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 50 dB SPL. CM thresholds measured with RW stimulation ranged from -14 to 35 dBmV with an intact middle ear chain and from -7 to 36 dBmV after lateral ossicular chain
disarticulation. Acoustically, stapes velocity maxima was observed at approximately 700 Hz and minima at approximately 2.65 kHz. With application of the
AMEP to the RW, peak stapes velocity was observed at 2-3 kHz. The equivalent ear canal sound pressure level (L(E)(max)dB SPL) evoked by RW stimulation with the
AMEP was 60-105 dB SPL for the intact middle ear and 70-100 dB 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.