Long-term Outcome of Sound Localization with Baha^® Attract System

Background:

Spatial hearing is a critical feature in daily life. However, there is quite a range in hearing loss patients regarding the effect of bone conduction device on localization performance.

Objectives:

To analyze the localization performance in patients with bilateral conductive or mixed hearing loss fitted with one Baha® Attract system.

Materials and Methods:

This prospective study included 12 patients who had followed up for more than one year. The parameters analyzed included (1) audiological results: sound field threshold, speech discrimination scores (SDSs), and sound localization test, and (2) functional results: scores for the Speech, Spatial and Qualities of Hearing Scale (SSQ) and the Chinese translation of the Spatial Hearing Questionnaire (C-SHQ).

Results:

The audiological assessments showed a reduction of 28.5 dB in the mean sound field thresholds and improvements of 61.7% in the SDSs for disyllabic words. The root mean square error improved slightly with the Baha® Attract system. Patients showed promising results in the functional questionnaire assessments, with significant improvements in the SSQ and C-SHQ scores.

Conclusions:

Although most patients were not able to localize sound accurately after surgery, the change in the scores of the SSQ and C-SHQ indicated that the Baha® Attract system could improve spatial hearing.

The first case describing the implantation of bone-anchored hearing aids (BAHAs) was reported by Tjellstrom and Granstrom in 1977.¹ The traditional percutaneous BAHA system has been proven effective in treating unilateral or bilateral, conductive or mixed hearing loss, and single-sided deafness.^2,3 Still, it has some disadvantages, such as the risk of local skin complications and aesthetic concerns.⁴ The Baha^® Attract system (Cochlear Ltd., Sydney, Australia) has been introduced to overcome these disadvantages. Unlike the percutaneous BAHA system, in which a sound processor is attached to a skin-penetrating abutment, the Baha^® Attract system works by a system of magnets instead of an abutment. The internal magnet is attached to a regular titanium implant underneath the skin, while the sound processor is attached to an external magnet with a soft pad.

The Baha^® Attract system has been proven to be efficacious in hearing performance and quality of life.^5–7 However, spatial hearing is another critical feature in daily life besides hearing performance. Sound localization is a simple and effective method to evaluate the capability of spatial hearing. Nevertheless, there is little knowledge about the long-term outcome of localization performance with the Baha^® Attract system. Thus, this study aims to assess the long-term localization performance with Baha^® Attract by objective and subjective measurements.

MATERIALS AND METHODS

Participants

The study protocol was approved by the Institutional Review Board of Beijing Tiantan Hospital (No. QX2016-016-01). A total of 13 patients (including ten male and three female patients) were enrolled at Beijing Tiantan Hospital from September 2017 to September 2018. The inclusion criteria were as follows: (1) age>8 y, (2) average bone conduction threshold<40 dB HL at frequencies of 0.5, 1, and 2 kHz, and<30 dB HL at frequency 4 kHz. Twelve patients completed all visits, while one male participant who could not participate in all postoperative measurements was excluded. Among these 12 patients (age, 10 to 64 y; mean age, 28 y), 8 had bilateral atresia, and 4 had bilateral chronic otitis media.

Surgery and Fitting

Preoperative high-resolution computed tomography (CT) scans of the temporal bone were obtained to measure the bone thickness and confirm the implant position. The surgery was performed using the typical protocol with a C-shaped incision under general anesthesia. The same senior author performed all surgeries. All patients received the same devices with a Baha^® 5 sound processor (SP) 4 weeks after surgery.

Audiological Evaluation

The audiological evaluation included sound field threshold, speech discrimination scores (SDSs), and sound localization test.

Audiometry tests evaluated sound field thresholds at the following frequencies (kHz): 0.25, 0.5, 1, 2, 3, 4, and 6, during which the loudspeaker was placed 1 m in front of the patients. The SDSs (in quiet) at 65 dB SPL was measured using the disyllabic test from the Mandarin Speech Test Materials (MSTM).⁸ The average sound field thresholds at 0.5, 1, 2, and 4 kHz were compared with the unaided condition and with the Baha® Attract system.

The sound localization test was performed in a sound-treated room pre- and post-operation (3 mo and 6 mo after surgery). Twelve loudspeakers, mounted at the head level and labeled 1 through 13, were set up in a semi-circular array at 15-degree intervals. The patient was seated such that the patient’s head was in the center of and in the same plane as the array of loudspeakers at a distance of approximately 1.0 m from each speaker. The pure tones of 500 and 3000 Hz were used as stimulus sounds at 65 dB SPL. Patients were asked not to move their heads during a sound presentation but were free to move their heads to identify the speaker number after the sound presentation. Our main criterion of judgment was the root mean square error (RMSE).

Functional Evaluation

Functional evaluation was performed using the following questionnaires pre- and post-operation (3 mo, 6 mo, and 12 mo after surgery).

• The Speech, Spatial, and Qualities of Hearing Scale (SSQ),⁹ which includes 49 questions about daily situations, evaluates the patient’s ability to listen to the language and locate sound and assesses sound input quality.

• The Chinese translation of the Spatial Hearing Questionnaire (C-SHQ),¹⁰ which consists of 24 questions covering eight aspects, namely, perception of male, female, and children’s voices, music perception, sound localization, and speech perception in quiet and in noise with the target and noise sources from the front and spatially separate. Subjective comparisons can be performed using a total score for all 24 items or the scores for any of the eight subscales.

