Combining electromyography and Raman spectroscopy: optical EMG

Abstract

Introduction/aims: Electromyography (EMG) remains a key component of the diagnostic work-up for suspected neuromuscular disease, but it does not provide insight into the molecular composition of muscle which can provide diagnostic information. Raman spectroscopy is an emerging neuromuscular biomarker capable of generating highly specific, molecular fingerprints of tissue. Here, we present "optical EMG," a combination of EMG and Raman spectroscopy, achieved using a single needle.

Methods: An optical EMG needle was created to collect electrophysiological and Raman spectroscopic data during a single insertion. We tested functionality with in vivo recordings in the SOD1^G93A mouse model of amyotrophic lateral sclerosis (ALS), using both transgenic (n = 10) and non-transgenic (NTg, n = 7) mice. Under anesthesia, compound muscle action potentials (CMAPs), spontaneous EMG activity and Raman spectra were recorded from both gastrocnemius muscles with the optical EMG needle. Standard concentric EMG needle recordings were also undertaken. Electrophysiological data were analyzed with standard univariate statistics, Raman data with both univariate and multivariate analyses.

Results: A significant difference in CMAP amplitude was observed between SOD1^G93A and NTg mice with optical EMG and standard concentric needles (p = .015 and p = .011, respectively). Spontaneous EMG activity (positive sharp waves) was detected in transgenic SOD1^G93A mice only. Raman spectra demonstrated peaks associated with key muscle components. Significant differences in molecular composition between SOD1^G93A and NTg muscle were identified through the Raman spectra.

Discussion: Optical EMG can provide standard electrophysiological data and molecular Raman data during a single needle insertion and represents a potential biomarker for neuromuscular disease.

Keywords: ALS; EMG; Raman spectroscopy; biomarker; muscle; optical EMG.

FIGURE 1

Optical EMG: electrophysiological data. (A) CMAP waveforms from both SOD1^G93A and NTg mice using the optical EMG needle. (B) Spontaneous EMG activity (PSWs; example circled) from the optical EMG needle. Note that the PSWs here have an irregular inter‐wave interval as the run was coming to an end. See supplemental information for further examples. (C) Comparison of CMAP amplitude in NTg and SOD1^G93A mice using the optical EMG needle. (D) Comparison of CMAP amplitude in NTg and SOD1^G93A mice using a standard concentric EMG needle.

FIGURE 2

Optical EMG: Raman spectroscopy data. (A) Mean Raman spectra (± SD) for NTg (top, orange) and SOD1^G93A mice (bottom, blue), both at 90 days. The two spectra have been arbitrarily offset on the y‐axis so that the two plots can be clearly visualized. Basic biochemical assignments are shown by colored bars. (B) Difference of the mean spectrum (SOD1^G93A minus NTg). This highlights the spectral regions at which the two groups differ the most. (C, D) Peak ratios exploring differences in the molecular composition of SOD1^G93A and NTg muscle.

FIGURE 3

In vivo intra‐muscular Raman spectroscopy does not alter CMAP amplitude. (A) Example CMAPs and Raman spectra from SOD1^G93A mice, recorded using the optical EMG needle. No significant difference was seen between the pre‐ and post‐Raman CMAP amplitudes. For convenience, right and left leg data are shown on the same plot. (B) Example CMAPs and Raman spectra from NTg mice. No significant difference was seen between the pre‐ and post‐Raman CMAP amplitudes. As above, right and left leg data are shown on the same plot.