Infrared neural stimulation of human spinal nerve roots in vivo

Abstract

Infrared neural stimulation (INS) is a neurostimulation modality that uses pulsed infrared light to evoke artifact-free, spatially precise neural activity with a noncontact interface; however, the technique has not been demonstrated in humans. The objective of this study is to demonstrate the safety and efficacy of INS in humans in vivo. The feasibility of INS in humans was assessed in patients ([Formula: see text]) undergoing selective dorsal root rhizotomy, where hyperactive dorsal roots, identified for transection, were stimulated in vivo with INS on two to three sites per nerve with electromyogram recordings acquired throughout the stimulation. The stimulated dorsal root was removed and histology was performed to determine thermal damage thresholds of INS. Threshold activation of human dorsal rootlets occurred in 63% of nerves for radiant exposures between 0.53 and [Formula: see text]. In all cases, only one or two monitored muscle groups were activated from INS stimulation of a hyperactive spinal root identified by electrical stimulation. Thermal damage was first noted at [Formula: see text] and a [Formula: see text] safety ratio was identified. These findings demonstrate the success of INS as a fresh approach for activating human nerves in vivo and providing the necessary safety data needed to pursue clinically driven therapeutic and diagnostic applications of INS in humans.

Keywords: electromyography; infrared neural stimulation; neurophotonics; optics; spinal nerve roots.

Figures

Fig. 1

Schematic diagram of optical box for modifying high-power and high-frequency clinical system to optimal parameters for infrared neural stimulation (INS). High-energy laser light entered the box from a 550-μm optical fiber [multimode fiber (FG550LEC), Thor Labs, NA=0.22] connected to the clinical system. A biconvex lens (f=25  cm, Thor Labs) was used to focus the divergent beam from the input fiber through a 20/80% beam splitter and couple the attenuated beam of laser light into the delivery fiber (d=550  μm). A photodiode, placed between the input fiber and lens, detected high-frequency input light pulses and triggered an optical shutter to adjust the output frequency to 2 Hz using a pulse generator. An external micromanipulator attached to the optical fiber mount inside the black box allowed for fine control of the light coupling efficiency and, thus, the amount of light entering the output 550-μm core optical fiber (NA=0.22). The output end of this fiber was mounted onto a sterilized, handheld optical fiber probe (600  μm, NA=0.22) for ease of use by the neurosurgeon during stimulation. A red HeNe aiming beam output from the clinical system was maintained through the probe output, providing a known stimulation site. The laser-probe system was footswitch controlled.

Fig. 2

Pulsed infrared light evokes compound muscle action potentials through stimulation of human dorsal root. Electromyogram (EMG) recordings from 12 muscles in the lower extremities (top six traces = left leg, bottom six traces = right leg) in response to electrical stimulation (20 Hz, 100  μs, 1 s) and INS (2 Hz, 350  μs, 10 s) on a left dorsal root L4. (a) Responses obtained from 20 Hz train of electrical stimulation (∼2.0  mA) visualized on a voltage scale of ±500  μV. EMG recordings indicate a response in all left and right side muscles. (b) Reponses obtained from INS (1.03  J/cm2) of same nerve with a voltage scale of ±500  μV. A single response is observed in the left adductor muscle (denoted by black arrows).

Fig. 3

Histological comparison of safe versus nonsafe of optically stimulated experimental sites in dorsal lumbar nerve roots. (a) Experimental site (500× magnification) resulting in the stimulated muscle recordings using 0.91  J/cm2 for a total of 20 laser pulses (damage score=0). Numerous axons (arrows) can be seen in the nerve fibers in this image. (b) An overview of the thermal lesion produced within the dorsal root nerve (200× magnification) using 1.32  J/cm2 (damage score=3). The lesion is generally hyperchromatic and the endoneurial tubes are straightened out in the center. The arrow represents the interface between the thermally damaged tissue and the underlying normal tissue. (c) The demarcated area from (b) at 500× magnification. This lesion has the following features characteristic of thermal damage: swelling and hyperchromasia of the collagen in the endoneurium (W), granular degeneration of myelin (X), straightening of the nerve fibers as compared to the intact fibers at the bottom of the image, vacuolization, and expansion of the endoneurial tubes in some areas (X). Although some axons show axonal thermal damage (Y), others in the lesion (Z) do not at this level of magnification.

Fig. 4

Identification of safe radiant exposures for stimulation human dorsal roots. Results from the safety and efficacy study from seven patients (102 stimulation sites) graded on a three-point scale. Laser radiant exposure used for each stimulation (20 pulses at 2 Hz recorded for 10 s) is graphed as a function of the thermal damage score assigned to each site based on histopathology from optical stimulation over a range of laser energies (0.46 to 2.05  J/cm2). Probability of damage is given by cumulative distribution function from statistical analysis of binary damage data. Successful stimulations are indicated by blue diamonds, no stimulation is indicated by red dots, and stimulation sites with no EMG recordings are indicated by black triangles. Compound muscle action potentials are first observed at 0.53  J/cm2. Stimulation range is identified as 0.53 to 1.28  J/cm2. Safe stimulation range is identified as 0.53 to 1.00  J/cm2.