Back in the days when I was fitting hearing aids in my own clinic in Germany, I found it particularly challenging to fit clients with severe-to-profound hearing loss who have worn another older device for many years. Any new instrument that I tried in a follow-up fitting sounded different than the old one and this was often a reason for initial rejection by my client. For the client, the new power hearing aid must be "the same but better".
There can be different reasons for testing a new instrument: the client is looking for some improvement, or the HCP is convinced that the new instrument will provide more benefits for his client, be it because of new feature functionality or the ability to wirelessly connect to external sound sources. And sometimes, the old instrument is simply broken because it was worn for so many hours per day over so many years.
When I talk to my students, I get the impression that the difficulties when performing a follow-up fit in the severe-to-profound hearing loss category have not changed very much; although it can be expected that these end users today have already experienced the often challenging transition from analog to digital technology.
To provide some help for this fitting process, I would like to make a proposal for how to start a follow-up fitting in cases where the old instrument is still functional. It is a rather simple, yet audiologically comprehensible method, based on my experience and the discussions with other clinicians, to create the starting point for the new fitting:
Step 1: Take the old instrument and measure its gain and maximum output characteristics in a test box. If possible, use a speech test signal with a level of 65 dB for the gain and a sine wave or warble tone at 90 dB for the maximum output.
Step 2: Take the new instrument (e. g., Bernafon Leox BTE SP or BTE UP) and turn off all feature functionalities that are not in the old instrument, or try to mimic the feature functionality of the old instrument as best as possible. You should consider keeping the feedback canceller switched on to make sure that the instrument does not generate feedback.
Step 3: Adjust the gain of the new instrument to match the measured gain curve of the old instrument (see Step 1). Make sure that the new instrument amplifies with a similar linear amplification or compression settings in the case where the old instrument had a non-linear amplification scheme. Adjust the MPO of the new instrument to match the measured output characteristics of the old instrument.
It is of course also possible to perform these measurements and adjustments by running REM. But for once, a test box could be useful in a fitting process as well, since the main goal here is to reproduce an existing sound experience of an old hearing aid and not to measure how the device performs on the real individual ear.
After you have created your initial setting of the new device, you have a good starting point that will most likely lead to your client accepting the new instrument. Of course, it is not necessarily the idea that this shall be the final setting, but it shall serve as support in the challenging transition phase from an old to a new instrument.
And it is also not possible to exactly hit the sound experience of the old device. But the steps described above shall create a setting in the new power hearing aid that gets as close as possible to the old device so that the audible change is as small as it can be. Depending on your client's willingness to change, and his feedback, you should consider modifying the gain settings and activating feature functionality over time to get the most out of the new instrument in more challenging listening situations. Basically, you are helping your client to adapt by taking him in small steps over time from an old to the new sound experience.
Good luck with your next power fitting.