Optical detection of peripheral nerves: an in vivo human study

A critical challenge encountered in interventional pain medicine procedures is to accurately and efficiently identify transitions to peripheral nerve targets. Current methods, which include ultrasound guidance and nerve stimulation, are not perfect. In this pilot study, we investigated the feasibility of identifying tissue transitions encountered during insertions toward peripheral nerve targets using optical spectroscopy. Using a custom needle stylet with integrated optical fibers, ultrasound-guided insertions toward peripheral nerves were performed in 20 patients, with the stylet positioned in the cannula of a 20-gauge stimulation needle. Six different peripheral nerves were represented in the study, with 1 insertion per patient. During each insertion, optical reflectance spectra were acquired with the needle tip in subcutaneous fat, skeletal muscle, and at the nerve target region. Differences in the spectra were quantified with 2 parameters that provide contrast for lipid and hemoglobin, respectively. The transition of the needle tip from subcutaneous fat to muscle was associated with lower lipid parameter values (P = 0.003) and higher hemoglobin parameter values (P = 0.023). The transition of the needle tip from the muscle to the nerve target region was associated with higher lipid parameter values (P = 0.008). The results indicate that the spectroscopic information provided by the needle stylet could potentially allow for reliable identification of transitions from subcutaneous fat to skeletal muscle and from the muscle to the nerve target region during peripheral nerve blocks.