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Abstract

There is currently a lack of prospective studies comparing multiple treatment options for single-sided deafness (SSD) in terms of long-term sound localization outcomes. This randomized controlled trial (RCT) aims to compare the objective and subjective sound localization abilities of SSD patients treated with a cochlear implant (CI), a bone conduction device (BCD), a contralateral routing of signals (CROS) hearing aid, or no treatment after two years of follow-up. About 120 eligible patients were randomized to cochlear implantation or to a trial period with first a BCD on a headband, then a CROS (or vice versa). After the trial periods, participants opted for a surgically implanted BCD, a CROS, or no treatment. Sound localization accuracy (in three configurations, calculated as percentage correct and root-mean squared error in degrees) and subjective spatial hearing (subscale of the Speech, Spatial and Qualities of hearing (SSQ) questionnaire) were assessed at baseline and after 24 months of follow-up. At the start of follow-up, 28 participants were implanted with a CI, 25 with a BCD, 34 chose a CROS, and 26 opted for no treatment. Participants in the CI group showed better sound localization accuracy and subjective spatial hearing compared to participants in the BCD, CROS, and no-treatment groups at 24 months. Participants in the CI and CROS groups showed improved subjective spatial hearing at 24 months compared to baseline. To conclude, CI outperformed the BCD, CROS, and no-treatment groups in terms of sound localization accuracy and subjective spatial hearing in SSD patients.

TRIAL REGISTRATION Netherlands Trial Register (https://onderzoekmetmensen.nl): NL4457, CINGLE trial.

Keywords

hearing loss unilateral cochlear implantation hearing aids sound localization follow-up studies

Introduction

Patients with single-sided deafness (SSD) have severe-to-profound hearing loss in one ear and normal or near-normal hearing in the other ear. SSD annually affects 12–27 per 100,000 adults (Zeitler & Dorman, 2019). Although SSD etiologies differ between patients, all experience problems with speech perception, sound localization and sometimes tinnitus (Zeitler & Dorman, 2019). Some of these problems are the result of a loss of binaural functionalities, including binaural squelch, binaural summation, and the head-shadow effect (Litovsky et al., 2019; Van Wanrooij & van Opstal, 2004). Especially for sound localization in the horizontal plane, SSD patients cannot use the essential interaural differences in loudness and time, resulting in poor spatial hearing (Grothe et al., 2010; Middelbrooks, 2015).

There are several available treatment options to improve hearing capabilities in SSD patients, such as contralateral routing of signals (CROS) hearing aids, a bone conduction device (BCD), and a cochlear implant (CI) (Zeitler & Dorman, 2019). The CROS and BCD systems route signals from the hearing-impaired ear to the normal-hearing ear. Both systems improve sound awareness from the deaf side. However, sound localization remains generally poor with BCD and CROS hearing aids as still a single auditory pathway is activated with the sound stimuli (Peters et al., 2015a; Snapp, 2019).

Since a CI provides input to the auditory nerve of the deaf ear, binaural input could be partially restored. In several studies, it has been demonstrated that SSD patients showed improved sound awareness and localization after cochlear implantation (Daher et al., 2023; Oh et al., 2023; Peters et al., 2021). Arndt et al. (2011) were one of the first to compare the effect of multiple treatment options (i.e., an unaided situation, a BCD on a headband, a CROS, and a CI) on sound localization capabilities in SSD patients and showed the best outcomes with a CI. The results of a randomized controlled trial (RCT) showed that SSD patients with a CI outperformed patients with a BCD, CROS, or no treatment on sound localization and quality of life (QoL) (Peters et al., 2021). However, most studies comparing the sound localization outcomes of different treatments for SSD had a limited follow-up period of six or 12 months (e.g., Arndt et al., 2011; Daher et al., 2023; Oh et al., 2023; Peters et al., 2021). It is important to weigh the short- and long-term benefits and harms for the patient to improve counselling. Besides, we hypothesize that long-term CI experience may further improve sound localization accuracy, because it takes time to learn to use the re-established binaural cues (Dillon et al., 2022; Körtje et al., 2020; Thompson et al., 2022).

Therefore, the aim of the current study was to assess the objective and subjective sound localization capabilities between CI, BCD, CROS, and no treatment in SSD patients after two years of follow-up, as part of an RCT on this topic (the CINGLE trial: Cochlear Implantation for siNGLE-sided deafness; Peters et al., 2015b). We hypothesize that SSD patients with a CI have better sound localization capabilities than those with a BCD, CROS, or no treatment.

Materials and Methods

Ethical Considerations

The research protocol of this RCT was approved by the Institutional Review Board of the University Medical Center Utrecht (NL45288.041.13) and is registered in the Netherlands Trial Register (www.trialregister.nl, NTR4580). In the Netherlands, a BCD or CROS is currently reimbursed as a treatment option for SSD patients, but a CI is not. The study budget was used to pay the costs of study interventions that were not covered by Dutch health insurers (such as cochlear implantation and related clinical care). For a detailed description of the CINGLE trial and the sample size calculation we refer to the study protocol (Peters et al., 2015b). Written informed consent was obtained by three authors (J.v.H., A.W., and J.P.) from all participants between July 2014 and February 2019. We reported the data according to the CONSORT statement (Schulz et al., 2010).

