| We present the investigation of diffuse optical signals, on a 100 ms time scale, in response to electrical stimulation of peripheral nerves. We studied potential clinical applications, as well as the spatial and spectral dependence of these signals measured on the sural and median nerves of healthy human subjects, using a two-wavelength near-infrared spectrometer and a broadband spectral system. The main focus of the studies presented in this work is to determine the physiological mechanism behind these 100 ms optical responses. We discuss several possible origins: (1) either dilation or constriction of blood vessels, (2) capillary bed hemodynamics, (3) blood vessel(s) displacement, and (4) fiber-skin optical coupling effects.;We have repeatedly measured an optical signal on the order of ~0.1% change in intensity and peaking at ~100 ms. The optical signals show a typical dose-response curve as well as delays in the peak times and larger pulse widths in subjects with diabetic neuropathy, indicating clinical relevance. Spatially dependent measurements along a line (16 positions, 2 mm intervals) perpendicular to the sural nerve showed significant changes where the nerve crossed the line of measurement, while spatially dependent measurements around a fixed point (12 source positions, at 30° intervals) showed repeatable positive (12-3 o'clock) and negative (5-11 o'clock) optical responses, indicating an anatomic origin. We argue that vessel dilation or constriction would exhibit only negative or only positive changes in optical intensity, respectively, and therefore could not be the origin of our signal. Venous and arterial occlusion experiments show that stimulated responses exhibit slight changes (~0.2-0.6%) during large tissue hemodynamic changes (~20-90%), indicating that the stimulated response is not due to the capillary bed hemoglobin affected by vascular occlusion. We used diffusion theory and Pade approximation to model blood vessel displacements within the tissue and were able to replicate both spatial and spectral experimental results. Broadband spectral results show characteristic hemoglobin absorption peaks, corroborating with a vascular absorption origin. Fiber-skin optical coupling effects did not show the spatially dependent positive and negative signals nor the spectral dependence seen in the stimulated optical response.;We conclude that the most probable origin of the optical response to peripheral nerve stimulation is from displacement of blood vessels within the optically probed tissue, as a result of muscle twitch. The results of this work suggest that the diffuse optical responses to peripheral nerve stimulation are a potential diagnostic tool for neuro-muscular diseases. |
