Round Window Reinforcement-Induced Changes in Intracochlear Sound Pressure

Abstract

Introduction: The round window membrane (RWM) acts as a pressure-relieving membrane for incompressible cochlear fluid. The reinforcement of the RWM has been used as a surgical intervention for the treatment of superior semicircular canal dehiscence and hyperacusis. The aim of this study was to investigate how RWM reinforcement affects sound pressure variations in the cochlea. Methods: The intracochlear sound pressure (ICSP) was simultaneously measured in the scala tympani (ST) and scala vestibuli (SV) of cadaveric human temporal bones (HTBs) in response to acoustic stimulation for three RWM reinforcement materials (soft tissue, cartilage, and medical-grade silicone). Results: The ICSP in the ST was significantly increased after RWM reinforcement for frequencies below 2 kHz. Between 400 and 600 Hz, all three materials demonstrated the highest median pressure increase. The higher the RWM stiffness, the larger the pressure increase: silicone (7 dB) < soft tissue (10 dB) < cartilage (13 dB). The ICSP in the SV was less affected by reinforcement. The highest median pressure increase was 3 dB. The experimental findings can be explained with numerical models of cochlear mechanics. Discussion and conclusions: RWM reinforcement increases the sound pressure in ST at lower frequencies but only has a minor influence on the SV pressure.

Intracochlear pressure in cadaver heads under bone conduction and intracranial fluid stimulation

“…The facial nerve and the chorda tympani were identified and a wide posterior tympanotomy was opened by removing the bone between the nerves as well as the facial nerve in the mastoidal segment. This also aided the intracochlear sound pressure (ICSP) measurements, by allowing the implantation (detailed in section 2.3) of custom hydrophones in scala vestibuli (SV) and tympani (ST), via cochleostomies performed similar to Liyanage et al (2021) . The ST cochleostomy (seen in Fig.…”

“…The intracochlear pressure sensors (ICAR) are inserted Prochazka et al, 2019 ;Liyanage et al, 2021 ) one at a time, starting with scala tympani (due to spatial and anatomical constraints). The P SV , P ST pressures are continuously monitored during the insertion procedure (see section 2.3 for details), together with P EC and P ICP .…”

“…The ICSP was measured in the ST and SV in accordance with a previously described method by our group ( Liyanage et al, 2021 ). The ICAR consisted of a sound receptor (SR, or tip) with a single polyimide diaphragm (1 μm thick, ∼0.375 -0 .…”

“…The sensors were calibrated prior to the experiments using a vibrating water column method as described in . While the ICARs are developed with the goal of providing a fully implantable cochlear receiver for hearing aids ( Prochazka et al, 2019 ), a version of the ICAR was optimized for research purposes ( Liyanage et al, 2021 ), specifically to reduce the mechanical coupling between the sensing part (MEMS microphone and tip) and the external support structure, under BC stimuli. This was done by removing the metal support structure connecting the two, and leaving only the relatively flexible electrical wiring (1.5 × 0.1mm 2 ).…”

“…Measuring intracochlear sound pressure has been established by several groups for air conduction (AC) ( Nakajima et al, 2009 ;Frear et al, 2018 ;Peacock et al, 2018 ;Raufer et al, al.,2021 ) and BC ( Mattingly et al, 2020 ;Stieger et al, 2018 ;Borgers et al, 2019 ), using of-the-shelf ( Grossöhmichen et al, al.,2016 ) or custom-made pressure probes ( Olson, 1998 ;Pfiffner et al, 2016 ;Liyanage et al, 2021 ). A potential confound in these measurements is the possible motion of the fiber optic pressure probe relative to otic capsule, into which cochleostomies are created, and/or the posterior wall of the external auditory canal Borgers et al, 2019 ).…”