Clinical Safety and Efficacy of Infrared Neural Stimulation During Nerve Transfers

A broad range of surgical procedures involve locating and identifying nerves and neural structures. In cases like brachial plexus reconstruction, the surgeon needs to identify specific nerve fascicles for grafting. While in other cases like dorsal root rhizotomies, the surgeon needs to identify hyperactive nerve rootlets for transection. The ability to accurately identify and localize nerves is critical in avoiding unintentional and detrimental consequences which can be costly source of medicolegal litigation. Surgeons commonly depend on their anatomical knowledge and visualization of the surgical field to identify and locate nerves. The distribution of nerves, however, varies from person to person often deviating from anatomical atlases. To account for this interpatient variability, surgeons also utilize electrical stimulation (ES) methods to identify and locate nerves and nerve fascicles intraoperatively. After ES, surgeons rely on electromyography or the presence of visible muscle contractions to confirm the identity and location of the stimulated nerve. ES, however, is constrained by inherent physical limitations. Current spread, in which electrical current disperses into the surrounding tissue, has long plagued ES methods. As a result of current spread, the electrical stimulus will extend beyond the point of contact with the stimulation probe activating adjacent nerve fascicles. Consequently, ES' poor stimulation focality makes it a non-ideal means to target small neural targets or to precisely monitor neural structures in immediate contact with the stimulation electrode. In clinical procedures, the activation of distant neural tissue or multiple neural structures can introduce ambiguity and uncertainty as to identity and location of specific neural structures that causes concern among surgeons. Thus, there is a need for a neural stimulation technique with a higher degree of spatial selectivity to improve nerve localization and identification during surgery.

Infrared neural stimulation (INS) is a label-free optical method used to excite neural tissue with low energy pulses of infrared light. As an optical neurostimulation technique, INS possesses a high degree of spatial specificity without the need for direct contact with the tissue (Figure 1). Our group has repeatedly demonstrated the spatial specificity of INS to activate individual nerve fascicles in rats, nonhuman primates, and humans in vivo. The initial findings from our group have also lead other groups to leverage the spatial selectivity of INS for other clinical applications such as nerve monitoring and cardiac pacing. The inherent spatial precision of INS is a direct result of its underlying biophysical mechanism. The deposition of infrared light into the neural tissue causes a transient thermal gradient that depolarizes the cell membrane through a thermally induced change in membrane capacitance. The thermal energy from the infrared pulses is spatially confined to the irradiated volume as determined by laser spot size and the penetration depth of the light into tissue. Our group as well as others have histologically proven that INS can safely and reliably excite nerves without inflicting damage. Given these advantages over traditional means of ES, the investigators believe INS is a viable alternative stimulation technique especially in surgical cases where there is a need for confined neurostimulation. The goal of this proposal is to take advantage of INS' intrinsic strengths and apply them to upper limb surgeries where spatially precise neurostimulation is necessary for nerve identification and localization. This study will build upon our group's previous clinical work and further establish INS as a valuable addition to clinical neurostimulation methods.

The objective of this proposal is two-fold: to demonstrate the efficacy of INS for spatially selective nerve stimulation in the upper extremity and to determine the histological safety of INS using diode laser systems in human patients. To do so, the investigators will recruit patients undergoing brachial plexus reconstruction (BPR) and nerve transfer surgeries wherein both the effectiveness and spatial selectivity of INS can be demonstrated and histological samples can be obtained without detriment to the patients' quality of care or recovery. To accomplish these objectives, the investigators propose the following aims:

Aim 1: Design and fabricate a clinical fiberoptic probe for a diode-based INS system Using our existing diode lasers, the investigators will create a clinical diode laser INS system by constructing INS fiberoptic probes. Fiberoptic probes will be designed and characterized to collect light from our diode lasers and deliver that light to the nerve. The probes will be sterilizable, ergonomic and maneuverable, transmissive at the relevant wavelengths, and provide consistent stimulation parameters. The probe design will be based on existing clinical ES probes and modified based on surgeon feedback.

Aim 2: Demonstrate the efficacy of INS in nerve transfer cases While INS with expensive clinical lasers has been shown to be an effective means of nerve stimulation, INS with diode lasers has yet to be demonstrated in human patients. Here, the investigator will stimulate nerves identified for transfer or grafting over a range of simulation parameters (pulse width, spot size, energy per unit area, etc) to determine the stimulation threshold. Only portions of the nerve that are no longer functionally required will be stimulated optically. Successful INS events will be determined by visual muscle contractions. Stimulation thresholds will be determined by fitting the data to a cumulative distribution function.

Aim 3: Determine the histological safety of the diode-based INS system In conjunction with Aim 2, stimulation sites on the nerve will be harvested and histologically examined for evidence of INS-induced damage. Stimulation sites will be marked with a tissue dye and excised for histological preparation. Regarding the tissue dye, the study surgeon will use a sterile surgical marking pen to mark the stimulation site. These sterile surgical marking pens are meant to be used in-vivo to mark tissue. The sterile surgical marking pens are used regularly in routine care to mark tissue intraoperatively. There is no additional risk to study participants who will have their stimulation site marked with a sterile surgical marking pen. The stimulation site (nerve segment) that will be marked with the sterile surgical marking pen will be excised for histological preparation.

Once the samples are fixed and sliced, they will be stained with toluidine blue and/or Luxol fast blue-Periodic acid Schiff stains and imaged. Imaged slides will be examined for evidence of myelin disruption, collagen hyalinization, and charring among other criteria. A histological damage grading scheme will also be developed based on the severity and depth of the damage with respect to the nerve itself. Damage thresholds will similarly be determined using Probit Analysis and compared to the stimulation threshold to determine the margin of safety.

INS has the potential to serve as valuable neural stimulation technique in the clinic. Due to its high degree of spatial selectivity, INS could improve upon current electrical methods of stimulation that can excite multiple nerves or fascicles at once due to current spread and therefore reduce uncertainty during nerve identification procedures. While INS has been successfully and safely utilized in humans, cost-effective laser diode systems have yet to be evaluated. This proposal will allow for the development and testing of a clinical diode-based INS system in terms of both efficacy and safety. This proposal brings together a complementary team of investigators with unique expertise in both the clinical and technical aspects of INS and its applications. The utilization of INS in BPR and nerve transfer cases will allow us to determine the stimulation and safety thresholds of diode-based INS in humans will pave the way for this technique to be employed in other procedures where improved spatial specificity is highly advantageous. By extending the use of INS to upper limb cases, this technique could significantly impact the standard of care in surgeries utilizing stimulation to identify specific nerves and nerve fascicles.