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

Abstract

Electrical impedance myography (EIM) is a non-invasive technique for the evaluation of neuromuscular disease that relies upon the application and measurement of high-frequency, low-intensity electrical current. EIM assesses disease-induced changes to the normal composition and architecture of muscle, including myocyte atrophy and loss, edema, reinnervation, and 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 major muscle fiber direction can provide a more complete picture of the condition of muscle. EIM holds the promise of serving as an indicator of disease status. It may be useful in clinical trials 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.

Figure 1

A basic approach to performing EIM measurements. In this figure, the two inner electrodes are for voltage measurement whereas the two outer ones inject the current. In other approaches, the current is injected further away, with electrodes placed on both hands and with the voltage electrodes in the same place. Each approach has certain advantages and disadvantages as discussed. The measurements here are being made with the ImpediMed Imp™ SFB7.

Figure 2

A. Basic concept of electrical impedance testing. Electrical current of a known frequency, amplitude is injected into a tissue. The tissue impacts the applied electrical current, reducing the measured voltage’s amplitude, secondary to the tissue’s resistance and slightly altering its timing, secondary to the tissue’s capacitance. The changes in electrical current flow reflect the properties of the tissue and thus can reveal information about tissue health or pathology. B. The “standard” basic equivalent circuit of bioelectrical impedance. The capacitor represents the cell membranes (in the case of muscle the sarcolemma) and the resistors the extra- and intra-cellular resistance. Although simplistic, this circuit diagram provides a basic handle with which to understand why changes in cellular structure may result in changes in the measured impedance.

Figure 3

Two- versus four-electrode impedance methods. In the two-electrode approach (A.), current is injected at the same location where the intervening voltage is measured. In the four-electrode approach (B.), current is injected separately (see Figure 1).

Figure 4

The basic concept of rotational EIM. The entire electrode array is rotated around a central point, allowing the anisotropy (directional dependence of current flow) of the muscle to be studied.

Figure 5

Archetypal changes in the multifrequency signature in normal muscle (circles) versus diseased, in this case inclusion body myositis (triangles). The normal low-frequency peak is lost, resistance increases and reactance decreases.

Figure 6

Anisotropy measurements on muscle using the approach outlined in Figure 4. A. Normalized data from a group of 15 normal subjects recording tibialis anterior; mean values +/− 1 standard deviation. B. Normalized data from a group of 7 patients with myopathy clinically affecting tibialis anterior; C. Normalized data from a group of 7 patients with ALS clinically affecting tibialis anterior.

Figure 7

Example of dynamic-EIM being performed on biceps and the data obtained.. The time course in the changes observed appear to be due to changes in muscle fiber morphology with contraction and not clearly due to the firing of motor unit potentials per se.