Introduction: Needle EMG is performed ‘blind’ with no sense of the position of the needle within the motor unit. As a result, no information on motor unit dimensions or internal structure is obtained. Scanning EMG was developed in the 1990s to solve this problem but never entered clinical use due to the complexity of the equipment and the time taken for recordings. We have developed a simple to use technique combining EMG and ultrasound to perform targeted motor unit transects in human muscles.
Methods: Ultrasound guidance was achieved using a Philips EPIQ with eL18-4 linear transducer, frequency 2-20MHz. The ultrasound recordings were captured using a video capture card and synchronised to EMG recording using the audio-in channel. We stimulated the peroneal nerve at the fibular head until alternation was visible in a single motor unit in the tibialis anterior muscle, and targeted this with a 28g concentric needle electrode. The needle was targetted towards the centre of the motor unit under ultrasound guidance and advanced until it had passed through the motor unit and into a region in which no movement was seen. It was then withdrawn in small increments by hand, with around 20 motor unit potentials recorded at each position. Needle position was determined post-hoc to allow a 2D motor unit scan to be reconstituted.
Results: Here, we present recordings from 6 healthy volunteers. We demonstrate that the movement resolution of the human operator was around 1-2mm, which was sufficient to allow a detailed transect of a motor unit. Across six completed transects of the motor unit by the EMG needle, the corridor length was measured between 1.5-11.3mm (median: 5.9mm). The recorded MUPs were of latency 4-37ms with a bimodal distribution around 4-10ms and 37ms, and were of duration 3.4-10.2ms (median: 5.7ms). Stimulation currents varied from 3.6-11.75mA, and recordings took between three and twelve minutes.
Discussion: This novel technique allows the dimensions and internal structure of human motor units to be easily determined. Unlike traditional scanning EMG, it is quick to perform, uses standard clinical equipment, and requires only a single needle. Future studies will explore its use both as a research tool and as a clinical diagnostic technique.
