Fitting Hearing Aids: Art or Science

Hearing Aid fitting with real ear measurements

Those of us that are in the business will likely recognise this as a popular essay question from training days. For those readers that are the users of our artfully and scientifically fitted hearing aids, I thought it would be useful to give an overview of what I like to think of as being a great way to fit hearing aids. Before I do, I want to jump right in and thank Gus Mueller for the wealth of work that he’s published and posted on the subject; and then apologise for any blatant borrowings from his pearls of wisdom.

As I mentioned ‘…Art or Science…’ is a pretty popular essay topic at schools of audiology. That means that many audiologists will have researched and written about it. I think that the answer is pretty well standard; I.e. ‘it’s both’, but to get a decent grade on that essay you’ll have to flesh it out with why. It’s surprising then that Gus has been polling audiologists, as well as conducting and collating research into what audiologists must know is best practice, only to find that the majority of audiologists are still not doing what research shows is best for our patients or clients. What’s more, the number of audiologists that are conducting best practice isn’t really increasing.

So, what’s best practice anyway? Every practitioner and every patient might have a different idea of best practice. I guess that’s where the ‘Art’ of hearing aids comes in (some like Expressionism, some like Pop-Art) so a protocol to follow which will suit every person that comes in for advice just can’t exist. The concept and implementation of ‘patient centred care’ addresses this nicely. Patient centered care means that you do what is best for your patient while being respectful of and working towards their individual goals and wishes. The Ida Institute has some great resources on patient centered care here.

So what about the science? This is the bit where we can be a little more protocol driven and when I say that many audiologists aren’t following best practice, it’s the verification of hearing aid performance with probe microphone measurements that I’m referring to. As I mentioned, Gus Mueller has been banging this drum for years and years- and for good reason. The aim of a hearing aid fitting is mainly to maximise speech intelligibility while maintaining comfort. It can sometimes be the case that these two factors are at odds with each other, especially for somebody who has not had hearing aids before. i.e. fitting a client with their first hearing aids on their full prescription can be overwhelming and they might find it uncomfortable. If they find it uncomfortable, they are less likely to wear it. For this reason, it’s perfectly normal to set a hearing aid quieter than would be optimal and gradually increase the hearing aid settings as the client acclimatises. For me and my team, that means setting the hearing aids as close to their prescription as we can get it with probe microphone measures and then reducing the gain (or the hearing aid acclimatisation setting) in the knowledge that our end point will be ‘on target’.

This brings us on to targets and prescriptions. Hearing aids are always fitted to one prescription or another and how much amplification somebody needs will be based on their hearing levels (audiogram) referenced against a group of research subjects. Within that group of research subjects, there will be some that need sounds a little louder than others for their best outcome, but essentially a ‘line of best fit’ through the research group delivers the prescription. With that in mind, it doesn’t matter if a client doesn’t agree with their prescription (they might align with one of the outliers), but it’s still the best place to start and even if they never end up liking their prescription, it’s still important to measure what their hearing aids are delivering while they are in their ears.

The prescriptions that have the most research behind them are NAL and DSL. Their research is ongoing meaning that new versions are published from time to time. Currently we are working with NAL NL2 and DSL v5. Each hearing aid manufacturer will also have their own proprietary prescription, developed from their own research. This proprietary prescription tends to be the one that hearing aid fitting software is set to by default and thus is often the one that audiologists stick to. There are a couple of problems with this however. Firstly, it is very difficult to use probe microphone measurements to verify that the settings are correct as the proprietary prescription is unlikely to be available in the probe microphone measurement system. Secondly, almost all proprietary prescriptions set the gain and output of the hearing aid to a level which gives a significantly lower speech intelligibility than NAL or DSL. You’ll remember from a previous paragraph that listening comfort and optimal speech intelligibility are sometimes at odds with each other. A more cynical man than I might propose that the proprietary prescriptions from hearing aid manufacturers are more heavily weighted towards hearing comfort than they are to hearing performance to reduce the number of hearing aids that are returned from unverified fittings. Studies have shown that manufacturers prescriptions are almost always returning a lower speech intelligibility than NAL or DSL, but even if NAL or DSL are selected in the fitting software, the only way to be sure that what’s shown on the computer is being delivered to the ear is to measure it. I can’t think of any occasion when I’ve checked a fitting and found that it can’t be improved based on the measured response.

There have been a few reasons offered by audiologists as to why they don’t use probe microphone verification; the cost of the equipment and the time it takes to carry out verification are the most common reasons. I’d suggest that verification is an essential part of a hearing aid fitting and so cost of the equipment should be factored into the business costs. Regarding the time it takes; it’s not an especially onerous process and the time saved in follow ups by doing a good initial fit certainly offsets the time to carry it out. If you need another reason; clients like it and it shows that we’re basing our work on science. It adds value!

I’m conscious that I meant to write this post as a guide for clients and patients, but it seems to have become very jargon heavy. I’ll upload it anyway and have a go at writing something a little less weighty soon.

Closing message though; if you’re a client looking for a good audiologist try to find someone that listens to what you want and helps you set a plan for what you want to achieve. When it comes to the hearing aid fitting, verification with probe microphone measures are best. This involves putting a slim tube into each ear along with the hearing aids and playing what sounds like a foreign language to you. Once you’ve had the measures, give some feedback to your audiologist so that they can tweak it to your personal preferences.