Electrical impedance myography: Background, current state, and future directions

Abstract

Electrical impedance myography (EIM) is a new non-invasive technique for the evaluation of neuromuscular disease that relies upon the application and measurement of high-frequency, lowintensity electrical current. EIM assesses disease-induced changes to muscle's normal composition and architecture, including myocyte atrophy and loss, edema, reinnervation, and the deposition of endomysial connective tissue and fat. With application of single-frequency electrical current, EIM can be used to help grade the severity of neuromuscular disease. Assessing electrical impedance across a spectrum of applied frequencies and with current flow at multiple orientations relative to the major muscle fiber direction can provide a more complete picture of muscle condition. EIM holds the promise of serving as an indicator of disease status, thus being useful in clinical trials work and in monitoring effectiveness of treatment in individual patients; ultimately, it may also find diagnostic application. Ongoing efforts have been focused on obtaining a deeper understanding of the basic mechanisms of impedance change, studying EIM in a variety of clinical contexts, and further refining the methods of EIM data acquisition and analysis. KeywordsElectrical impedance; muscle; neuromuscular disease; technique; anisotropy Electrical impedance myography (EIM) is a non-invasive, painless approach to muscle assessment based upon the application and measurement of high-frequency, low-intensity electrical current (Figure 1). In contrast to conventional needle electromyography (EMG) and most standard neurophysiological techniques, EIM does not focus on measuring the inherent electrical activity of the tissues. Rather, similar to diagnostic ultrasound, measurements are made over a small area of interest, with energy being applied to the body and the resultant surface patterns analyzed. Unlike ultrasound, however, in which energy is in the form of sound waves and the main interest is image reconstruction, in the case of EIM, electrical current is used and the output is a set of quantitative parameters describing the state of the muscle, with presently little emphasis on imaging (though this remains possible). Although still in a relatively early stage of development, EIM may have important clinical uses in the future, both as an indicator of neuromuscular disease status and as a diagnostic tool. This review will begin with a brief history of bioelectricity in neuromuscular disease, exploring why electrical impedance measurements have been essentially overlooked as a potential means of muscle assessment. It will then detail the basic concepts underlying impedance measurements in general and the substantial challenges of applying them effectively to muscle.Please address correspondence to: Seward B. Rutkove, MD, Beth Israel Deaconess Medical Center, Department of Neurology, 330 Brookline Avenue, Shapiro 810, Boston, MA 02215; srutkove@bidmc.harvard.edu, Tel: 617-667-8130; Fax: 617-667-8747. NIH Public Access It will then review the data tha...

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