Optical stimulation of the facial nerve: a new monitoring technique?
OBJECTIVE
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.
Teudt
Ingo Ulrik
IU
Department of Otolaryngology-Head and Neck Surgery, Northwestern University, Feinberg School of Medicine, Chicago, Illinois 60611-3008, USA.
Nevel
Adam E
AE
Izzo
Agnella D
AD
Walsh
Joseph T
JT
Richter
Claus-Peter
CP
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D004558:Electric Stimulation / Q000295:instrumentation; D004567:Electrodes, Implanted; D005154:Facial Nerve / Q000502:physiology* / Q000528:radiation effects*; D005336:Fiber Optic Technology; D005849:Gerbillinae; D007834:Lasers; D018482:Muscle, Skeletal / Q000502:physiology; D009801:Oculomotor Muscles / Q000502:physiology; D009900:Optic Nerve / Q000528:radiation effects; D055100:Optical Fibers