Neuromuscular diseases like amyloid lateral sclerosis (ALS) and muscular dystrophy often involve a progressive loss of muscle function, but tracking the health of muscles over time is not always easy or precise. The best way to diagnose and evaluate muscle degeneration involves an uncomfortable needle test; both this test and other approaches like questionnaires are subjective and not easy to reproduce over multiple sessions.
A new device, under development by Seward Rutkove, a neurologist and scientist at Harvard Medical School, and his colleagues at MIT could provide a painless, noninvasive, and quantitative alternative. The prototype handheld probe, similar to an ultrasound probe, measures electrical impedance in the muscle, which changes depending on the health of the tissue.
The approach, also known as electric impedance myography (EIM), is a modification of the basic technology used in body composition devices to measure the percentage of fat or muscle in the body. A high-frequency electric current is applied to the skin through a set of noninvasive electrodes, while another set of skin electrodes records the resulting voltages from the tissue. The properties of the current change depend on the composition and microscopic structure of the underlying tissue.
Muscles are made of long bundled fibers oriented in the same direction. An electrical current passes more easily when it travels parallel to the fibers; when it passes across the fibers, it encounters more cell membranes, which cause a greater delay or phase shift in the current. Rutkove’s group at Beth Israel Deaconess Medical Center has found that this phase shift varies depending on the health of the muscle, since diseased muscle has fewer cell membranes. In addition, energy is lost as the current flows through muscle, and more so when flowing across the fibers. Rutkove’s group has found that looking at both phase shift and energy loss can provide unique information on the health of the muscle, since diseased muscles have fewer muscle fibers, smaller cell membranes, and abnormal amounts of fat and water in the muscle, all of which impact these measurements.
Rutkove’s group initially made muscle measurements using off-the-shelf body composition devices modified to perform EIM. But the process required stick-on electrodes placed at several positions along a muscle, and a single body part might require multiple rounds placing the electrodes at various angles. The handheld probe, developed in collaboration with Joel Dawson’s electrical-engineering lab at MIT, makes it possible to take the measurements quickly without a need for electrodes.