Modeling and Reproducibility of Twin Concentric Electrical Impedance Myography

Objective: Electrical impedance myography (EIM) is a recent technology to assess muscle health. As of today, the clinical application of EIM has been applied only to evaluate muscle condition using non-invasive surface electrodes in contact with the skin; however, intermediate tissues at the recording site introduce confounding artifacts which reduce the technique’s performance as a biomarker of neuromuscular disorders (NMD). Here, we develop and test in humans a new approach using two concentric needles for intramuscular EIM recordings. Methods: First, we study the recording characteristics of dual concentric needle EIM via analytical models and finite element models (FEMs). Next, the validity of the models is verified by performing experiments on saline and agar phantoms. Finally, 8 subjects with various neuromuscular diseases were studied measuring tibialis anterior, biceps, deltoid, adductor pollicis brevis, first dorsal interosseous and flexor carpi radialis muscles. Results: Analytical and FEM simulations are in good agreement with a maximum experimental discrepancy $< $8% and $< $9% using gauge needles 26 and 30, respectively. The inter-session reproducibility, as measured by the intraclass correlation coefficients for all muscles studied, was 0.926, which is comparable or exceeds the reproducibility of other well-established electrophysiological tests to assess muscle health. Conclusion: The reproducibility of the technique support future clinical validation of needle EIM for assessment of disease status, either as part of standard patient care or as biomarker measure in clinical trials. Significance: Needle EIM has the potential of becoming a valuable diagnostic tool to evaluate NMD in adult population.

I. Introduction

Electrical impedance myography (EIM) is a technology that is finding increasing application to assess neuromuscular disorders (NMD) [1]–[3]. EIM relies on the four-terminal electrical impedance technique [4], namely the application of an alternating electrical current across a region of muscle and the measurement of the resulting electrical voltage response. The underlying scientific premise of EIM is that structural and morphological alterations in pathological tissue (e.g., myofiber atrophy, inflammation, edema, and fat infiltration) change the intrinsic electrical properties of muscle, and these changes can be detected in the measured impedance [5]. This important feature allows EIM to serve both as a primary diagnostic test as well as a method for tracking disease progression or response to therapy. In point of fact, EIM has already been employed to follow disease progression in a number of clinical studies including amyotrophic lateral sclerosis and Duchenne muscular dystrophy [6]–[8].