Study Design

Adult SSD patients could participate in the study if they had a duration of deafness of minimum three months and maximum 10 years, since degeneration of the auditory nerve could limit optimal hearing outcomes (Peters et al., 2015b). SSD was defined as a pure tone average hearing loss at 0.5, 1, 2, 4 kHz ≥70 dB for the hearing-impaired ear, and ≤30 dB for the normal-hearing ear. Patients with retrocochlear pathology, abnormal cochlear anatomy (e.g., ossification), or an implanted BCD were excluded.

Included participants were randomized into three groups using a web-based randomization tool (ratio 2:3:3, block randomization): 1: CI group; 2: ‘first BCD, then CROS’ trial periods group; 3: ‘first CROS, then BCD’ trial periods group. The study flowchart is presented in Figure 1.

Figure 1.

Study flow diagram. See the Appendix for participant allocation at each moment of follow-up. Abbreviations: BCD, bone conduction device; CI, cochlear implant; CROS, contralateral routing of signals hearing aid.

If randomized to the CI group, implantation of a CI (Cochlear™) was scheduled, followed by several visits to the clinic for activation, adjusting settings, and speech-language therapy (in accordance with standard clinical care). If randomized to one of the trial period groups, a BCD on headband (Cochlear™ Baha^® BP110 or 5 Power) or a conventional CROS hearing aid (Phonak Audéo) was adjusted based on the participant's hearing levels by one experienced audiologist (in accordance with standard clinical care). The participant took home each device for use over a six-week period. After the BCD and CROS trial periods, participants indicated their choice of treatment: BCD, CROS, or no treatment. If they opted for a BCD, surgical placement of the implant was scheduled with the fitting of the BCD 6 weeks later. If CROS was preferred, participants were fitted with their own CROS hearing device. If participants preferred no treatment, they remained unaided in the follow-up.

Study Procedures

In this paper, we reported on the data of all four groups (CI, BCD, CROS, and no treatment) regarding sound localization capabilities measured at baseline (i.e., the unaided condition at the inclusion visit) and at 3, 6, 12, and 24 months of follow-up. The follow-up period of the treatment groups (CI, BCD, and CROS) started at the moment of activation of the device.

Sound localization was measured in a sound-attenuated booth using the Crescent of Sound test setup (Kitterick et al., 2011; Smulders et al., 2015). The setup consisted of a horizontal arch from −90° to +90° with nine loudspeakers (Figure 2). Screens were mounted below the loudspeakers depicting the number of each loudspeaker. The participant was facing forward to the loudspeaker at 0° azimuth and was instructed not to turn the head during the tests. The radius of the loudspeakers to the participant was 1.45 m. The sound stimulus was the sentence “Hello, what's this?”, spoken by various female English speakers. The stimulus was presented in quiet conditions (three roving levels of 60, 65, and 70 dB SPL) by using loudspeakers in three configurations:

15-degree configuration: 5 loudspeakers separated by an angle of 15° (−30°, −15°, 0°, + 15°, and +30°);

30-degree configuration: 5 loudspeakers separated by an angle of 30° (−60°, −30°, 0°, + 30°, and +60°);

60-degree configuration: 3 loudspeakers separated by an angle of 60° (−60°, 0°, and +60°).

Figure 2.

Sound localization test setup. The setup consisted of a horizontal arch from −90° to +90° with nine loudspeakers. The radius of the loudspeakers to the participant was 1.45 m. Screens were mounted below the loudspeakers were numbers could be presented.

Thirty sentences per configuration were randomly presented from one of the loudspeakers. The participant had to say the loudspeaker number from which each stimulus was presented.

Primary and Secondary Outcomes

The primary outcome of the current study was sound localization accuracy, scored as the root-mean squared (RMS) error in degrees between the presented and the participant-identified sound source locations. A lower RMS error represents better sound localization accuracy. Chance performance was calculated using a Monte Carlo simulation with 1000 runs (Bonne et al., 2019). Chance performance for the 15-degree configuration was an RMS error of 23.9 degrees, for the 30-degree configuration 47.6 degrees, and for the 60-degree configuration 53.0 degrees.

The secondary outcomes were sound localization accuracy scored as the percentage of correct responses (% correct) and subjective spatial hearing outcomes. Chance performance was 20.0% correct for the two configurations with five loudspeakers and 33.3% correct for the 60-degree configuration with three loudspeakers. The subjective spatial hearing outcome was measured with the Spatial Hearing subscale of the Speech, Spatial and Qualities of hearing (SSQ) scale (Gatehouse & Noble, 2004) in Dutch (validated by Batthyany et al., 2023). By using this questionnaire the perceived hearing ability in different situations is scored using a numeric rating scale (range 0 to 10). Responses are averaged to derive an overall score for the 17-item Spatial Hearing subscale. A higher score reflects a subjective better spatial hearing performance.

Serious adverse events (SAEs) were surveyed at all follow-up moments.

