The bilateral fitting can be advantageous for both binaural interaction and directional hearing. Compared to unilateral fitting, bilateral fitting increases the aided ear's signal-to-noise ratio in spatially separated noise, due to the greater head shadow effect. This is because the head itself tends to diffract higher-frequency components of the contralateral noise. In short, bilateral fitting improves speech in noisy environments. So what are the benefits of bilateral fitting?
Binaural amplification
There are a few pros and cons to binaural amplification with bilateral hearing aid systems. One major benefit is that patients may have a better localization ability. Studies have shown that this ability is strongly related to speech intelligibility. However, it should be noted that this advantage is not always permanent. Some patients experience problems with localization even after receiving treatment. Therefore, the effectiveness of binaural amplification should be considered carefully before considering its use.
The main advantage of binaural amplification with bilateral hearing aid systems is that both ears are stimulated, making speech understanding easier. This helps reduce the likelihood of auditory deprivation, a side effect of unaided hearing patients. Patients also benefit from the added stimulation of both auditory pathways, which reduces their stress levels and improves their overall quality of life. While the costs of binaural amplification may be greater than those of single-ear devices, the benefits are more significant as patients age.
There are many other benefits of binaural amplification, but not all of these are objectively demonstrable. Among the major benefits of binaural hearing are improved speech intelligibility in noisy environments, increased sound quality, and the ability to tune the desired signal and minimize unwanted background noise. Another benefit is enhanced localization ability. In one study, participants with bilateral amplification showed significantly higher speech intelligibility than those with mono-channel hearing.
When using binaural amplification with bilateral hearing aid systems, two ear channels can work together to increase the amplification of both sides. The use of CROS devices is an example of binaural amplification. These devices do not introduce binaural cues to patients. Hence, this type of binaural amplification with bilateral hearing aids has multiple benefits.
Binaural interference
Aside from the obvious financial and aesthetic benefits, bilateral amplification can also have negative effects. One such side effect is binaural interference. Some studies have found that amplification of both ears can reduce the ability to detect speech and other binaural cues. Similarly, long-term monaural amplification can have detrimental effects. However, this isn't the only side effect.
Mussoi and Bentler studied the effects of binaural interference in three groups of older adults with symmetrical hearing loss. They also included subjects who were not wearing hearing aids. In each group, participants were tested separately and binaurally using the Hearing in Noise Test and the Connected Speech Test. While the results were inconsistent, the overall group showed a significant right-sided binaural advantage.
In these studies, one in six people had binaural interference, a problem that causes poor speech recognition when listening with both ears. But this shouldn't stop hearing professionals from recommending binaural fittings to their patients. The new findings shouldn't discourage hearing care professionals from offering binaural fittings, as some people still prefer to use one. It also doesn't mean that bilateral hearing aids aren't for everyone.
The findings from these studies suggest that binaural interference can also be caused by physical factors such as acoustic head shadow and directional hearing. These studies support the view that audiograms don't give an accurate assessment of binaural ability. Fortunately, BMLD measurements are more robust than earphones in determining whether hearing aids cause binaural interference. For those interested in finding out whether their hearing aids are working, these tests can help them make an informed decision.
Post-operative hearing gain
Results of post-operative hearing gain with bilaterally placed hearing aids have shown some variability. Although the average absolute change in hearing thresholds was not significantly different between patients, the post-operative gain in the hearing was. In the group of patients, the average change in hearing thresholds was 6 dB at 10 kHz, 11 dB at 12 kHz, and 14 dB at 14 kHz. Interestingly, the age and sex of the patients did not have any significant effect on post-operative hearing gain.
The incremental net budget impact after surgery was considerably lower than for hearing aids, with the former costing EUR3446.9 versus the latter at US$5995. However, there are some nuances in the cost of each procedure and the overall outcome, which is why patients must consider all possible risks and benefits before making the decision to undergo surgery. Post-operative hearing aid use has been associated with an overall cost of a third to one-half of the cost of surgery.
