Studies varied the spatial location of the speech and noise stimuli relative to the IE or non-IE (NE). Spatial configurations were classified as either creating a similar signal to noise ratio (SNR) at the two ears (IE = NE), a more favorable SNR at the IE (IE > NE), or a less favorable SNR at the IE (IE < NE). Consistent assessment methodologies were only observed among studies that used the HINT sentence test and meta-analyses were conducted only where identical speech and noise locations were used across studies.
Meta-analyses of data from the HINT sentence test identified significant benefits from both BCD and ACD when speech was presented from in front and noise was presented toward the NE (Fig. 2; Table 5, “HINT-S0Nne”). SRTs were also impaired significantly by both device types when noise was presented toward the IE (“HINT-S0Nie”). No significant effect of rerouting was identified when speech and noise were colocated (“HINT-S0N0”; see Figure, Supplemental Digital Content 5 and 6 (http://links.lww.com/EANDH/A268 and http://links.lww.com/EANDH/A269), which list the individual study estimates for ACD and BCD, respectively).
Meta-analyses were conducted using data from studies that compared APHAB before and after use of a rerouting device separately for BCD and ACD devices (Fig. 2). Significant benefits from BCD use were found for all subscales except for aversion to loud sounds (AV) and significant benefits for ACD were found only for background noise and reverberation (Table 5, “APHAB”; see Figure, Supplemental Digital Content 8 and 9 (http://links.lww.com/EANDH/A271 and http://links.lww.com/EANDH/A272), which lists the individual study estimates for BCD and ACD, respectively). Additional meta-analyses were conducted on SSQ data but no significant effects were identified for either bone or air conduction (Table 5, “SSQ”; see Figure, Supplemental Digital Content 10 and 11 (http://links.lww.com/EANDH/A273 and http://links.lww.com/EANDH/A274), which list the individual study estimates for ACD and BCD, respectively).
The scientific literature was searched using a systematic approach to identify studies that evaluated benefits of hearing instruments in adults with a unilateral sensorineural severe to profound hearing loss. Studies primarily examined changes in self-reported difficulties with listening and behavioral measures of speech perception before and after providing patients with a device that either rerouted sounds from the impaired to the NE or restored aspects of hearing to the IE.
When the SNR was less favorable at the IE, rerouting actually hindered performance. The meta-analysis revealed that the average size of the degradation in performance on the HINT sentence test (increase in SNR: ACD: 3.9 dB, BCD: 2.3 dB) was similar to or larger than the size of the benefit observed when the SNR was more favorable at the IE (decrease in SNR: ACD: 2.6 dB, BCD: 2.5 dB). Patients may, therefore, require counseling to form appropriate expectations about the situations in which benefit may be obtained and those in which the use of a rerouting device may be counterproductive to listening. The fact that the effect of a rerouting device can be highly dependent on the listening situation in which it is used may also explain why the meta-analysis found significant effects on one self-reported outcome (APHAB) but not another (SSQ), and also why moderate-to-high levels of heterogeneity (I2 >4 0%) were observed on many of their subscales.
The evidence also suggests that rerouting devices do not aid the listener to localize sounds. This conclusion is compatible with the fact that localization abilities are largely dependent on binaural cues (Akeroyd 2006) and rerouting devices do not restore two-eared hearing. Some have speculated that these devices may provide cues that enable a listener to distinguish sounds on the left from sounds on the right (lateralization) by distorting the spectral content of sounds transmitted through the device (Harford & Barry 1965; Vaneecloo et al. 2001). The ability of the current review to synthesize evidence for the effects of rerouting devices on spatial listening was hindered by heterogeneity in the assessment methodologies used across studies. However, even when multiple studies assessed spatial listening using the same self-report questionnaire (SSQ), the meta-analysis identified that there were only small and nonsignificant effects for both ACD (effect size, 0.4) and BCD (effect size, 0.6) with significant heterogeneity in effect sizes across studies for the latter (BCD, I2 = 79%). Therefore, the current review identified a lack of evidence to suggest that devices that reroute sounds to the NE provide reliable cues to support spatial hearing.
