The fact that we have two ears does not mean that both of them hear the same way. So should the hearing aids on both ears be the same?
Maybe yes, but maybe no. We really need to test to know. I wear eyeglasses and I know that my eyes are not the same. If my prescription were the same for both eyes I would be able to see out of only one eye and would not have binocular vision. If we do not fit hearing aids separately to each ear then the child will not have binaural hearing.
Why do we need binaural hearing?
Binaural hearing is important for two reasons. It enables us to localize (tell where sound is coming from) and, even more important, to understand speech when there is competing noise. If you hear equally well from both ears (at the same loudness and with the same clarity), you are able to separate speech from noise and turn in the direction of what you need to hear, tuning out the noise. So how to we arrange all this?
Don’t assume that the same hearing aid with the same settings will be a best fit for both ears. It is often not the same. Even if hearing is the same or similar in both ears, it does not mean that the settings should be the same. One ear may have recruitment making things sound louder in one ear at some or all frequencies, or speech perception may be different in each ear requiring different settings to maximize performance. If ears are very different, then the two ears may require different hearing aids. Parents, and many adults, want the smallest hearing aids. But the small aid that is good for one ear may not be appropriate for an ear that has greater hearing loss. Trying to match the hearing aids can mean that one ear is not fit optimally.
Most audiologists fit hearing aids via prescriptive formulas – either Desired Sensation Level (DSL) or National Acoustics Laboratory (NAL). These are designed to determine the appropriate amount of amplification needed to make speech audible and minimize discomfort. Prescriptive formulas are a great start but not the end. As I have said a number of times in this blog, it is important to validate the performance. After starting with prescriptive formulas, the child (or adult) needs to be tested with the hearing aids to be sure that the hearing aid is doing what we think it is doing. Soundfield-aided thresholds will tell us if the child is hearing well enough throughout the frequency range in each ear.
|250 Hz||500 Hz||1000 Hz||2000 Hz||3000 Hz||4000 Hz|
|Right aid||20 dB||20 dB||30 dB||35 dB||40 dB||40 dB|
|Left aid||20 dB||20 dB||20 dB||20 dB||20 dB||25 dB|
Table 1 shows one child’s aided test results. Results for the left ear are excellent with thresholds at 20-25 dB throughout the frequency range. Thresholds in the right ear are not so good. This child ears well through 500 Hz, but his hearing is outside of expected levels starting at 1000 Hz. If testing was performed only binaurally, we would not know that performance with the right ear was not failing to meet expectations. With these thresholds in the right ear this child will not hear soft speech– between 30 and 35 dB, and will have trouble hearing normal conversation in noise.
Speech in noise
In addition to information about thresholds with hearing aids, it is important to know that speech perception is good in both ears. Testing needs to include speech perception performance at normal and soft conversation in quiet and also in noise. Table 2 shows test results for the child described above.
|50 dBHL||35 dBHL||50 dB+5 SNR|
Left-ear performance is excellent for normal conversation and good for soft speech and speech in noise. Right-ear performance is fair for normal conversation and poor for soft speech and speech in noise. Binaural hearing demonstrates excellent performance for normal conversation, which drops to fair for soft speech and poor for speech in noise. Even though the left ear is good, the right ear is causing the performance to drop in the binaural condition.
Can we fix this?
By testing in different conditions, we can identify the problem and try to determine what is needed to improve performance. It may be as simple as changing hearing aid settings; it may require changing the hearing aid for the ear not providing sufficient benefit; it may require therapy to improve skills; or it may require some combination of the three. We need to test in all conditions and we need to do whatever it takes to improve performance.