Part of the problem lies in the difficulty clinical researchers face in collecting accurate data on skeletal muscle. This is partly due to the subjective nature of existing non-invasive assessments of disease status and partly to the painfully invasive character of electromyography (EMG) and biopsy, the standard objective tools. Two Northeastern University professors have recently developed an objective, but non-invasive and painless technique based on long known but previously overlooked properties of muscle tissue. The new technique, called "electrical impedance myography" (or EIM), was developed by professors Ronald Aaron and Carl Shiffman of Northeastern's Physics department, and it is now being studied at the Beth Israel Deaconess Medical Center in collaboration with Seward B. Rutkove, M.D., chief of the division of neuromuscular disease.
Skeletal muscle consists of tightly bundled, basically cylindrical fibers, so that electrical currents flowing perpendicular to the fibers must cross many more cell membranes than those flowing parallel to them. EIM takes advantage of this directional dependence, using very weak high frequency currents, well out of the range that can affect the nervous system. In particular, it measures the time delay between oscillations of the current and oscillations of the voltage at the skin surface (the "phase-shift"). This serves as a direct indicator of the health of the underlying muscle cells membranes.
"The goal of our research on EIM is to refine it as a test that can assist in the treatment of neuromuscular disease," said Aaron. "Our early results show that it will be particularly useful in drug trials and generally in following the course of disease in individual subjects, but also in rehabilitation programs. Hopefully, it will also prove to be useful in diagnosis."
With the support of a grant from the National Institutes of Health, EIM is currently being tested in a clinical setting:
- To establish the range of normal values and to confirm the reproducibility of the technique under clinical conditions, via tests on a large sampling of healthy subjects of both sexes and various ages;
- To evaluate patients with different disease and disuse states, to determine EIM patterns associated with myopathic and neuropathic diseases and to compare the sensitivity and specificity of these patterns as diagnostic tools;
- As a continuation of research on EIM as a tool for assessing disease progression and remission.
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