Statistical Analyses

Statistical analyses were performed using R statistical software (version 4.2, R Core Team, Vienna, Austria) and SPSS software (version 25, IBM Corp., Armonk, NY, USA). Participant data were analyzed ‘as treated’. The data was checked for normal distribution with the Shapiro-Wilk test and Q-Q plots. We reported differences in continuous variables between groups at 24 months of follow-up. Within groups, we reported differences between baseline and 24 months as additional information. To compare between patient and disease-specific characteristics and treatment groups at baseline (Table 1), the Fisher's exact test and One-way ANOVA were used in normally distributed data. The Independent-Samples Kruskal-Wallis was used when data was not normally distributed. The Pearson correlation or Spearman rank correlation was used to analyze the correlation between the objective and subjective spatial hearing outcomes.

Table 1.

Participant Characteristics After Choice of Treatment.

Characteristic	CI (n = 28)	BCD (n = 25)	CROS (n = 34)	No treatment (n = 26)	Statistics
Sex, No.
Male	13	10	21	9	ns^a
Female	15	15	13	17
Age at inclusion, y
Mean (SD)	52.5 (12.9)	56.0 (8.4)	52.1 (12.0)	51.5 (12.9)	ns^b
Deaf ear, No.
Left ear	15	16	18	17	ns^a
Right ear	13	9	16	9
Duration of deafness, y
Median (range)	1.9 (0.3–10.0)	2.3 (0.3–10.0)	1.3 (0.3–10.0)	1.7 (0.3–10.0)	ns^c
PTA better ear (0.5–4 kHz), dB
Median (range)	15.0 (5.0–30.0)	12.5 (3.8–28.8)	16.3 (5.0–27.5)	15.6 (2.5–30.0)	ns^c
PTA poor ear (0.5–4 kHz), dB
Median (range)	96.3 (75.0–120.0)	92.5 (80.0–116.3)	93.8 (73.8–120.0)	93.8 (70.0–117.5)	ns^c
SSD etiology, No.
Iatrogenic	1	0	0	1	ns^a
Infection	0	0	2	1
Labyrinthitis	4	2	5	1
M. Ménière	3	3	3	1
Traumatic	0	1	1	2
Unknown	20	19	23	20

Abbreviations: BCD, bone conduction device; CI, cochlear implant; CROS, contralateral routing of signals hearing aid; ns, not significant; PTA, pure tone average; SD, standard deviation; y, years.

Fisher’s exact test, ^bOne-way ANOVA, ^cIndependent-Samples Kruskal-Wallis Test.

We used generalized estimating equations (GEE) to compare spatial hearing outcomes between groups at 24 months, and between baseline and 24 months for each group. Factors included in the GEE were ‘group’ (1 = CI, 2 = BCD, 3 = CROS, 4 = No treatment), ‘time’ (0 = baseline, 4 = 24 months), and ‘group*time’. To correct for multiple testing, a Bonferroni correction was applied (Armstrong, 2014). Based on our hypothesis that includes three comparisons, p-values < 0.017 (0.05 / 3 = 0.017; two-sided) were considered to be statistically significant.

Results

Participant Allocation

A total of 120 SSD patients were included and randomized (for characteristics see Table 1, for the study flowchart see Figure 1). 29 participants were allocated to the CI group, of which one participant withdrew from the study before implantation. Forty-five and 46 participants were allocated to a trial period of BCD-CROS and CROS-BCD, respectively (for outcomes of the trial period, see Wendrich et al., 2023). Six participants dropped out before the start of the trial period. After the trial period, 25 (29%) participants chose BCD, 34 (40%) chose CROS, and 26 (31%) chose no treatment.

The Appendix contains detailed information on participant allocation and non-use during follow-up. Total missing data (missing measurements and participants lost to follow-up) at 3 months was 10.8% (13/120), at 6 months 10.0% (12/120), at 12 months 16.7% (20/120), and at 24 months 18.3% (22/120). During the two-year follow-up period, three participants in the CI group indicated that they no longer used their CI: one experienced unexplained pain and had the CI removed; one because of unrelated health issues; one because of annoying amplification of sounds and no subjective benefit from the CI. Four participants stopped using their BCD, of which three experienced no benefit, two had recurrent skin infections, and one reported that using the BCD made the tinnitus more bothersome. Four participants stopped wearing their CROS hearing aids because they experienced no benefit and/or annoying amplification of sounds.

Device Characteristics

In the CI group, the first 12 participants were implanted with a Nucleus^® CI422 electrode array, the latter 16 participants were implanted with a Nucleus^® CI512 electrode array. In the BCD group (n = 25), the first three patients were fitted with a Baha^® BP110, the remaining 22 patients were fitted with the Baha^® 5 Power. In the CROS group (n = 34), participants were fitted with their own CROS hearing device (i.e., Phonak Audéo Q70 or V70).

Sound Localization Accuracy

Table 2 and Figure 3 show the sound localization accuracy outcomes. We found statistically significant results for the factors included in the GEE (‘group’, ‘time’, ‘group*time’) for all sound localization accuracy outcomes (p < 0.001).

