Electrical Impedance Methods in Neuromuscular Assessment: An Overview

Abstract

Electrical impedance methods have been used as evaluation tools in biological and medical science for well over 100 years. However, only recently have these techniques been applied specifically to the evaluation of conditions affecting nerve and muscle. This specific application, termed electrical impedance myography (EIM), is finding wide application as it can provide a quantitative index of muscle condition that can assist with diagnosis, track disease progression, and assess the beneficial impact of therapy. Using noninvasive surface methods, EIM has been studied in a number of conditions ranging from amyotrophic lateral sclerosis to muscular dystrophy to disuse atrophy. Data support that the technique is sensitive to disease status and can offer the possibility of performing clinical trials with fewer subjects than would otherwise be possible. Recent advances in the field include improved approaches for using EIM as a "virtual biopsy" and the development of combined needle impedance-electromyography technology.

Figure 1.

Electrical impedance myography (EIM) system (Myolex). (A) EIM consists of three components: a standard laptop, a power convertor, and the impedance measuring device itself with an attached electrode array. (B) EIM being performed on the forearm flexors.

Figure 2.

Sketch of proposed impedance-electromyography (I-EMG) needle. The red electrodes are the impedance current emitting electrodes and the blue are for voltage measurement. The dark gray tip represents the active EMG electrode that would be referenced against a surface electrode with a ground electrode.