Optical stimulation of the facial nerve: a new monitoring technique?

Abstract

Objectives/ hypothesis: One sequela of skull base surgery is iatrogenic damage to cranial nerves, which can be prevented if the nerve is identified. Devices that stimulate nerves with electric current assist in nerve identification. Contemporary devices have two main limitations: 1) the physical contact of the stimulating electrode and (2) the spread of the current through the tissue. In contrast to electrical stimulation, pulsed infrared optical radiation can be used to safely and selectively stimulate neural tissue and might be valuable for screening.

Methods: The gerbil facial nerve was exposed to 250 microsecond pulses of 2.12 microm radiation delivered via a 600-microm-diameter optical fiber at a repetition rate of 2 Hz. With use of 27 GA, 12-mm intradermal electrodes, muscle action potentials were recorded. Nerve samples were examined for possible tissue damage.

Results: Eight facial nerves were stimulated with radiant exposures between 0.71 and 1.77 J/cm, resulting in compound muscle action potentials (CmAPs) that were simultaneously measured at the m. orbicularis oculi, m. levator nasolabialis, and m. orbicularis oris. Resulting CmAP amplitudes were 0.3 to 0.4 mV, 0.15 to 1.4 mV, and 0.3 to 2.3 mV, respectively, depending on the radial location of the optical fiber and the radiant exposure. Individual nerve branches were also stimulated, resulting in CmAP amplitudes between 0.2 and 1.6 mV. Histology revealed tissue damage at radiant exposures of 2.2 J/cm but no apparent damage at radiant exposures of 2.0 J/cm.

Conclusions: The experiments showed that selective muscle action potentials can be evoked optically in the gerbil facial nerve without direct physical contact.

Figures

Fig. 1

(A) Laterofacial view of a gerbil head after cutis and subcutaneous fat and parotid gland are removed. Facial nerve is exposed, and its trunk and branches are labeled as shown. Round dots (counted from top) mark insertion site for muscle electrodes m. orbicularis oculi, m. levator nasolabialis, and m. orbicularis oris, respectively. Rectangles represent nerve recording positions for the buccal and mandibular branches of the facial nerve. Arrows point to the region of optical and electric stimulation at the facial nerve. (B) Preparation of gerbil for optical stimulation. Facial nerve trunk is exposed, and optical fiber is lowered to the nerve. Recording electrodes are in place (as explained in A). During stimulation, twitching of the gerbil’s whiskers could be seen while recording compound muscle action potentials from m. levator nasolabialis.

Fig. 2

Measurements of laser beam profile. Optical fiber, which is coupled to the laser, is placed approximately 2 mm away from the bottom of the Petri dish (right column). The Petri dish contains a thin glass window below the tip of the optical fiber. Radiation energy is measured with an energy sensor below the dish. During measurements, a razor blade is moved in 50-μm increments into the optical path, thereby blocking radiation to the energy sensor. The razor blade is moved along traces with increasing distance from the dish. Resulting plots show incremental energy change for each advancement of the razor blade. Results reveal a gaussian laser beam profile. Measurements were made while the dish was filled with air (upper left), with Ringer’s lactate (middle left), and with a 1.5-mm thick slice of muscle tissue immersed in Ringer’s lactate placed below the optical fiber (lower left). The width of the laser beam becomes wider when tissue is placed between the optical fiber and the energy sensor.

Fig. 3

Compound muscle action potentials recorded in m. levator nasolabialis after facial trunk stimulation with 70 μA electric and 1.13 J/cm2 optical stimulation. Note optical radiation and electrical current generated similar response amplitudes.

Fig. 4

Compound muscle action potentials (CmAPs) measured in three facial muscles. CmAP response evoked by an electrical stimulus (70 μA) on facial nerve trunk (left column). CmAP responses after optical radiation (0.88 J/cm2) at the same stimulation site for electrical stimulation (4 right columns). Both CmAPs are an average of 10 recordings. After the first recording (nerve center), the optical fiber was moved by 50 μm steps radially away from the nerve center. After each step, additional measurements were performed. Note the isolated maximal response of the m. levator nasolabialis after moving the fiber 50 μm from the initial nerve center.

Fig. 5

Compound muscle action potential responses generated by optical radiation of the facial nerve trunk (top row) and facial nerve branches (bottom row). The facial nerve trunk was stimulated at 0.88 J/cm2, and branches were stimulated at 1.13 J/cm2. The fascia overlying nerve branches, which is not present on nerve trunk, might explain the difference in radiant exposure to evoke similar responses.

Fig. 6

Light microscopy photograph of facial nerve cross section after optical stimulation with 20 pulses of 2.1-μm radiation at 4.0 J/cm2 (left). Enlarged section reveals obvious disruption and carbonization of neural tissue. Light microscopy photograph of facial nerve after stimulation with 2.0 J/cm2 (right). Visible damage caused by optical radiation is not observed.

Fig. 7

Traces show the width of the laser beam for different conditions described in Figure 6. The width of the beam was determined as the full width at 6 dB below the maximum optical energy for each of the traces. The width is very similar for the beam in air and in Ringer’s lactate. Tissue in the optical path, however, widens the beam.