Figure 3.

A-C. Sound localization accuracy calculated as RMS error in degrees per configuration. D-F. Sound localization accuracy calculated as % correct per configuration. The boxplots display the minimum, first quartile, median, third quartile, and maximum. After Bonferroni correction, p-values < 0.017 were considered to be statistically significant. Statistically significant differences between groups at 24 months of follow-up: ** = p < 0.017; *** = p < 0.001. For the statistical difference within groups at 24 months compared to baseline: •• = p < 0.017; ••• = p < 0.001. Abbreviations: BCD, bone conduction device; BLN, baseline; CI, cochlear implant; CROS, contralateral routing of signals hearing aid; RMS, root-mean squared localization error in degrees.

Table 2.

Sound Localization Outcomes.

		Median (IQR)					Difference		GEE Wald
Characteristic		Baseline	3 m	6 m	12 m	24 m	24 m – BLN	p-value	Chi-Square
Primary outcome		Localization, RMS error in degrees
15 degrees	CI	16.5 (8.5)	11.0 (4.5)	9.0 (2.5)	10.0 (3.5)	7.5 (4.0)	−9.0	<0.001	51.8
	BCD	19.5 (8.4)	18.5 (7.0)	18.0 (6.5)	18.8 (8.0)	19.3 (5.9)	−0.2	0.903	0.0
	CROS	18.3 (7.9)	18.0 (9.8)	18.5 (10.5)	19.0 (13.5)	16.8 (8.4)	−1.5	0.166	1.9
	No treatment	23.3 (8.1)	21.0 (7.4)	21.5 (7.0)	20.0 (9.5)	17.8 (8.6)	−5.5	0.002	9.6
30 degrees	CI	25.5 (16.3)	13.0 (9.0)	11.0 (8.0)	10.0 (6.0)	10.0 (7.5)	−15.5	<0.001	41.5
	BCD	27.0 (26.5)	31.0 (18.0)	27.0 (14.3)	24.0 (11.0)	30.5 (20.0)	3.5	0.475	0.5
	CROS	26.0 (21.3)	32.0 (23.5)	31.0 (17.0)	27.0 (18.0)	26.0 (24.3)	0.0	0.989	0.0
	No treatment	36.0 (17.0)	33.5 (25.5)	36.0 (23.0)	28.0 (17.8)	26.5 (19.0)	−9.5	0.014	6.1
60 degrees	CI	22.0 (21.5)	8.0 (12.0)	4.0 (10.0)	2.0 (6.0)	2.0 (6.0)	−20.0	<0.001	49.2
	BCD	22.0 (37.0)	28.0 (20.0)	28.0 (22.0)	24.0 (21.5)	31.0 (29.5)	9.0	0.104	2.6
	CROS	21.0 (30.0)	26.0 (26.0)	28.0 (22.0)	24.0 (36.0)	24.0 (41.0)	3.0	0.565	0.3
	No treatment	32.0 (26.5)	28.0 (37.5)	32.0 (35.0)	28.0 (32.5)	23.0 (28.0)	−9	0.058	3.6
Secondary outcomes		Localization, % correct
15 degrees	CI	26.7 (10.0)	43.3 (10.0)	50.0 (10.0)	46.7 (16.7)	53.3 (18.3)	26.6	<0.001	41.6
	BCD	26.7 (16.7)	26.7 (16.7)	26.7 (16.7)	28.3 (15.0)	23.3 (13.3)	−3.4	0.867	0.0
	CROS	23.3 (13.3)	26.7 (15.0)	26.7 (16.7)	26.7 (20.0)	26.7 (19.2)	3.4	0.340	0.9
	No treatment	21.7 (6.7)	23.3 (10.0)	20.0 (13.3)	25.0 (13.3)	30.0 (16.7)	8.3	0.059	3.6
30 degrees	CI	36.7 (28.3)	60.0 (23.3)	63.3 (23.3)	66.7 (16.7)	70.0 (20.0)	33.3	<0.001	43.7
	BCD	33.3 (36.7)	33.3 (16.7)	33.3 (13.3)	36.7 (20.0)	36.7 (23.3)	3.4	0.467	0.5
	CROS	35.0 (28.3)	30.0 (23.3)	36.7 (23.3)	36.7 (26.7)	36.7 (35.8)	1.7	0.890	0.0
	No treatment	31.7 (20.0)	30.0 (32.5)	33.3 (23.3)	31.7 (26.7)	38.3 (26.7)	6.6	0.044	4.0
60 degrees	CI	65.0 (34.2)	86.7 (20.0)	93.3 (16.7)	96.7 (10.0)	96.7 (10.0)	31.7	<0.001	53.6
	BCD	66.7 (51.7)	56.7 (26.7)	60.0 (23.3)	60.0 (28.3)	50.0 (36.7)	−16.7	0.064	3.4
	CROS	66.7 (40.0)	63.3 (30.0)	66.7 (33.3)	63.3 (53.3)	66.7 (52.5)	0.0	0.830	0.0
	No treatment	55.0 (37.5)	56.7 (49.2)	56.7 (40.0)	56.7 (46.7)	65.0 (38.3)	10.0	0.140	2.2
		SSQ Spatial Hearing subscale
SSQ score	CI	2.3 (2.2)	6.1 (1.6)	5.5 (2.4)	6.2 (2.1)	6.3 (2.6)	4.0	<0.001	96.3
	BCD	2.2 (1.4)	5.3 (2.5)	3.7 (3.4)	2.7 (3.0)	2.4 (3.2)	0.2	0.062	3.5
	CROS	2.6 (2.3)	4.9 (1.8)	3.8 (2.6)	4.0 (2.9)	3.9 (2.4)	1.3	<0.001	12.8
	No treatment	2.8 (2.6)	3.7 (2.4)	2.6 (2.6)	3.2 (3.0)	3.1 (1.9)	0.3	0.565	0.3

