Neuromuscular disorders (NMD) such as amyotrophic lateral sclerosis affect muscle health, which in turn affects its electrical impedance. The traditional technique for recording skeletal muscle impedance and providing diagnostic information, known as surface Electrical Impedance Myography (EIM), involves placing two pairs of surface electrodes on the skin over the muscle of interest: the outer pair of electrodes sends an electrical current signal that travels through the muscle, and the inner one collects the resulting voltage. However, this surface EIM approach can only measure the most superficial muscles and is prone to measurement errors induced by the thickness of the subcutaneous fat tissue between the skin and the muscle at the measurement site.

To overcome these limitations, recent investigations have focused on developing minimally-invasive unipolar and bipolar needles, containing respectively one and two electrodes, to record the impedance. However, the polarization and dimensions of the electrodes in these configurations can have a major impact on measurements of muscle impedance.

A team led by Benjamin Sanchez from Harvard University therefore turned to theory and modelling to design a tetrapolar needle that would solve these problems. The needle was subsequently manufactured and tested in vivo by recording the muscle impedance in rats, and the results showed a good agreement with reference muscles values previously reported in the literature.

Combining theory, modelling and experiments

The researchers considered many different parameters to design the needle with one major objective in mind: to minimize the contribution of skin and subcutaneous fat tissues that usually distort surface EIM. Different depths and angles of penetrations, changing the distance between the electrodes, and different sampling frequencies were all tested in 3D finite element model simulations to obtain the optimal set of needle parameters.

Exploiting theory and modelling to design the needle and optimize its recordings is a novel approach, since previous needle-based studies have only judged their results qualitatively. And the results speak for themselves: while the contribution to the electrical impedance from muscle is typically ranging between 8 and 32% for surface EIM, the engineered tetrapolar needle increased this contribution to more than 97%.

Although this experiment has been performed on a small sample of rats and needs further testing in both healthy and diseased muscles, the results of this study are promising. Compared to surface EIM, needle EIM offsets the contributions from both skin and subcutaneous fat tissues, thus providing more accurate readings in overweight patients. They are also unaffected by changes in muscle size or shape and consequently generate more robust data, plus they can provide a local assessment of diseased muscle with higher spatial resolution. Taken together, these results show that needle EIM has the potential of becoming a powerful biomarker for the neurology community to assess muscle condition and therapy effect.

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