Lightdosimetric quantitative analysis of the human petrous bone: Experimental study for laser irradiation of the cochlea

Background and Objective: Application of laser irradiation targeting the inner ear has to be investigated for therapeutic effectiveness in cochlear injury and dysfunction. In vitro data demonstrate low-level laser-induced photochemical and photobiologic cell response, depending on cell type and irradiation parameters such as light dose. The aim of the presented study was to determine the light dose received by the cochlear hair cells by using different irradiation modalities for the human petrous bone. Study Design/Materials and Methods: Lightdosimetric assessment was performed in human cadaver temporal bones (n = 13) after removing the cochlear membranous labyrinth. The external auditory meatus, the tympanic membrane (quadrants), and the mastoid bone were illuminated by a helium-neon laser (λ = 593 nm) and diode lasers of different wavelengths (λ = 635, 690, 780, and 830 nm). The spatial distribution of transmitted light in the cochlear windings was measured by means of a retro-cochlearly positioned endoscopic CCD camera for image processing and was assigned to acoustic frequencies according to the tonotopic organization of the cochlea. For an estimation of the corresponding space irradiance in an intact cochlea, correction factors have been calculated by a Monte Carlo procedure on the basis of experimentally determined optical properties of skull bone. Results: The transmission of light across the tympanic cavity and the promontory depends strongly on wavelength of the laser and the position of the radiator. Trans-tympanal irradiation results in spatial intensity variations of a factor 4 to 10 within the cochlear windings. The space irradiance in an intact cochlea is 10 to 20 times the measured irradiance. For an irradiation of the mastoid, the light transmission within the cochlea is 10 3 to 10 5 times smaller compared with an irradiation of the tympanic membrane and is extremely variable for different specimens. Conclusion: The strong dependence of the cochlear light distribution on various irradiation parameters demonstrates the impact of preclinical lightdosimetric investigations for effective individual laser irradiation of the human cochlea. Because of the observed spatial intensity variations, the optimal external light dose has to be chosen with regard to the tonotopy of the human cochlea. The obtained results are enabling us to apply defined laser light doses to different cochlear winding areas. Mastoidal irradiation leads to therapeutically insufficient light doses within reasonable treatment times, whereas transmeatal irradiation is recommendable. Further studies are mandatory for development of clinical devices for transmeatal irradiation of the cochlea. Lasers Surg. Med. 28:18-26, 2001.