Bold values indicate statistically significant differences. After Bonferroni correction, p-values < 0.017 were considered to be statistically significant. Abbreviations: BCD, bone conduction device; BLN, baseline; CI, cochlear implant; CROS, contralateral routing of signals hearing aid; IQR, interquartile range; RMS, root-mean squared; SSQ, Speech, Spatial and Qualities of hearing scale.

There were no statistically significant differences between the groups at baseline. Participants in the CI group showed statistically significantly better sound localization accuracy than the participants in the BCD, CROS, and no-treatment groups in terms of RMS error in degrees and % correct for all three configurations at 24 months of follow-up (Table 3). We found no statistically significant differences in sound localization accuracy between the participants in the BCD, CROS, and no-treatment groups at 24 months.

Table 3.

Between-Group Differences in Sound Localization Outcomes at 24 Months.

	CI vs. BCD		CI vs. CROS		CI vs. NT		BCD vs. CROS		BCD vs. NT		CROS vs. NT
Characteristic	diff.	p-value	diff.	p-value	diff.	p-value	diff.	p-value	diff.	p-value	diff.	p-value	GEE Wald Chi-Square	p-value
Primary outcome	Localization, RMS error in degrees
15 degrees	−11.8	<0.001	−9.3	<0.001	−10.3	<0.001	2.5	0.378	1.5	0.561	−1.0	0.715	121.8	<0.001
30 degrees	−20.5	<0.001	−16.0	<0.001	−16.5	<0.001	4.5	0.638	4.0	0.835	−0.5	0.730	102.4	<0.001
60 degrees	−29.0	<0.001	−22.0	<0.001	−21.0	<0.001	7.0	0.251	8.0	0.295	1.0	0.836	102.4	<0.001
Secondary outcomes	Localization, % correct
15 degrees	30.0	<0.001	26.6	<0.001	23.3	<0.001	−3.4	0.488	−6.7	0.731	−3.3	0.702	67.5	<0.001
30 degrees	33.3	<0.001	33.3	<0.001	31.7	<0.001	0.0	0.557	−1.6	0.409	−1.6	0.868	66.7	<0.001
60 degrees	46.7	<0.001	30.0	<0.001	31.7	<0.001	−16.7	0.494	−15.0	0.254	1.7	0.686	108.5	<0.001
	SSQ Spatial Hearing subscale
SSQ score	3.9	<0.001	2.4	<0.001	3.2	<0.001	−1.5	0.119	−0.7	0.667	0.8	0.015	56.7	<0.001

Bold values indicate statistically significant differences. After Bonferroni correction, p-values < 0.017 were considered to be statistically significant. Abbreviations: BCD, bone conduction device; CI, cochlear implant; CROS, contralateral routing of signals hearing aid; diff., difference; NT, no treatment; RMS, root-mean squared; SSQ, Speech, Spatial and Qualities of hearing scale; Wald, GEE Wald Chi-Square.

Compared to baseline, participants in the CI group showed a statistically significant improvement in sound localization accuracy calculated as the RMS error in degrees in all three test configurations at 24 months of follow-up (difference at the 15-degree configuration: −9.0°, p < 0.001; at 30 degrees: −15.5°, p < 0.001; and at 60 degrees: −20.0°, p < 0.001). The %-correct score of the CI group at the 15-degree configuration increased between baseline and 24 months from 26.7% to 53.3% (difference: 26.6%, p < 0.001); at 30 degrees from 36.7% to 70.0% (difference: 33.3%, p < 0.001); and at 60 degrees from 65.0% to 96.7% (difference: 31.7%, p < 0.001). Figure 4 shows the response patterns for the 15-degree configuration.

Figure 4.

Response patterns for the 15-degree configuration at baseline, 3, and 24 months of follow-up. The sound source location is indicated on the x-axis, and the response location is indicated on the y-axis. The shading indicates the frequency of each observed stimulus-response pair. The contrast ranges from white at the lowest frequency to black at the highest frequency. Abbreviations: BCD, bone conduction device; CI, cochlear implant; CROS, contralateral routing of signals hearing aid.

For the participants in the BCD and in the CROS group, no statistically significant change in sound localization accuracy was found between 24 months and baseline.

