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 transcu taneous electrical stimulation to activate the central and peripheral nervous systems. This technique is widely used in neurology and otolar yngology for non-invasive stimulation of the brain and facial nerve. E MS uses electromagnetic field pulses which pass unimpeded through the cranium and soft tissues to activate excitable membranes of volume con ductors. In this series of studies, the effects and side-effects of el ectromagnetic stimulation on the auditory system of humans and experim ental animals were investigated. In the first study, 18 profoundly har d-of-hearing and deaf patients who were candidates for cochlear implan ts 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) a uricle, (2) mastoid process, and (3) the temporal lobe area. EMS elici ted auditory sensations in 26 ears (of 14 patients/subjects). The lowe st 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 max imum level (2.0 Tesla), and with equal sensitivity in each coil positi on. There was no correlation between the EMS/TAS and the immediate pos toperative psychoacoustic tests in ten patients receiving cochlear imp lants. A prominent side effect of EMS was found to be the high intensi ty, high frequency impulse noise generated by the coil which causes se vere cochlear damage and permanent sensorineural hearing loss in exper imental animals. Measurements of the sound pressure level (SPL) of the magnetic coil acoustic artifact (MCAA) at the tympanic membrane of th e rabbit ear showed levels of up to 160 dB for maximum EMS. Measuremen ts of the spectral content and SPL of the MCAA in the ear canal of lif e 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 los s. Studies on computer simulated impulse noises showed that the peak s ound pressure rather than the rise time (in the range 0.1 - 1.0 ms) de termined the permanent threshold shift (PTS). The MCAA was more harmfu l than a 128 dB SPL continuous noise with 100 times more energy. The e ar may be protected from the most damaging effects of the MCAA by the use of ear protectors. However, artifically activating the acoustic re flex with a contralateral broad band noise during exposure to the inte nse magnetic coil artifact reduced the PTS and protected the ear again st noise-induced hearing loss. The findings on extensive and long term exposure of experimental animals to EMS showed no harmful effects on the brain or auditory system. It was concluded that EMS may induce sou nd perception in deaf ears and that the risk of acoustic trauma in nor mal ears due to the MCAA must be considered. It was further suggested that the EMS technique may be developed to be used in the selection of cochlea implant candidates and in stimulating surviving auditory neur ons in deaf patients.