Hearing Aid Treatment for Patients with Mixed Hearing Loss. Part II: Speech Recognition in Comparison to Direct Acoustic Cochlear Stimulation.

BACKGROUND: The conventional therapy for severe mixed hearing loss is middle ear surgery combined with a power hearing aid. However, a substantial group of patients with severe mixed hearing loss cannot be treated adequately with today's state-of-the-art (SOTA) power hearing aids, as predicted by the accompanying part I of this publication, where we compared the available maximum power output (MPO) and gain from technical specifications to requirements for optimum benefit using a common fitting rule. Here, we intended to validate the theoretical assumptions from part I experimentally in a mixed hearing loss cohort fitted with SOTA power hearing aids. Additionally, we compared the results with an implantable hearing device that circumvents the impaired middle ear, directly stimulating the cochlea, as this might be a better option. OBJECTIVES: Speech recognition outcomes obtained from patients with severe mixed hearing loss supplied acutely with a SOTA hearing aid were studied to validate the outcome predictions as described in part I. Further, the results obtained with hearing aids were compared to those in direct acoustic cochlear implant (DACI) users. MATERIALS AND METHODS: Twenty patients (37 ears with mixed hearing loss) were provided and fitted with a SOTA power hearing aid. Before and after an acclimatization period of at least 4 weeks, word recognition scores (WRS) in quiet and in noise were studied, as well as the speech reception threshold in noise (SRT). The outcomes were compared retrospectively to a second group of 45 patients (47 ears) using the DACI device. Based on the severity of the mixed hearing loss and the available gain and MPO of the SOTA hearing aid, the hearing aid and DACI users were subdivided into groups with prediction of sufficient, partially insufficient, or very insufficient hearing aid performance. RESULTS: The patients with predicted adequate SOTA hearing aid performance indeed showed the best WRS in quiet and in noise when compared to patients with predicted inferior outcomes. Insufficient hearing aid performance at one or more frequencies led to a gradual decrease in hearing aid benefit, validating the criteria used here and in the accompanying paper. All DACI patients showed outcomes at the same level as the adequate hearing aid performance group, being significantly better than those of the groups with inadequate hearing aid performance. Whereas WRS in quiet and noise were sensitive to insufficient gain or output, showing significant differences between the SOTA hearing aid and DACI groups, the SRT in noise was less sensitive. CONCLUSIONS: Limitations of outcomes in mixed hearing loss individuals due to insufficient hearing aid performance can be accurately predicted by applying a commonly used fitting rule and the 35-dB dynamic range rule on the hearing aid specifications. Evidently, when outcomes in patients with mixed hearing loss using the most powerful hearing aids are insufficient, bypassing the middle ear with a powerful active middle ear implant or direct acoustic implant can be a promising alternative treatment.

Feasibility of Round Window Stimulation by a Novel Electromagnetic Microactuator.

INTRODUCTION: Most implantable hearing aids currently available were developed to compensate the sensorineural hearing loss by driving middle ear structures (e.g., the ossicles). These devices are successfully used in round window (RW) stimulation clinically, although this was initially not the intended use. Here, a novel microactuator, specifically designed for RW stimulation, was tested in human temporal bones to determine actuator performance and applicability. METHODS: Stapes footplate response to RW stimulation was determined experimentally in human temporal bones and the obtained sound pressure output level was estimated. RESULTS: The actuator had a flat displacement response between 0.125 and 4 kHz, a resonance between 4 and 7 kHz, and a roll-off above. At increasing contact force, the stapes footplate displacement decreased by 5-10 dB re µm for forces ≥ 2 mN. The equivalent sound pressure level between 0.125 and 4 kHz amounted to 87-97 eq dB SPL and increased to 117 eq dB SPL for frequencies of 4-7 kHz. The total harmonic distortion (THD) of the actuator ranged within 15-40% for static forces of 5 mN. CONCLUSION: The feasibility of an electromagnetic actuator that may be placed into the RW niche was demonstrated but requires further optimization in terms of THD and static force sensitivity.