Statistical Analysis

Statistical analyses were performed using SPSS 23.0 software (SPSS Inc., Chicago, IL). Mean values (± standard deviation [SD] or standard error of mean [SEM]) were used to describe outcomes. Differences between the aided and unaided situations for hearing and SDSs were performed using a non-parametric Wilcoxon paired test. Mann-Whitney test was used to analyze the result of the sound localization test. Friedman test was conducted to compare the results of questionnaires. P values<0.05 were considered to be statistically significant.

RESULTS

Surgical Outcomes

Patients with bilateral atresia did not undergo simultaneous auricle reconstruction. One patient had a hematoma 8 days post-operatively, and two patients developed seroma 4 and 6 months after the operation, respectively. They recovered after being treated with puncture and pressure dressing for three days. The average daily use was 8.9 hours (range, 5 to13 h). None of the patients reported excessive pain at the implant side, nor did they experience major complications such as skin necrosis or implant loss.

Audiological Results

The sound field threshold audiometry results are shown in Figure 1. The mean sound field thresholds improved significantly by 28.5 dB after surgery (from 52.5 dB SPL unaided to 24.0 dB SPL with the Baha® Attract system; P<0.001). Significant improvements also showed in speech understanding in quiet after surgery. The mean SDSs for disyllabic words was 26.3% in the unaided condition and 87.9% with the Baha^® Attract system, yielding a mean improvement of 61.7% (P<0.001). Our study’s audiological performance results were similar to those reported previously.^6,11,12

FIGURE 1.

The result of sound field threshold on individual frequencies with Baha® Attract (aided) or without (unaided). Error bars represent the standard error of mean.

The results of the localization test in each follow-up time point are shown in Figure 2. The RMSE changed from 76.88° in non-aided conditions to 73.79° at 3 months post-operation and finally to 68.15° at 6 mo post-operation. There was no significant difference between pre-op and 3 mo or 6 mo post-op (P=0.450 and P=0.061, respectively).

FIGURE 2.

The result of the localization test at preoperative and postoperative follow-up. (median=horizontal lines; whiskers=min to max)

Questionnaire Assessments

The detailed results of the SSQ questionnaire are shown in Supplemental Table 1, Supplemental Digital Content 1, http://links.lww.com/SCS/F48. There were significant improvements in all subscales 3 months after surgery (Fig. 3). Significant progress was also found 12 months post-operatively in all subscales compared with the result at 3 months post-operation. Similar to the results of the SSQ questionnaire, the C-SHQ scores also showed a significant improvement 3 months after surgery and was further improved at 12 months post-operation (Fig. 4). The comparison between 3- and 6-month post-operation or between 6- and 12-month post-operation for all questionnaire measurements showed no significant differences.

FIGURE 3.

The result of the SSQ questionnaire at preoperative and postoperative follow-up. (*P<0.05; ***P<0.001). Error bars represent the standard deviation.

FIGURE 4.

The result of the C-SHQ at preoperative and postoperative follow-up. (median=horizontal lines; whiskers=min to max; *P<0.05; ***P<0.001).

DISCUSSION

Spatial hearing evaluates the central auditory system’s ability to integrate and process binaural information. Speech recognition test and sound localization test are used to assess spatial hearing. Although speech recognition test has been widely used in clinical, sound localization test can more directly demonstrate spatial hearing.

Sound localization was not known as a benefit of percutaneous BAHA, and there was quite a range in hearing loss patients regarding the effect of BAHA on localization performance.^13–16 This might be due to various measurements and analysis methods used for sound localization. Few studies have reported the localization outcome with Baha® Attract, most of which were based on short-time follow-ups.

In this study, we showed the change in localization performance over a more extended follow-up period. Meanwhile, objective and subjective information concerning sound localization were collected. Our findings of the sound localization test indicated that localization ability slightly improved after Baha® Attract implantation, and most patients could not localize sounds accurately. Kong et al reported a study concerning the localization ability after Baha® Attract implantation in unilateral conductive or mixed hearing loss patients.¹⁷ They found that the localization ability at low frequencies was significantly improved during 6 mo postoperation. It is generally assumed that the interaural time difference is the primary cue to determine the sound location at low frequencies. The interaural level difference is the primary cue at high frequencies. Bilaterally fitted BAHAs might be able to use time difference as well as level difference to estimate the sound direction.¹⁶ In contrast, patients with bilateral conductive or mixed hearing loss cannot access binaural cues when listening with only one bone conduction device (BCD).

Besides the localization test, subjective information about auditory localization was also collected, including the SSQ and C-SHQ. The SSQ includes questions on spatial hearing, and the quality of speech or music, whereas the C-SHQ focuses only on spatial hearing. The C-SHQ also assesses subjective hearing ability by stimuli with different frequency content. Significant improvements were observed in the SSQ and C-SHQ three months after surgery and within the one-year follow-up period. Although the sound localization accuracy did not significantly improve, both SSQ and C-SHQ scores demonstrated that the Baha® Attract system improved spatial hearing.

Bilateral conductive or mixed hearing loss patients with two BCDs may gain increased access to binaural cues, allowing for improved localization abilities.^16,18 In addition, localization training might also improve localization skills in this type of hearing loss patient.^19,20 Thus, a prospective study with two Baha^® Attract implantation, and long-term localization training might be needed to further reveal the potential benefit of bilateral fitting of Baha^® Attract on localization ability in patients with bilateral conductive or mixed hearing loss.

CONCLUSION

In patients with bilateral conductive or mixed hearing loss fitted with one Baha^® Attract, localization ability slightly improved after surgery, and the localization performance was stable over time. Although most patients were not able to localize sound accurately, the scores of the SSQ and C-SHQ significantly increased after surgery indicating that the Baha® Attract system could improve spatial hearing. It is necessary to conduct more research on the variability of the localization performance, as well as the methods to further improve the localization ability.