates of motor unit number in dilator naris and FDI. Panel A. Representative record showing increments in the dilator naris EMG obtained by graded, sub-maximal stimulation of the facial nerve (upper recordings in Panel A). The lower recording in Panel A shows the maximal evoked potential (M-wave) in the same subject. As discussed in Results, we estimated 53 motor units in this subject. Panel B. Bland-Altman plot showing the difference in motor unit number estimates in dilator naris (trial 1 – trial 2), plotted as a function of the average number of motor units for that subject [(mean trial 1 + mean trial 2)/2]. The solid line shows a difference of zero, and the dashed lines represent ± 2 SDs of the differences (see text for explanation). Panel C. Bland-Altman plot for dilator naris M-waves recorded in each subject. Panel D. Motor unit number estimates in dilator naris and FDI in six subjects. Numbers inside rectangles show the group average value for dilator naris and FDI.