Differential Intracochlear Sound Pressure Measurements in Human Temporal Bones with an Off-the-Shelf Sensor.

The standard method to determine the output level of acoustic and mechanical stimulation to the inner ear is measurement of vibration response of the stapes in human cadaveric temporal bones (TBs) by laser Doppler vibrometry. However, this method is reliable only if the intact ossicular chain is stimulated. For other stimulation modes an alternative method is needed. The differential intracochlear sound pressure between scala vestibuli (SV) and scala tympani (ST) is assumed to correlate with excitation. Using a custom-made pressure sensor it has been successfully measured and used to determine the output level of acoustic and mechanical stimulation. To make this method generally accessible, an off-the-shelf pressure sensor (Samba Preclin 420 LP, Samba Sensors) was tested here for intracochlear sound pressure measurements. During acoustic stimulation, intracochlear sound pressures were simultaneously measurable in SV and ST between 0.1 and 8 kHz with sufficient signal-to-noise ratios with this sensor. The pressure differences were comparable to results obtained with custom-made sensors. Our results demonstrated that the pressure sensor Samba Preclin 420 LP is usable for measurements of intracochlear sound pressures in SV and ST and for the determination of differential intracochlear sound pressures.

Do you hear the noise? The German matrix sentence test with a fixed noise level in subjects with normal hearing and hearing impairment.

OBJECTIVE: The aim of this study was to determine the relationship between hearing loss and speech reception threshold (SRT) in a fixed noise condition using the German Oldenburg sentence test (OLSA). DESIGN: After training with two easily-audible lists of the OLSA, SRTs were determined monaurally with headphones at a fixed noise level of 65 dB SPL using a standard adaptive procedure, converging to 50% speech intelligibility. STUDY SAMPLE: Data was obtained from 315 ears of 177 subjects with hearing losses ranging from -5 to 90 dB HL pure-tone average (PTA, 0.5, 1, 2, 3 kHz). RESULTS: Two domains were identified with a linear dependence of SRT on PTA. The SRT increased with a slope of 0.094 ± 0.006 dB SNR/dB HL (standard deviation (SD) of residuals = 1.17 dB) for PTAs < 47 dB HL and with a slope of 0.811 ± 0.049 dB SNR/dB HL (SD of residuals = 5.54 dB) for higher PTAs. CONCLUSION: The OLSA can be applied to subjects with a wide range of hearing losses. With 65 dB SPL fixed noise presentation level the SRT is determined by listening in noise for PTAs < ∼47 dB HL, and above it is determined by listening in quiet.

Electro-Mechanical Stimulation of the Cochlea by Vibrating Cochlear Implant Electrodes.

INTRODUCTION: Electro-acoustic stimulation (EAS) of the cochlea uses the preserved residual low-frequency hearing for acoustic stimulation in combination with electrical stimulation. The acoustic low-frequency component is amplified and high-frequency hearing is enhanced by a cochlear implant (CI). In this work, the feasibility of EAS by the floating mass transducers (FMTs) firmly attached to the implanted electrode was investigated and the achieved stapes displacement was compared with sound stimulation. METHODS: Experiments were performed in eight fresh human temporal bones compliant to the ASTM standard (F2504-5). Four EAS custom-made prototypes (EAS-CMP) were tested, consisting of standard MED-EL CI electrodes with Vibrant Soundbridge (VSB) FMTs or a Bonebridge (BB) FMT tightly molded to the electrode in different orientations. The stapes footplate (SFP) response to EAS-CMP stimulation and sound stimulation was measured using a Laser Doppler Vibrometer (LDV). RESULTS: The SFP displacement amplitudes achieved by EAS-CMP stimulation were calculated to 1 VRMS FMT input and were pair-wise statistically compared between prototypes yielding no significant differences at frequencies ≤1 kHz. At frequencies ≤1 kHz stimulation by the BB FMT resulted in a flat and potentially highest SFP displacement amplitude of approximately -40 dB re µm at 1 VRMS input voltage. Estimated equivalent sound pressure levels achieved by the BB FMT prototype were approximately 83-90 eq. dB SPL at frequencies ≤1 kHz. CONCLUSION: The feasibility of cochlear stimulation by vibrating electrodes was shown although the achieved output level at frequencies ≤1 kHz was too low for EAS applications.

