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

Abstract

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.

Keywords: Active middle ear implant; Gain; Hearing aid; Maximum power output; Mixed hearing loss; Stapes surgery.

Fig. 1

Unaided pure tone average (PTA) and hearing thresholds at 4 separate frequencies (0.5, 1, 2, and 4 kHz). Upper panel: bone conduction (BC) thresholds of the conventional hearing aid users (white) and direct acoustic cochlear implant (DACI) users (light gray). Lower panel: air conduction (AC) thresholds of the conventional hearing aid users (white) and DACI users (light gray), as well as after correction by mean improvement with a theoretical stapes surgery (dark gray). The medians are depicted by bold black horizontal lines and the boxes mark the interquartile ranges (i.e., 25th and 75th percentiles). After exclusion of outliers (circles), the error bars indicate the minimum-to-maximum range. Statistically significant differences (top: Mann-Whitney U test; bottom: Wilcoxon signed-rank test) are indicated by brackets.

Fig. 2

Comparison of the word recognition score (WRS) obtained with the walk-in (WI) hearing aid (HA) (x axis) to the WRS obtained with the newly fitted state-of-the-art (SOTA) HA (y axis). a WRS in quiet, obtained with the Freiburg monosyllable test. b WRS in noise, obtained with the Hochmair-Schulz-Moser test. Symbols indicate individual results within the three different subgroups G0 (circles), G1 (triangles), and G2 (crosses). The dashed lines indicate limits for 95% critical differences, calculated according to Thornton and Raffin [1978].

Fig. 3

Aided monaural speech intelligibility in the sound field for patient subgroups G0, G1, and G2 of hearing aid users (state-of-the-art device; white) and direct acoustic cochlear implant (DACI) users (gray). a Word recognition score (WRS) (Freiburg monosyllable test) in quiet. b WRS (Hochmair-Schulz-Moser test) at a fixed signal-to-noise ratio (SNR). c The SNR determined with the International Matrix Test after exclusion of outliers with an SNR >15 dB (not shown; 1 hearing aid user in G2 and 1 DACI user in G1). The median is depicted by black horizontal lines and the boxes mark the interquartile ranges (i.e., 25th and 75th percentiles). The error bars indicate the minimum-to-maximum range. Significance levels were determined using the Kruskal-Wallis test and subsequent Bonferroni correction.