The present experiments verify our hypothesis that the facial nerve can be stimulated with pulsed infrared laser radiation, thus presenting optical stimulation as a potential alternative to electrical stimulation for facial nerve monitoring and the detection and stimulation of nerve structures during surgery. This method of optical nerve stimulation, which occurs without direct contact between the stimulator and the nerve tissue, does not appear to damage the nerve when using low optical energy levels. Moreover, stimulation with optical radiation is more spatially selective than electrical stimulation.
Interaction of the laser radiation with the tissue determines the spatial distribution of light in the tissue. Primary light-tissue interactions fall into two broad categories, absorption and scattering, the prevalence of which is highly dependent on the wavelength of the light and the tissue characteristics.16,17
At first approximation, absorption dominates the light-tissue interaction in the mid-infrared wavelength range. The axial distribution of a primarily absorbed wavelength can be determined using Beer’s Law, a description of the exponential decrease of laser energy over the optical path. However, scattering does play some small role in the light-tissue interaction at mid-infrared wavelengths. Spatial scatter is caused by random spatial variations in tissue density, refractive index, and dielectric constant. Although it has been reported that the scattering decreases monotonically with wavelength, a rigorous description of the multiple scattering events that occur as a collimated beam propagates through tissue is extremely difficult.17
At present, we have determined that the infrared wavelength of the radiation, which we used to stimulate the facial nerve, spreads little in air (). Although tissue in the radiation path increases the scatter (), the width of the beam is less than 1 mm at the level of the nerve. The addition of focusing optics could potentially decrease this value to the desired spot size. A more thorough study of this issue is in progress and will be published separately.
Contemporary cranial nerve monitors inject current into the tissue and thereby stimulate nerves. The physical properties of the tissue, however, result in a wide spread of the electric current and prevent the devices from being spatially selective. Moreover, the current spread can lead to “false responses.” For example, a facial nerve that has been severed may still be stimulated more distally from the damage because of the current spread through the surrounding tissue.2
The neural response to the electrical stimulation could lead the surgeon to believe that the nerve is unscathed because of a positive response to electrical stimulation. However, these false recordings could be avoided with optical stimulation because a response is only evoked at the site of irradiation; the light does not spread through the tissue.
The spatial selectivity of optical nerve stimulation has been demonstrated in our experiments at the facial nerve trunk: different nerve bundles within the main nerve trunk were individually stimulated when the radial location of the stimulation site was varied. Similar spatial selectivity has been reported by Wells et al.14
for the rat sciatic nerve, in which the authors were able to individually stimulate single muscles (i.e., hamstring muscle) while scanning the small diameter optical beam over the sciatic nerve. In contrast to optical stimulation, electric stimulation evoked strong and unselective responses in all muscle groups innervated by the sciatic nerve ().
Moreover, we have shown for the auditory nerve that selective stimulation is possible.18,19
Here, the tip of the optical fiber is immersed in fluid. The optical radiation has to penetrate the fluids and the modiolar bone before it reaches the target structures, the auditory neurons. The optical stimulation has been significantly more selective when compared with electrical stimulation.20
Moving the optical fiber across the opening of the round window stimulated different locations along the cochlea, thereby “scanning” the cochlea. Consequently, these data further suggest that a device using optical radiation to stimulate neurons can be used as a tool for nerve scanning. The parameter space that is useful for scanning cranial nerves, however, must first be optimized.
One might argue that stimulation with optical radiation might not be possible during difficult approaches, including approaches via the posterior fossa. The beam has to be directed toward the undersurface of acoustic neurinoma. This challenge is not necessarily a limitation because optics can be used, similar to optics in endoscopes, that are able to direct the optical beam toward the target, even in difficult to access locations.
Although the experiments confirmed our hypothesis, some shortcomings must be considered. The Holmium: YAG laser only emits 2.1 μ
m radiation. This wavelength corresponds to a penetration depth of approximately 400 μ
m, which describes the distance over which the incident optical energy is reduced by 67%. Despite this short penetration depth, we have been able to stimulate facial nerve branches through the overlying few-micrometer thick facial fascia. A longer penetration depth would appear to be beneficial when stimulating the nerve through surrounding tissue. It is not expected that thick tissue layers, on the order of several millimeters, could be penetrated with 2.1 μ
m radiation. Other wavelengths with longer penetration depths might be advantageous and should be tested in the future. Wells et al.14
and Izzo et al.15
showed that CmAPs and CnAPs can also be elicited with optical radiation at wavelengths from 1.844 to 1.873 μ
m, which corresponds to penetration depths of approximately 300 to 1,100 μ
The threshold of optical stimulation was approximately 0.71 J/cm2
for the facial nerve trunk and approximately 0.88 J/cm2
for the facial nerve branches, with both being stimulated through the facial fascia, which corresponds in the gerbil to several micrometers. After the histologic examination of the nerve samples, the nerve-damage threshold was determined to be above 2.0 J/cm2
; above this threshold, one saw carbonization and disruption within the nerve tissue, as shown in . The ratio between the ablation threshold and stimulation threshold revealed a safety ratio of approximately 2.6 for optical stimulation with the Ho:YAG laser radiation. Wells et al.14
determined a safety ratio of 6.25 using the Ho:YAG laser. Note that no difference exists between the damaging threshold in the two experiments: the radiant exposure for both sets of experiments was 2.0 J/cm2
. However, a difference exists in the stimulation threshold: 0.71 J/cm2
versus 0.32 J/cm2
in the experiment by Wells et al.14