Auditory brainstem and cortical responses following extensive transcranial magnetic stimulation

The long term effects of transcranial electromagnetic stimulation (TEMS) on auditory brainstem and cortical evoked responses and on neuroanatomical structures in the auditory tract were investigated over a 12 month period in rabbits exposed to 1000 stimuli at 100% maximum stimulation level (2.0 tesla instrument output) with a clinical magnetic coil positioned over the cranium. (1) The tone and click audiograms of the pre and post TEMS-exposed plugged ears were normal and did not differ significantly, suggesting that the protected cochlea is unaffected by TEMS. (2) The mean absolute and interwave latencies of auditory brainstem evoked responses (ABR) and the peak amplitudes of the vertex positive waves P1, P3, and P4 in the exposed rabbits were within normal limits, and comparable those of the normal, pre-exposed animals. Wave P5 in the exposed animals was more variable and significantly different from the normal data in mean latency and amplitude. (3) The mean latencies and amplitudes of the post exposed cortical (late) auditory evoked responses (CAER) were not significantly different from the non-exposed ears. Light microscopic examination of sections of the cochlear nucleus and inferior colliculus, possible sources of waves P2 and P5, respectively, of the ABR, showed no EMS-related changes in cellular organization or histological damage. In conclusion, no deleterious effects of TEMS were observed on the protected ear or the peripheral and central auditory system of rabbits after extensive exposure to long term, high intensity, low frequency time-varying magnetic field stimulation with a clinical instrument.