Participants in the no-treatment group showed a statistically significantly improved sound localization accuracy scored as the RMS error in degrees at 24 months of follow-up compared to baseline for two out of three configurations (i.e., 15 and 30 degrees). For the 15-degree configuration, the RMS error difference was −5.5° (p = 0.002). For the 30-degree configuration, the RMS error difference was −9.5° (p = 0.014). The difference in sound localization accuracy scored as % correct between 24 months and baseline was not statistically significant for the no-treatment group.

Subjective Spatial Hearing

Table 2 and Figure 5 show the outcomes of the SSQ Spatial Hearing subscale. We found statistically significant results for the factors included in the GEE (‘group’, ‘time’, ‘group*time’) (p < 0.001).

Figure 5.

Subjective spatial hearing assessed with the SSQ spatial hearing subscale. The boxplots display the minimum, first quartile, median, third quartile, and maximum. After Bonferroni correction, p-values < 0.017 were considered to be statistically significant. Statistically significant differences between groups at 24 months of follow-up: ** = p < 0.017; *** = p < 0.001. For the statistical difference within groups at 24 months compared to baseline: •• = p < 0.017; ••• = p < 0.001. Abbreviations: BCD, bone conduction device; BLN, baseline; CI, cochlear implant; CROS, contralateral routing of signals hearing aid; SSQ, Speech, Spatial and Qualities of hearing scale.

There were no statistically significant differences between the groups at baseline. At 24 months of follow-up, participants in the CI group showed statistically significantly higher scores on the SSQ Spatial Hearing subscale than those in the BCD (difference: 3.9, p < 0.001), CROS (difference: 2.4, p < 0.001), and no-treatment groups (difference: 3.2, p < 0.001) (Table 3). Despite the worse subjective spatial hearing scores in the CROS group compared to the CI group, it showed better results than the no-treatment group at 24 months (CROS vs. no treatment: difference: 0.8, p = 0.015).

Participants in the treatment groups CI and CROS scored statistically significantly higher on the SSQ Spatial Hearing subscale at 24 months compared to baseline (Table 2, Figure 5). For CI, the difference between the score at 24 months and baseline was 4.0 (p < 0.001) and for CROS the difference was 1.3 (p < 0.001).

The subjective sound localization outcome (SSQ Spatial Hearing subscale) was statistically significantly correlated to the objective sound localization outcomes. At 24 months of follow-up, the Spearman's rank correlation coefficient between scores of all participants on the SSQ Spatial Hearing subscale and the RMS error in degrees was −0.57, −0.55, and −0.51 (15, 30, and 60 degrees, respectively; p < 0.001), and between scores on the SSQ Spatial Hearing subscale and the %-correct scores it was 0.54, 0.53, and 0.55 (15, 30, and 60 degrees, respectively; p < 0.001).

Serious Adverse Events (SAEs)

There were two related SAEs during two years of follow-up: one participant in the BCD group had an implant extrusion and chose to be reimplanted. One participant in the CI group experienced unexplainable pain and eventually had the CI removed.

There were also unrelated SAEs: one participant in the CI group had a transient ischemic attack several months after implantation; in the CROS group, one participant had a myocardial infarction for which he underwent percutaneous coronary intervention, and one participant underwent sinus surgery due to nasal polyps; in the no-treatment group, one participant had an arm fracture that required surgery, and one participant developed leukemia.

Discussion

In this RCT, we demonstrated that both the sound localization accuracy and subjective spatial hearing of SSD patients were significantly better after cochlear implantation than with a BCD, CROS, or no treatment after two years of follow-up. Compared to the no-treatment group, the CROS group showed better subjective spatial hearing at 24 months of follow-up.

So far, high-level evidence studies are missing comparing the effect of multiple treatment options on sound localization accuracy and subjective spatial hearing in SSD patients with a follow-up period longer than six months. In a recent meta-analysis (Daher et al., 2023), results from twelve observational studies on sound localization outcomes in SSD patients (n = 247) treated with a CI were included, also of studies with a longer follow-up. The reported improvement in RMS error between the unaided situation and the situation after cochlear implantation was 25.3° (p < 0.001) after a follow-up period of six to twelve months, compared to an improvement in RMS error of 6.5°, 15.5°, or 20.0° (depending on the test configuration) after one year of follow-up in our study. A substantial heterogeneity among the included studies in this meta-analysis was reported, explaining the difference in magnitude of the improvement in sound localization accuracy (Daher et al., 2023). Overall, comparisons between studies are partly constrained by the variability of the sound materials and setups for localization tasks used in individual studies.

Despite the statistically significant improvement in objective sound localization, SSD CI users do not reach the same level of sound localization accuracy as normal-hearing listeners (Dorman et al., 2016; Peter et al., 2019; Smulders et al., 2015; Thompson et al., 2022). This suggests that the spatial hearing mechanisms in SSD CI patients, who must integrate and process signals from an acoustically and an electrically stimulated auditory pathway, are not as accurate as the binaural acoustic system in normal-hearing patients (Litovsky et al., 2019). Nonetheless, over two years of follow-up, the CI group's median RMS error decreased from 22.0° to 2.0° and their median %-correct score increased from 65.0% to 96.7% in the localization test with the 60-degree configuration. Based on this, we can conclude that most SSD CI users developed excellent lateralization skills, which can be of importance in daily life of SSD patients.

