Discriminating neurogenic from myopathic disease via measurement of muscle anisotropy

Abstract

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.

FIGURE 1

Technique for measuring skeletal muscle using the tibialis anterior. The electrode array is placed at multiple angles ranging from −90° to +90° (relative to the primary muscle fiber direction). The array here is shown at 0°. Current is applied via the outer electrodes, and the voltages are measured via the inner electrodes.

FIGURE 2

Raw anisotropy plots for the 15 subjects in a study of biceps brachii. Data show mean values ±1 standard deviation for R, X, and θ vs. angle ϕ.

FIGURE 3

Raw anisotropy plots for tibialis anterior. Data show mean values ±1 standard deviation for R, X, and θ vs. angle ϕ.

FIGURE 4

Normalized anisotropy plots for biceps. Data show mean values ±1 standard deviation for R_n, X_n, and θ_n vs. angle ϕ.

FIGURE 5

Normalized anisotropy plots for tibialis anterior. Data show mean values ±1 standard deviation for R_n, X_n, and θ_n vs. angle ϕ. Note the similarity between these curves and those for biceps (Fig. 4).

FIGURE 6

Internal reproducibility of measurements taken from the last normal subject studied, a 21-year-old man: (A) biceps and (B) tibialis anterior. Repeat measurements were made immediately after completing the first set on each muscle.

FIGURE 7

Normalized anisotropy plots for tibialis anterior from 6 patients with ALS plotted against θ, showing the elevation and distortion of the normal anisotropic pattern in these individuals.

FIGURE 8

Normalized anisotropy plots for tibialis anterior from 6 patients with myopathy, showing the diminution of the anisotropic pattern in these individuals.