Distinguishing neuromuscular disorders based on the passive electrical material properties of muscle.

INTRODUCTION: The passive electrical properties of muscle, including conductivity and permittivity and their directional dependence, may be altered in neuromuscular disease; however, the character of these alterations is unknown. METHODS: Fifteen wild-type mice, 13 amyotrophic lateral sclerosis mice, 9 muscular dystrophy (mdx) mice, and 15 mice with induced disuse atrophy were euthanized, and the gastrocnemius was excised. A 50-kHz current was applied immediately to the ex vivo muscle, and its material properties were calculated. RESULTS: The disease groups showed distinct material property values [F(12, 119) = 14.6, P < 0.001] according to MANOVA. Post-hoc tests confirmed that differences existed between all 4 groups. They were most pronounced in the mdx mice, which had markedly increased conductivity. Direction-dependent properties of current flow also were significantly different among the groups (P < 0.001). CONCLUSIONS: These data confirm that the inherent passive electrical properties of muscle differ by disease type. We anticipate that similar data could eventually be obtained via surface measurements, providing an innovative approach to muscle disease diagnosis.

Optimizing electrode configuration for electrical impedance measurements of muscle via the finite element method.

Electrical impedance myography (EIM) is a technique for the evaluation of neuromuscular diseases, including amyotrophic lateral sclerosis and muscular dystrophy. In this study, we evaluated how alterations in the size and conductivity of muscle and thickness of subcutaneous fat impact the EIM data, with the aim of identifying an optimized electrode configuration for EIM measurements. Finite element models were developed for the human upper arm based on anatomic data; material properties of the tissues were obtained from rat and published sources. The developed model matched the frequency-dependent character of the data. Of the three major EIM parameters, resistance, reactance, and phase, the reactance was least susceptible to alterations in the subcutaneous fat thickness, regardless of electrode arrangement. For example, a quadrupling of fat thickness resulted in a 375% increase in resistance at 35 kHz but only a 29% reduction in reactance. By further optimizing the electrode configuration, the change in reactance could be reduced to just 0.25%. For a fixed 30 mm distance between the sense electrodes centered between the excitation electrodes, an 80 mm distance between the excitation electrodes was found to provide the best balance, with a less than 1% change in reactance despite a doubling of subcutaneous fat thickness or halving of muscle size. These analyses describe a basic approach for further electrode configuration optimization for EIM.

Assessment of alterations in the electrical impedance of muscle after experimental nerve injury via finite-element analysis.

The surface measurement of electrical impedance of muscle, incorporated as the technique of electrical impedance myography (EIM), provides a noninvasive approach for evaluating neuromuscular diseases, including amyotrophic lateral sclerosis. However, the relationship between alterations in surface impedance and the electrical properties of muscle remains uncertain. In order to investigate this further, a group of healthy adult rats, a group of rats two weeks postsciatic crush, and a group of animals six months postcrush underwent EIM of the gastrocnemius-soleus complex. The animals were then killed and the conductivity and permittivity of the extracted muscle measured. Finite-element models based on MRI data were then constructed for each group. The characteristic EIM parameter, 50 kHz phase (±standard error), obtained with surface impedance measurements was 17.3° ± 0.3° for normal animals, 13.8° ± 0.7° for acutely injured animals, and 16.1° ± 0.5° for chronically injured animals. The models predicted parallel changes with phase values of 24.3°, 18.8°, and 21.2° for the normal, acute, and chronic groups, respectively. Other multifrequency impedance parameters showed similar alterations. These results confirm that surface impedance measurements taken in conjunction with anatomical data and finite-element models may offer a noninvasive approach for assessing biophysical alterations in muscle in neuromuscular disease states.

Assessing electrical impedance alterations in spinal muscular atrophy via the finite element method.

Electrical impedance myography (EIM) is a surface-based, non-invasive technique of evaluation of muscle health, involving the application of high frequency, low-amplitude current to the skin over a muscle of interest. Results from a previous animal study suggest that the finite element method can relate disease-induced changes in electrical properties of the muscle to alterations in surface impedance measurements; however, whether such an approach will prove useful in human models is uncertain. Therefore, to further investigate this question, we have created a single finite element model of the human biceps muscle using data from one healthy subject and one with spinal muscular atrophy (SMA), each of whom had comparable age, limb girth, muscle size, and subcutaneous fat thickness. Since healthy human tissue was unavailable, permittivity and conductivity measurements were obtained from five healthy and five advanced amyotrophic lateral sclerosis rat gastrocnemius muscles immediately after sacrifice; their data were input into the human biceps model and the expected surface voltages calculated. We then compared the results of this model to the actual surface EIM data for both individuals. Although the actual resistance and reactance values varied and the peak values were displaced, the resulting maximum phase predicted by the model approximated that obtained with surface recordings. These results support that alterations in the primary characteristics of muscle impact the surface impedance measurements in meaningful and likely predictable ways.