In the present study, the corresponding ear implant patients underwent either BAHA or Sophono surgery. In general, post-operative hearing gain with BAHA was higher than with Sophono. In addition, the patients with a BAHA had significantly higher AC hearing levels than the Sophono group. The difference in air-bone gap during a post-operative time was 18.2 dB for BAHA and 67.9 dB for the Sophono group.
The differences in the AB gap between the two groups were statistically significant. The BAHA and Sophono implants had significantly lower AB gap at 0.5 kHz and at four kHz, respectively. These findings highlight the potential for these implant devices to improve post-operative hearing gain. Nevertheless, more clinical trials are needed before any recommendations can be made. This is a study in which it would be better to combine BAHA and Sophono in a patient.
Efficacy of bilateral amplification in the domain of listening effort
Efficacy of bilateral amplification has been demonstrated in several studies, but there is limited evidence of its clinical benefits. The use of a second HA in mildly hearing-impaired subjects significantly improves localization, listening effort, and speech reception in noise, especially when used in combination with a single HA. However, the effects of bilateral amplification on the other domains are unclear.
Objective measurements of listening effort have been made possible thanks to recent advances in neurophysiology and neuropsychology. The use of simple rating scales and validated subjective scales has also helped to establish objective listening effort measures. However, objective measures are still lacking in many studies, and their validity has been challenged by advanced signal processing strategies. These limitations may prevent objective assessment of listening effort. A future study of bilateral amplification in mildly hearing-impaired patients may require further studies.
In this study, 27 articles reported 56 relevant findings, with the quality of the evidence for both research questions very low. In addition, studies did not use standardized measures and lacked statistical power. Further research is needed to establish the best methods for clinical use. Efficacy of bilateral amplification is most beneficial to patients with mild hearing impairment. However, further research is needed to determine whether bilateral amplification has significant benefits for these two domains.
There is no strong correlation between subjective and objective measures of listening effort. A recent study by Gosselin and Gagne (2011) used a dual task paradigm combining tactile recognition with sentence recognition. In this study, participants' listening efforts were assessed using the Listening Effort Scale questionnaire. In this study, the effect sizes were not significantly correlated, suggesting that the subjective and objective measures are unrelated.
Comparison of bilateral versus unilateral hearing aids
One of the biggest challenges in any hearing aid study is comparing the effectiveness of unilateral and bilateral hearing aids. The three main interventions, bilateral and unilateral hearing aids, were each used by about half of the participants. This means that the difference in hearing aids between the two groups is largely due to individual preferences. However, studies do not have to be definitive. Researchers should test both models and make recommendations.
But what are the differences between the two?
While bilateral hearing aids have been shown to improve speech quality and speech recognition, these devices are not suitable for everyone. A patient may benefit from a bilateral aid if their hearing loss is caused by a condition other than hearing loss. However, the effectiveness of unilateral hearing aid will not be evident unless the use of a hearing aid is clinically indicated. This study is an attempt to answer this question, using a clinical trial.
A study of children with hearing impairments found that a bilateral aid was more effective for speech recognition than a unilateral device. Although this benefit was asymmetric, it remained consistent when using the bimodal device with either ear. Bimodal use of hearing aids resulted in a higher benefit for both groups. Bilateral users received more than twice as much benefit as patients with unilateral hearing loss. The researchers compared the benefits of bilateral versus unilateral devices in a clinical trial, but they found that a unilateral hearing aid was more effective for speech perception in noisy settings than a bilateral one.
A comparison of bilateral versus unilateral hearing aids in children who have hearing loss is an important step toward finding the right hearing device for their unique situation. Although bimodal devices may be more effective for children with bilateral hearing loss, the results of this study do not prove that the asymmetric approach is better for children with comparatively low residual hearing. There is no consensus, but based on other studies, the results support expanding the criteria for implantation of bilateral devices.
