Dawson says that the main technical challenge in developing the device was to find a way to deliver electric currents at varying angles without requiring complex machinery. “We came up with the idea of having a lot of little pixel probes and connecting them together,” he says. The head of the device contains two rings of small electrodes: one to send current, and one to measure voltage. These individual electrodes can be electrically connected in different combinations to act as single larger electrodes, or can be isolated individually to give a finer resolution. This allows the researchers to program the specific angles that they want to measure. The device is connected to a computer that calculates impedance measurements and displays the results graphically.
Rutkove is currently testing EIM in patients with ALS and in children with spinal muscular atrophy. He says that the biggest challenge for making EIM useful is knowing how to interpret the data. His work has shown that neuromuscular diseases can have unique EIM “signatures” that can be used to diagnose and treat the disease, but it’s an ongoing research effort “to find the right signature or impedance profile that tells you it’s one type of disease versus another.” The technique must also be tested in enough patients to understand the normal range of individual variability.
“The idea of having a tool that is noninvasive and painless to assess muscle function is very attractive,” says Michael Benatar, a neurologist at Emory University, who is testing the device in patients. Currently, the best test for muscle function is electromyography (EMG), which involves placing a needle into the muscle and having the patient contract the muscle. Benatar has been testing the EIM method in patients with ALS to see if the technique could be used for early detection of disease. “We’re hoping we might be able to detect abnormalities with EIM that aren’t apparent clinically or with conventional techniques,” he says. But he adds that EIM is not ready to be used more widely in the clinic until it’s clear how to interpret the results.
Rutkove hopes that in the meantime, EIM will prove useful as a research tool. His group is also conducting studies on animals with neuromuscular diseases to understand in more detail how EIM readings relate to the underlying tissue changes with disease.