The outperformance in sound localization abilities after cochlear implantation compared to BCD, CROS, or no treatment in SSD patients could be explained by the (partial) restoration of binaural hearing with a CI. To explain the mechanisms involved in the restoration of binaural hearing in SSD CI users over time, some studies evaluated sound localization accuracy with low- and high-frequency stimuli. It has been demonstrated that SSD CI users show better sound localization for high-frequency sounds. This suggests that the improvement is primarily due to sensitivity to interaural level differences (ILDs) (e.g., Dillon et al., 2022; Ludwig et al., 2021; Seebacher et al., 2023), similar to bilaterally implanted CI users (Dillon et al., 2017). Nonetheless, a limited ability to localize low-frequency sounds has been observed in SSD CI users, which could indicate sensitivity to interaural time difference (ITD) cues as well (Dillon et al., 2022; Seebacher et al., 2023). Auditory aging, frequency-to-place mismatches, the signal coding strategy, and the duration of CI use could reduce the sensitivity to ITDs (Dillon et al., 2022). Long-term CI experience may improve ITD-sensitivity in lower-frequency acoustic hearing in some cases (Dillon et al., 2022; Körtje et al., 2020; Thompson et al., 2022), which might explain the improvement of sound localization capabilities that seem to occur during two years of follow-up in the CI group (see Figure 3). These results suggest that it takes time to learn to use the re-established binaural ITD cues and emphasize the need for long-term follow-up of SSD treatments to be able to judge their benefits (or lack thereof). Further improvements are not ruled out during a longer follow-up period.

Our findings demonstrated that a BCD, a CROS, or no treatment did not consistently improve sound localization accuracy in SSD patients after two years of follow-up compared to the baseline (i.e., unaided) condition. This is in line with the results reported in the literature, although these results are mainly obtained in lower-level evidence studies and during shorter follow-up periods (Jakob et al., 2022; Peters et al., 2015a). A BCD or a CROS improves sound awareness from the deaf side, but sound localization remains poor as still a single auditory pathway is activated with the sound stimuli.

Remarkably, in our study, a few individuals in the BCD, the CROS, and the no treatment groups showed very good localization accuracy at baseline and at all moments of follow-up. However, at each moment of follow-up, a large variability in localization accuracy was seen within these three groups in contrast to the CI group (Table 2, Figure 3). It has been described that some unaided SSD patients can learn to use the monaural level cues to localize in the horizontal plane depending on the high-frequency pure-tone thresholds in their normal-hearing ear, duration of deafness, and age (Agterberg et al., 2014; Firszt et al., 2015; Kitoh et al., 2023; Liu et al., 2018; Van Wanrooij & van Opstal, 2004). Both a BCD and a CROS influence the monaural cues as they (partially) lift the head-shadow effect via bone- and air-conduction, respectively. Due to the interaural attenuation with a BCD, performance with a BCD could be slightly worse than with a CROS, especially in the case of mild high-frequency hearing loss in the better ear.

In the group without intervention, we found an improvement in sound localization accuracy for two out of three configurations at two years of follow-up compared to baseline (only statistically significant when scored as RMS error in degrees, but not when scored as % correct). However, the difference was small (i.e., difference in RMS error between −5.5° and −9.5°). Based on current literature it is unclear whether these changes are considered to be clinically relevant. Additionally, the no-treatment group reported no improvement in subjective spatial hearing in daily life at two years of follow-up.

Participants in the CI and CROS group showed improved subjective spatial hearing at 24 months compared to baseline. The median difference in the SSQ Spatial Hearing subscale was 4.0 for the CI group and 1.3 for the CROS group. Noble et al. (2009) considered average differences in SSQ scores between unaided and aided conditions of one to two SSQ scale points as a proportional change in score and a moderate effect. By extension, they considered a change of two to four scale points to be a large effect, and a change of more than four scale points to be a substantial effect. Olsen et al. (2012) considered a difference of more than two SSQ scale points to be the minimal, clinically important difference. If we apply this to our results, only the improvements in subjective spatial hearing observed in the CI group between baseline and two years of follow-up are to be qualified as clinically significant. Kitterick et al. (2016) performed a meta-analysis with the results of the SSQ Spatial Hearing subscale comparing CI, BCD, and CROS in SSD patients to the unaided condition. They reported a statistically significant improvement of scores on the SSQ Spatial Hearing subscale only for the CI group with a standardized mean difference of 1.3.

Although the SSD patients implanted with a CI outperformed the other groups, still, the scores of SSD CI users on the SSQ Spatial Hearing subscale indicate limitations in localization in daily life. This is of importance for the counselling of SSD patients. Note that for the BCD, CROS, and no-treatment groups, the scores on the SSQ Spatial Hearing subscale seem to decrease between three and 24 months (Figure 5). Especially the BCD group initially showed an increase in the subjective spatial hearing score at 3 months of follow-up, which almost returned to baseline scores afterwards. This could be the result of treatment bias and again emphasizes the importance of a longer duration of follow-up.

