Electromagnetic stimulation of the auditory system: effects and side-effects.

Extracranial electromagnetic stimulation (EMS) is a recently developed clinical technique which may be used in place of conventional transcutaneous electrical stimulation to activate the central and peripheral nervous systems. This technique is widely used in neurology and otolaryngology for non-invasive stimulation of the brain and facial nerve. EMS uses electromagnetic field pulses which pass unimpeded through the cranium and soft tissues to activate excitable membranes of volume conductors. In this series of studies, the effects and side-effects of electromagnetic stimulation on the auditory system of humans and experimental animals were investigated. In the first study, 18 profoundly hard-of-hearing and deaf patients who were candidates for cochlear implants were examined by non-invasive EMS in an effort to determine whether EMS could stimulate residual neurons in the cochlea, 8th nerve proper, or higher auditory brain centers, and evoke auditory sensations. The patients were stimulated with a magnetic coil positioned at the (1) auricle, (2) mastoid process, and (3) the temporal lobe area. EMS elicited auditory sensations in 26 ears (of 14 patients/subjects). The lowest threshold of auditory sensation (TAS) at each stimulus position was found to be at the 20% EMS level, with a range of 20-50% of the maximum level (2.0 Tesla), and with equal sensitivity in each coil position. There was no correlation between the EMS/TAS and the immediate postoperative psychoacoustic tests in ten patients receiving cochlear implants. A prominent side effect of EMS was found to be the high intensity, high frequency impulse noise generated by the coil which causes severe cochlear damage and permanent sensorineural hearing loss in experimental animals. Measurements of the sound pressure level (SPL) of the magnetic coil acoustic artifact (MCAA) at the tympanic membrane of the rabbit ear showed levels of up to 160 dB for maximum EMS. Measurements of the spectral content and SPL of the MCAA in the ear canal of life size models of the human cranium with the stimulating coil placed at standard clinical positions indicated that the major acoustic energy of the pulse is concentrated in the 2-5 kHz range, and that the SPL of the pulse at some positions may place persons at risk for hearing loss. Studies on computer simulated impulse noises showed that the peak sound pressure rather than the rise time (in the range 0.1-1.0 ms) determined the permanent threshold shift (PTS). The MCAA was more harmful than a 128 dB SPL continuous noise with 100 times more energy.(ABSTRACT TRUNCATED AT 400 WORDS)