Characterization of New Optical Imaging Biomarkers of Peripheral Nerve Overstimulation

Abstract

An animal study was designed to explore the potentially damaging effects of electrical overstimulation on peripheral nerves using a polarization‐sensitive optical coherence tomography (PS‐OCT) imaging and stimulation platform. Lewis rats (n = 15) were divided into 3 groups with 2 different levels of stimulation applied to the exposed sciatic nerve: Group 1 (sham control): 0.0 mA/0 Hz, Group 2: 3.4 mA/50 Hz, and Group 3: 6.8 mA/100 Hz. On day 1, the protocol contains Von Frey and walking track behavioral tests, followed by sciatic nerve exposure surgery (left leg). Electrical stimulation of the nerve was performed for one hour, with a one‐hour recovery period. PS‐OCT and optical coherence tomography angiography (OCT‐A) imaging were performed both prior to and during these periods. On day 7, this protocol was repeated without stimulation. OCT imaging was performed along a 5‐mm length of the sciatic nerve, including sites proximal and distal to injury. Both the stimulated and contralateral sciatic nerves were then resected and processed for histology.Image analysis quantified the effects of electrical stimulation at the two time points and allows comparison between stimulation levels. In all cohort groups, PS‐OCT image features remained consistent on day 1. On day 7, we observed relatively few PS‐OCT or OCT‐A changes compared to day 1 in the sham control animals. Groups 2 and 3 showed a clear increase in OCT‐A perfusion in the stimulated region, while there is a drop in PS‐OCT signal in the same area. The PS‐OCT signal drop‐out appears more pronounced in Group 2 compared to Group 3. It is not yet clear if the PS‐OCT signal drop‐out is related to demyelination or from shadowing from hyperperfused tissue. Walking track video was analyzed frame‐by‐frame for animal foot spread to determine sciatic and tibial function indices. This data, along with Von Frey withdrawal forces, were analyzed with statistical comparison tests (ANOVA) and indicated no significant differences between the groups nor significant interactions when compared across stimulation group, pre‐ and post‐exposure, and ipsilateral‐contralateral leg. Histological samples were processed for histomorphometry and axonal damage proximal and distal to the injury, as well as for immunohistochemistry of axons, macrophages, and nuclei in the stimulated and unstimulated regions in all groups.PS‐OCT and OCT‐A proffer real‐time observation of nerve changes with overstimulation that are undetectable by behavioral testing.Support or Funding InformationNIH SPARC