Evidence for the relative effectiveness of the various interventions identified in the review was sparse. A meta-analysis of two studies (Niparko et al. 2003; Wazen et al. 2003) suggested that BCD conferred additional benefits to speech perception compared with ACD but only when speech and noise were colocated. The effect was of medium size (SMD = 1.0) and consistent across the two studies (I2 = 7%). However, the nature of the improvement was that listeners could tolerate an additional decrease in SNR of only 1.5 dB on average and it is unclear whether the size of this effect would be considered clinically meaningful.
There are physical differences between BCD and ACD devices that could plausibly give rise to preferences for one over the other including whether the patient has to wear two devices (ACD) or only one (BCD) and whether the NE is occluded (ACD) or not (BCD). Differences in the intensity of rehabilitation, which was likely to be higher after abutment-mounted BCD provision compared with ACD provision, may also produce differences in outcome that are not a direct function of the conduction modality. However, no study was identified that compared the relative benefits of an abutment-mounted BCD and an ACD where these potential confounding factors were controlled for, and only one study compared CI to either ACD or BCD (Arndt et al. 2011a, 2011b). There is, therefore, a lack of evidence for the relative benefits of different rerouting modalities and for the additional benefits, if any, of CI over the rerouting of signals to the NE.
The lack of prospective controlled trials and the exclusion of patients who provided incomplete data raise concerns about the potential for selection bias to influence the observed effects. As a result, no recommendations for the management of unilaterally deaf adults can be based on the current evidence. The following recommendations, therefore, aim to ensure that future studies are of sufficient quality to strengthen the evidence base for hearing instruments for unilateral deafness. The primary recommendation is that randomized controlled trials should be conducted to compare ACD and BCD devices (whether mounted on a head-band or abutment) to the unaided condition and to each other, and also to compare CI with rerouting devices. If further cohort or case–control studies are undertaken, they should be well designed, appropriately powered, planned prospectively with detailed inclusion criteria, and should recruit from multiple sites. The effect sizes provided by the current meta-analysis (Table 5) could be used to inform the sample size calculations for future studies.
It is also recommended that those conducting further studies of the effectiveness of hearing instruments for unilateral severe to profound sensorineural hearing loss consider supporting future efforts to synthesize new and existing evidence when selecting outcome measures and when reporting the results of any further studies. Where studies seek to measure the benefits to speech perception in noise, it is recommended that outcomes should include SRTs for IE < NE, IE > NE, and IE = NE conditions as the current evidence suggests they may be useful in demonstrating both the benefits and potential drawbacks of different hearing instruments. Where studies aim to assess benefits to spatial listening abilities, it is recommended that the methodology should include the presentation of sounds to the left and right of straight ahead to permit the reporting of performance in terms of percent correct sound lateralization. Few studies have assessed lateralization explicitly and uncertainty remains as to whether certain hearing instruments may only aid gross spatial judgments rather than finer judgments of spatial location. Where studies wish to assess these finer judgments using localization tasks, it is recommended that the reporting of results includes the mean unsigned localization error. The current literature has employed a range of localization performance metrics that cannot be compared directly and mean unsigned error represents a summary statistic that is straightforward to compute and to interpret.
In the absence of a patient-reported outcome instrument that has been validated specifically for use in unilaterally deaf adults and to detect the effects of rerouting and restorative devices, it is recommended that future studies should assess hearing-related quality of life using at least one of the two most commonly used instruments, the APHAB and the SSQ. These instruments appear to be sensitive to the impact of device use and may indicate whether improvements on laboratory-based tests generalize to everyday listening situations. On a similar basis, it is recommended that all future studies include a generic preference-based instrument for measuring health-related quality of life that is sensitive to interventions for hearing, such as the HUI3, as only a single study was found to collect and report data on health-related quality of life. The use of generic instruments to provide preference-based valuations of the health states associated with the use of different hearing instruments (“utility” values) is necessary to inform the health-economic evaluations that are increasingly underpinning commissioning decisions in publicly funded healthcare systems.
The authors thank Dr. Susan Arndt for providing access to individual-level data for two studies (Arndt et al. 2011a, 2011b) to facilitate their inclusion in the meta-analyses.
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