Electrocochleographic and mechanical assessment of round window stimulation with an active middle ear prosthesis

Abstract

Mechanical stimulation of the round window (RW) with an active middle ear prosthesis (AMEP) has shown functional benefit in clinical reports in patients with mixed hearing loss (MHL). Further objective physiological data on the efficacy of RW stimulation is needed, however, to demonstrate that RW stimulation with an AMEP can generate input to the inner ear comparable to acoustic input.Cochlear microphonic (CM) and mechanical (stapes velocity) responses to sinusoidal stimuli were measured by electrode and laser Doppler vibrometry in eight chinchillas in response to normal acoustic stimulation via sealed calibrated insert earphones and to AMEP stimulation (Otologics MET, Boulder, CO, USA) of the RW with and without lateral ossicular chain disarticulation.CM thresholds for acoustic stimulation were frequency dependent and ranged from 16 to 50dB SPL. CM thresholds measured with RW stimulation ranged from −14 to 35dBmV with an intact middle ear chain and from −7 to 36dBmV after lateral ossicular chain disarticulation. Acoustically, stapes velocity maxima was observed at ∼700Hz and minima at ∼2.65kHz. With application of the AMEP to the RW, peak stapes velocity was observed at 2–3kHz. The equivalent ear canal sound pressure level (LEmaxdB SPL) evoked by RW stimulation with the AMEP was 60–105dB SPL for the intact middle ear and 70–100dB SPL after ossicular chain disarticulation.Stimulating the inner ear through the RW with an AMEP produces evoked responses (CM) comparable to normal acoustic input. When adjusted for threshold (due to unit differences, dB SPL or dB mV), the sensitivity of the CM (slope) for acoustic was comparable to sensitivities obtained by AMEP stimulation of the RW. Mechanical stimulation of the RW with an AMEP produces cochlear responses (CMs) and stapes velocities that are functionally equivalent to acoustic stimulation.