Skeletal muscle is electrically anisotropic, with a tendency for applied electrical current to flow more readily along muscle fibers than across them. In this study, we assessed a method for non-invasive measurement of anisotropy to determine its potential to serve as a new technique for distinguishing neurogenic from myopathic disease. Measurements were made on the biceps brachii and tibialis anterior muscles in 15 normal subjects and 12 patients with neuromuscular disease (6 with amyotrophic lateral sclerosis and 6 with various myopathies) using 50 kHZ applied current. Consistent multi-angle anisotropic patterns were found for reactance and phase in both muscles in normal subjects. Normalized anisotropy differences for each subject were defined, and group average values identified. The amyotrophic lateral sclerosis (ALS) patients demonstrated increased and distorted anisotropy patterns, whereas myopathic patients demonstrated normal or reduced anisotropy. These results suggest that non-invasive measurement of muscle anisotropy has potential for diagnosis of neuromuscular diseases. Keywordsangle; anisotropy; current flow; electrical impedance; electrode; muscle Electrical impedance myography (EIM) is a non-invasive method of muscle evaluation that relies on application of high-frequency, low-intensity alternating electrical current to a limb and measurement of the consequent surface voltages over a muscle or muscle group of interest. 10,13 Abnormalities in muscle fiber structure and muscle membrane health are reflected in the impedance changes measured in this way. 5,10,15 The major parameters measured include resistance (R) and reactance (X), and, from these, the phase (θ) of the tissue can be calculated. Although R and X are both potentially valuable variables in their own right, the phase tends to help account for size and shape effects and has thus been the focus of much of our EIM work to date. Indeed, phase declines in both localized and generalized neuromuscular disorders and has the potential to serve as a new approach for assessing disease severity. 5,10,11,15 EIM measurements are influenced by two distinct aspects of electrical conduction of muscle: (1) the properties of individual fibers, in particular via the capacitance of their membranes; and (2) the high degree of columnar order in the arrangement of the fibers. Indeed, the tightly bundled fiber structure of muscle confers an important additional feature accessible to EIM: the characteristic that the ease of electrical current flow varies according to its angle relative to the muscle fibers. 1,2,4,8 This directional dependence of the impedance to current flow, termed anisotropy, impacts the three major EIM variables, X, R, and θ. anisotropy of muscle has been appreciated for decades, to our knowledge its potential measurement in the assessment of neuromuscular disorders has only recently been considered. 3 Both muscle and nerve disorders might be expected to produce substantially different effects. The columnar structure is more disrupted...

Objective To quantify disuse atrophy using electrical impedance myography (EIM), a noninvasive technique that we have used successfully to study neurogenic and myopathic atrophy. Design We performed EIM of the tibialis anterior of 10 subjects with disuse atrophy secondary to cast immobilization and in their contralateral normal leg. Subjects were studied shortly after cast removal and again several weeks to months after the cast was removed and normal mobility was restored. Setting Outpatient neurology and orthopedic practices at a tertiary care medical center. Participants 10 otherwise healthy subjects with unilateral leg fracture. Main Outcome Measures Resistance, reactance, and phase measured at 50 kHz. Results We found that the main EIM outcome parameter, phase at 50 kHz, was lower in the immobilized leg in 9 of 10 cases. Additionally, when normal mobility was restored, the phase of the casted leg increased relative to its initial measurement in all 10 cases, while it increased inconsistently in the contralateral leg. Conclusions EIM may be a powerful tool for the assessment of disuse atrophy.

Skeletal muscle is electrically anisotropic, with applied high-frequency electrical current flowing more easily along than across muscle fibers. As an early step in harnessing this characteristic for clinical use, we studied approaches for maximizing the measured anisotropy by varying electrode size and applied current frequency in the tibialis anterior of 10 normal subjects. The results were compared to those from two patients with amyotrophic lateral sclerosis (ALS). Current was applied percutaneously, first parallel and then perpendicular to the major fiber direction of the muscle at frequencies ranging from 20 kHZ to 1 MHZ, using a fixed voltage-electrode length and varying the current-electrode length. The measured anisotropy was most pronounced using the longest length current electrodes and with a 125-kHZ applied frequency for the major outcome parameter phase. In addition, the two ALS patients showed very distinct anisotropic patterns. These results support the belief that, with the appropriate measurement technique, non-invasive assessment of electrical anisotropy of muscle may have useful clinical application.