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 in ner ear has to be investigated for therapeutic effectiveness in cochlear in jury and dysfunction. in vitro data demonstrate low-level laser-induced pho tochemical and photobiologic cell response, depending on cell type and irra diation parameters such as light dose. The aim of the presented study was t o determine the light dose received by the cochlear hair cells by using dif ferent irradiation modalities for the human petrous bone. Study Design/Materials and Methods: Lightdosimetric assessment was performe d in human cadaver temporal bones (n = 13) after removing the cochlear memb ranous labyrinth. The external auditory meatus, the tympanic membrane (quad rants), and the mastoid bone were illuminated by a helium-neon laser (lambd a = 593 nm) and diode lasers of different wavelengths (lambda = 635, 690, 7 80, and 830 nm). The spatial distribution of transmitted light in the cochl ear windings was measured by means of a retrocochlearly positioned endoscop ic 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 fact ors have been calculated by a Monte Carlo procedure on the basis of experim entally determined optical properties of skull bone. Results: The transmission of Light across the tympanic cavity and the promo ntory depends strongly on wavelength of the laser and the position of the r adiator. Transtympanal irradiation results in spatial intensity variations of a factor 4 to 10 within the cochlear windings. The space irradiance in a n intact cochlea is 10 to 20 times the measured irradiance. For an irradiat ion of the mastoid, the light transmission within the cochlea is 10(3) to 1 0(5) times smaller compared with an irradiation of the tympanic membrane an d is extremely variable for different specimens. Conclusion: The strong dependence of the cochlear light distribution on var ious irradiation parameters demonstrates the impact of preclinical Lightdos imetric investigations for effective individual laser irradiation of the hu man cochlea. Because of the observed spatial intensity variations, the opti mal 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 lea ds to therapeutically insufficient light doses within reasonable treatment times, whereas transmeatal irradiation is recommendable. Further studies ar e mandatory for development of clinical devices for transmeatal irradiation of the cochlea. Lasers Surg. Med. 28:18-26, 2001. (C) 2001 Wiley-Liss, Inc .