Question
I am working with an 8 year old patient who has a unilateral conductive hearing loss that has not been able to be corrected with surgery. The referring audiologist has recommended a hearing aid with a directional microphone. Is there any research to support the benefit of using directional microphones with unilateral losses?
Answer
An interesting question, which in my mind is really three separate, but important questions:
- Should I consider directional hearing aids for unilateral hearing losses?
- Does the fact that the hearing loss is conductive change my mind?
- Should I consider directional hearing aids for children?
In answer to the first question, there is some research indirectly supporting directional benefit in patients with unilateral hearing loss. We did a study on adults with fairly symmetrical hearing loss that showed that the amount of directional benefit (improvement in speech recognition when you switch from omnidirectional to directional modes) was essentially identical for unilateral and bilateral fittings (though there was a bilateral advantage to performance). If you are interested in reading more, or are having trouble sleeping, see Ricketts (2000). The impact of head angle on monaural and binaural performance with directional and omnidirectional hearing aids. With one normal ear, the issue is a little bit trickier though and I would guess it would have to do with the amount of SNR loss in the unilateral ear. Since it is conductive in this case, I would guess that the same change in SNR in both ears (once audible) would lead to about the same improvement in word recognition, so at least the potential for benefit is there.
2.) Does the fact that the hearing loss is conductive change my mind?
I don't know of any research on conductive impairment and directional hearing aids specifically, but those with conductive impairment are expected to get the same increase in speech recognition from improving signal-to-noise ratio as those with normal hearing... So I would expect at least as much directional benefit, if not more, for those with conductive impairment as for those with sensorineural loss. I am assuming the conductive component may range across frequencies if so, there is a better potential for improved SNR than with a sloping loss with essentially normal hearing in the lows. The only caveat I would offer is if their ability to understand speech in noise is already very good in the environments they are in, you obviously might not see much from a directional.
3.) Should I consider directional hearing aids for children?
So far so good, it looks like directional hearing aids may lead to benefit in this case if we were fitting an adult, so let's consider the fact they are a child. We just finished a large 3-4 year study examining the potential for directional benefit in school aged children. The bottom line is (as with adults) in some noisy environments children get a significant directional advantage and in other environments the directional mode is no better (and occasionally it make things worse). That is, school aged children are expected to benefit from hearing aids that include a directional mode if they are in the right microphone mode for the right condition. Children (or adults) don't always switch appropriately, and the appropriateness of switching in automatic systems (based on research to date) is probably no better than 90% (10% of the time the wrong mode will be selected). We are still doing further work at this time to look at the switching question, but my opinion at this time is that directional hearing aids are not as good as we hoped for kids, but the additional advantages (especially for automatic ones) probably outweigh the potential disadvantages.
References
Ricketts, T.A. (2000). The impact of head and body angle on monaural and binaural performance with directional and omnidirectional hearing aids. Ear and Hearing, 21, 318-329.
Todd Ricketts, Ph.D. is an Associate Professor in the Department of Hearing and Speech Sciences at Vanderbilt Bill Wilkerson Center.
