| Abstract #589, Date 2/16/99, Session R4, Poster (B89) |
| Damages to the mammalian cochlea caused by low frequency ultrasound |
| *R. Dieler (Dept. Otolaryng. - HNS, U. of Wuerzburg); R. Hagen (Dept. Otolaryng. - HNS, Katharinenhospital); H. Bressmer, L. Joppek (Inst. Biomed. Eng., U. of Stuttgart); D. Luecke (Dept. Otolaryng. - HNS, Katharinenhospital); J. Nagel (Inst. Biomed. Eng., U. of Stuttgart) |
The question if exposure to low frequency ultrasound (US) in the 20-100 kHz range causes significant hazards to biological systems needs urgent clarification due to expanding applications in industry and medicine. Though both damaging effects and safe levels of exposure have been established for high frequency US used in medicine, almost no knowledge is available regarding the biological compatibility of low frequency US. Potential damages to the inner ear were examined. Pigmented guinea pigs were anesthetized and placed into a ventilated, acoustically shielded box where they were exposed to a continuous-wave sinusoidal signal. The US pressure level was controlled and kept constant by measuring the rms-voltage generated by a pressure microphone (B&K 4136). The animals were placed to the transducer so that the acoustical axis fell between the ears. Low frequency US was generated using a specifically developed compound transducer providing an acoustic field with a symmetrical drop of < 2 dB in sound pressure at a distance of 1 cm perpendicular to the acoustical axis, providing a uniform field with equal exposure of both ears. To determine US damages as a function of frequency, harmonics produced by the transducer had to be minimized. By proper design of transducer and signal generator, harmonics were kept at levels 30 dB below those of the fundamental frequency. Air and fur temperature was monitored throughout the experiments. Temperature rose < 1 degree C. Animals were observed during the whole experiment with a CCD camera. In the present experiments, guinea pigs were exposed to US at 27.3 kHz for two hours at a sound pressure of 91 ± 1 Pa[eff] (133 dB SPL) measured in the acoustical axis. Subsequently, the animals were sacrificed and their inner ears processed for light and electron microscopy. The major changes observed were vacuolization of the outer and, to a lesser extent, inner hair cells in all cochlear turns. These findings indicate that the amount of energy fed into the ears was sufficient to damage high and low frequency areas of the guinea pig cochlea. They also suggest that it appears imperative to establish safe limits of exposure as well as safeguards to prevent health damages resulting from low frequency US. |