Ultrasound-guided motor unit scanning electromyography

Abstract

Introduction/aims: Measuring the spatial dimensions of a single motor unit remains a challenging problem, and current techniques, such as scanning electromyography (EMG), tend to underestimate the true dimensions. In this study we aimed to estimate more accurately the dimensions of a single motor unit by developing a clinically applicable scanning EMG protocol that utilizes ultrasound imaging to visualize and target a transect through the center of a single motor unit.

Methods: Single motor unit twitches in the tibialis anterior muscles of healthy volunteers were elicited via stimulation of the fibular nerve, visualized with ultrasound, and targeted with an intramuscular EMG electrode. The electrode was moved by hand in small steps through the motor unit territory. Ultrasound video output was synchronized to EMG capture, and the needle position was tracked at each step.

Results: Eight recordings from six participants were collected. The technique was quick and easy to perform (mean time, 6.1 minutes) with reasonable spatial resolution (mean step size, 1.85 mm), yielding motor unit territory sizes between 1.53 and 14.65 mm (mean, 7.15 mm).

Discussion: Ultrasound-guided motor unit scanning EMG is a quick and accurate method for obtaining a targeted motor unit transect. This combination of two readily available clinical tools provides insights into the dimensions and internal structure of the motor unit as a marker for neuromuscular conditions.

Keywords: biomarker; motor unit territory; scanning EMG; ultrasound.

FIGURE 1

Needle in situ (white linear structure from top left to labeled needle tip) in a motor unit within tibialis anterior (TA). The needle has been targeted through the center of the motor unit and advanced through it until the motor unit potential is no longer detectable, before being withdrawn along the same trajectory by hand. For a video demonstrating the movement of the needle through the muscle, see Data S1.

FIGURE 2

Example electromyographic scans from healthy human tibialis anterior muscle. For each scan, the Y origin indicates the position of the needle from the deepest starting position; a higher Y value indicates a shallower position. Stimulus artifact can be seen at time = 0. Each trace (n = 20) is shown at each position. A, Diffuse cannula potential at deeper positions giving way to triphasic motor unit potential (MUP), inset to highlight alternation and MUP shape (recording ID = 8). B, Evolving activity within the motor unit (recording ID = 7). Both the deep and superficial boundaries are included, permitting the motor unit territory to be estimated. C, Example of both alternation and MUP latency variability (recording ID = 4). D, Enhanced view of a single motor unit transect, with traces at each position averaged (recording ID = 2).