Oval window membrane vibroplasty for direct acoustic cochlear stimulation: treating severe mixed hearing loss in challenging middle ears.

OBJECTIVE: To date, all the Vibrant Soundbridge (VSB) applications have managed to stimulate the inner ear indirectly. Our objective was to present a new VSB application for direct inner ear stimulation. STUDY DESIGN: Prospective cohort study. SETTINGS: Tertiary, referral center PATIENTS: Three patients with previous middle ear surgery and moderate-to-severe ipsilateral, mixed hearing loss. INTERVENTIONS: Oval window membrane vibroplasty (OWMV) for direct acoustic cochlear stimulation. A total ossicular replacement prosthesis (TORP) was attached to the floating mass transducer (FMT). Then, the stapes footplate was perforated, and the tip of the FMT-TORP assembly was advanced approximately 1 mm into the inner ear. A silicon ring was placed around the TORP's tip to prevent it from slipping deeper into the inner ear. MAIN OUTCOME MEASURE: Audiologic assessment involving pure-tone audiometry, aided and unaided free-field audiometry, Freiburg monosyllabic word test, and registration of any complications. RESULTS: OWMV resulted in an average functional hearing gain of 36.1 dB (range, 24.2-47.5 dB). Although the greatest amplification was observed in the higher frequencies, there also was a significant improvement in the lower frequencies. The surgery was not related to any difficulties; vertigo, inner ear trauma, or further complications did not occur. CONCLUSION: We present a new method for direct acoustic cochlear stimulation using an active middle ear implant. The preliminary results show that OWMV is a promising and safe option for treating moderate and severe hearing loss, even in challenging cases with previous middle ear operations or fixed stapes footplate.

Leukoencephalopathy upon disruption of the chloride channel ClC-2.

ClC-2 is a broadly expressed plasma membrane chloride channel that is modulated by voltage, cell swelling, and pH. A human mutation leading to a heterozygous loss of ClC-2 has previously been reported to be associated with epilepsy, whereas the disruption of Clcn2 in mice led to testicular and retinal degeneration. We now show that the white matter of the brain and spinal cord of ClC-2 knock-out mice developed widespread vacuolation that progressed with age. Fluid-filled spaces appeared between myelin sheaths of the central but not the peripheral nervous system. Neuronal morphology, in contrast, seemed normal. Except for the previously reported blindness, neurological deficits were mild and included a decreased conduction velocity in neurons of the central auditory pathway. The heterozygous loss of ClC-2 had no detectable functional or morphological consequences. Neither heterozygous nor homozygous ClC-2 knock-out mice had lowered seizure thresholds. Sequencing of a large collection of human DNA and electrophysiological analysis showed that several ClC-2 sequence abnormalities previously found in patients with epilepsy most likely represent innocuous polymorphisms.

Imaging the living inner ear using intravital confocal microscopy.

Confocal laser scanning microscopy permits detailed visualization of structures deep within thick fluorescently labeled specimen. This makes it possible to investigate living cells inside intact tissue without prior chemical sample fixation and sectioning. Isolated guinea pig temporal bones have previously been used for confocal experiments in vitro, but tissue deterioration limits their use to a few hours after the death of the animal. In order to preserve the cochlea in an optimal functional and physiological condition, we have developed an in vivo model based on a confocal microscopy approach. Using a ventral surgical approach, the inner ear is exposed in deeply anaesthetized, tracheotomized, living guinea pigs. To label the inner ear structures, scala tympani is perfused via an opening in the basal turn, delivering tissue culture medium with fluorescent vital dyes (RH 795 and calcein AM). An apical opening is made in the bony shell of cochlea to enable visualization using a custom-built objective lens. Intravital confocal microscopy, with preserved blood and nerve supply, may offer an important tool for studying auditory physiology and the pathology of hearing loss. After acoustic overstimulation, shortening and swelling of the sensory hair cells were observed.