Oto-traumatic Effects of Computer Simulated Magnetic Coil Impulse Noise: Analysis of Mechanisms

Abstract

The brief impulse noise artifacts of 1.0 ms or less generated by some magnetic coils used in extracranial magnetic stimulation may induce acoustic trauma. We investigated the effects of these magnetic coil acoustic artifacts (MCAA) on the inner ear by exposing rabbits to computer simulated impulse noise designed to mimic the impulse noise of the coil in spectrum and acoustic energy. The simulated impulse noise stimuli (50 impulses) were varied in maximum peak sound pressure (160, 157, and 155 dB re: 20μ Pascal), rise-time (100 μs and 1,000 μs) and duration. The frequency spectrum of the simulated impulse noises were kept constant at 0.5 kHz to 7 kHz with peak energy in the 2-5 kHz range. The results indicated that the simulated magnetic coil impulse noise caused extensive cochlear damage and permanent threshold shifts largely equal to those induced by the MCAA. The MCAA created slightly greater PTS than the simulated impulse of the same peak sound pressure. Each of the 3 experimental stimuli induced similar PTS in the auditory range of 0.5 to 16 kHz, with the higher peak sound pressure stimuli (157 and 160 dB) causing greatest hearing loss. Increasing the rise-time of the simulated brief impulse noise from 100 μs to 1,000 μs did not reduce the level of PTS significantly. The results suggest that for brief acoustic signals of around 1 ms or less, the peak pressure and spectral content rather than the rise-time and duration were the important factors in the development of noise-induced hearing loss. A critical rise-time may exist, but is beyond the temporal range of this investigation. We also compared the PTS resulting from the simulated impulse noises with that caused by a continuous noise exposure of 128 dB SPL for 15 min. Although the continuous noise exposure had 100 times greater acoustic energy, it caused considerably less PTS than the magnetic coil and simulated impulse noises, further supporting the significance of peak pressure of impulse noise in inducing permanent threshold shifts.

Key Words:

• hearing loss
• cochlea
• magnetic stimulation
• 8th nerve
• deafness