Remote Hearing Aid Fitting: Factors that Affect Real Ear Measurements (1/3)

Jack Bennett, BSc (Hons), International Clinical Trainer from the Interacoustics Academy discusses the physical and environmental factors that effect the quality of measurements achieved when performing insitu real ear measures. He also discusses the patient types that might pose these problems for the clinician. This is the first video in a three part series on remote aid fitting.

You can read the full transcript below.

Introduction

The British Society of Audiology have produced practice guidance on the verification of hearing devices using probe microphone measurements. They state that fitting to prescriptive target levels will lead to more comfortable listening and significantly improved speech quality and intelligibility than fittings that deviate significantly from target.

This is extremely important for clinicians fitting hearing devices. After all, some core goals for most patients are to improve speech quality and intelligibility while maintaining a comfortable listening level.

REM process

Let's review the REM process and discuss the factors needed to ensure a quality fitting.

First is the probe placement. We must place the probe close to the eardrum as we are most interested in the response of the hearing aid at eardrum level. This is the level of sound actually presented to the patient.

Keeping the probe within 5 mm of the eardrum will avoid standing wave errors. For example, if the probe microphone is 6 mm from the eardrum, then an error of 2 decibels can be induced at 6 kHz. For closed fittings, probe placement can induce errors in frequencies above 2 kHz, and for open fittings, we can see errors above 1 kHz.

We should avoid reflective surfaces in the test environment. We want to control the stimulus level and only measure the hearing aid response to specific, repeatable levels. If there are reflective surfaces, such as solid walls or even people in the test environment, then reflected sounds can also be presented to the hearing aid and their effects will be measured in the hearing aid response. These reflections may not be consistent across subsequent measurements and therefore should be avoided.

By using the International Speech Test Signal, you can keep all adaptive features of the hearing aid switched on. This means we can verify the hearing aids in the same settings as they will be used when the patient leaves the clinic. It is recommended to run the signal for at least 10 seconds. This allows the long-term average speech spectrum to stabilize and consistency across measurements can be maintained.

We should present the ISTS at multiple levels to ensure fit and quality. This should be done at a normal speech level, say 65 decibels, a louder speech level, say 75 to 80 decibels, and finally a quiet speech level, like 55 or 50 decibels. You can perform a much louder sweep at more than 85 decibels to verify MPO levels if you wish, but this is sometimes uncomfortable for the patient.

REM challenges

There are of course some challenges posed to the REM process:

• You may not be able to place the probe at a suitable position in the ear canal. This may be due to the size or shape of the ear canal or other environmental factors.
• You may not be able to control the test environment to avoid reflective surfaces.
• Perhaps you have to be in a room that isn't suitable or the child's family have to be close.
• If you're in an environment where you can't control the noise level for the entire test period, then it's impossible to maintain consistency across all test runs and presentations.
• Your patient may be confined to a bed or immobile. This could make it unsafe for the clinician or patient to place the probe or perform any other part of the REM process.
• The patient could also be non-compliant, moving too much or making noise through the testing, rendering the tests unreliable.

Challenging patient populations

So, there are some key patient populations that may pose challenges to the REM process:

• Pediatrics, who are often non-compliant.
• Immobile patients that may be in situations that make it unsafe to perform REMs or mean they cannot come to the clinic.
• You may want to reduce the time you're in close contact with patients or reduce footfall through your clinic, for example in flu season or during an outbreak of infectious diseases.
• You may also be fitting patients in a domiciliary setting where the environment or patient health can have an effect.
• Adults with complex or additional needs.

However, all these patient populations deserve to be given the best possible care and especially as they may not be able to advocate for themselves. We need alternative methods for verifying hearing aid output to maximize the quality of these fittings.