This study filled a gap in the literature where high-quality evidence of longer-term outcomes is scarce when comparing the sound localization capabilities of different SSD treatments. With the results of this RCT in a large sample of SSD patients, we were able to compare outcomes between treatments, including a no-treatment group, and within treatments over a long time period. By doing so, not only improvements in effectiveness in different domains could be analyzed, but also the benefits and harms of treatments over time. Another strength of this study is that we reported not only the %-correct score but also calculated the RMS localization error, providing a more complete assessment of localization scores. To be noted, a participant who is only lateralizing can have the same %-correct score as someone who indicates the wrong sound speakers close to the real sound source each time. Therefore, we recommend to include the RMS localization error score in future studies on this topic as it provides more detailed information.

Some limitations could have influenced our outcomes. First, missing data increased to 18.3% after two years of follow-up. However, we expect the effect of this on the between-group comparisons to be very minimal, as the amount of missing data was equally distributed over all groups. Besides this, participants who switched to the no-treatment group or stopped study participation could have been less satisfied with their treatment option, resulting in more positive outcomes for the remaining treatment groups and more negative outcomes for the no-treatment group. Additionally, although there were no statistically significant differences in sound localization outcomes between the groups at baseline, in the no-treatment group higher RMS errors and lower %-correct scores were observed than in the other groups. An explanation for this variation in results could be that, although participants were randomized to one group, poor-performing participants who were allocated to the trial periods may have opted for no treatment, influencing both baseline and follow-up scores (e.g., the between-group differences at 24 months). Therefore, following the completion of the RCT after five years of follow-up, we will examine the effect of choice and the change in outcomes over time, for example by correcting for scores at baseline.

In this paper, we presented the outcomes of a RCT that has been performed to evaluate the effect of a CI, BCD, CROS, or no treatment on objective and subjective sound localization after two years of follow-up. This RCT is still ongoing, with a maximum duration of five years of follow-up. Outcomes of sound localization, speech perception, tinnitus burden, and quality of life will follow. Cost-utility analyses are needed to evaluate benefits and harms compared to societal costs, as cochlear implantation for SSD is not reimbursed in all countries. For this, long-term outcomes are important to balance the benefits in the short and long term with the costs for the years of usage.

To conclude, sound localization accuracy and subjective spatial hearing in SSD patients were significantly better in patients implanted with a CI than with a BCD, CROS, or no treatment after two years of follow-up after initiation of treatment. Compared to baseline, subjective spatial hearing in SSD patients was most improved with a CI.

Supplemental Material

sj-docx-1-tia-10.1177_23312165241287092 - Supplemental material for Sound Localization in Single-Sided Deafness; Outcomes of a Randomized Controlled Trial on the Comparison Between Cochlear Implantation, Bone Conduction Devices, and Contralateral Routing of Signals Hearing Aids

Supplemental material, sj-docx-1-tia-10.1177_23312165241287092 for Sound Localization in Single-Sided Deafness; Outcomes of a Randomized Controlled Trial on the Comparison Between Cochlear Implantation, Bone Conduction Devices, and Contralateral Routing of Signals Hearing Aids by Jan A. A. van Heteren, Hanneke D. van Oorschot, Anne W. Wendrich, Jeroen P. M. Peters, Koenraad S. Rhebergen, Wilko Grolman, Robert J. Stokroos and Adriana L. Smit in Trends in Hearing

Footnotes

Data Availability Statement

Raw data that support the findings of this study are available from the corresponding author upon reasonable request.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article: This study is partly funded by Cochlear Ltd. as an unrestricted research grant. By research contract, Cochlear Ltd. did not have influence on the study design, data collection, analysis, data interpretation, and publication. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

ORCID iDs

Jan A. A. van Heteren

Hanneke D. van Oorschot

Anne W. Wendrich

Jeroen P. M. Peters

Supplemental Material

Supplemental material for this paper is available online.

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Supplementary Material

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Sound Localization in Single-Sided Deafness;Outcomes of a Randomized Controlled Trial on the Comparison Between Cochlear Implantation,Bone Conduction Devices,and Contralateral Routing of Signals Hearing Aids

Abstract

Keywords

Introduction

Materials and Methods

Ethical Considerations

Study Design

Study Procedures

Primary and Secondary Outcomes

Statistical Analyses

Results

Participant Allocation

Device Characteristics

Sound Localization Accuracy

Subjective Spatial Hearing

Serious Adverse Events (SAEs)

Discussion

Supplemental Material

sj-docx-1-tia-10.1177_23312165241287092 - Supplemental material for Sound Localization in Single-Sided Deafness; Outcomes of a Randomized Controlled Trial on the Comparison Between Cochlear Implantation, Bone Conduction Devices, and Contralateral Routing of Signals Hearing Aids

Footnotes

Data Availability Statement

Declaration of Conflicting Interests

Funding

ORCID iDs

Supplemental Material

References

Supplementary Material