Question
What kind of results will you expect while doing insertion gain measurements in children with a Type C tympanogram where the eardrum is retracted? What are the important things to keep in mind while performing these measurements in children with Type C or Type Adeep tympanograms?
Answer
The status of the middle ear affects the impedance of the middle ear system. Anything that affects the impedance affects the SPL a hearing aid generates in the ear canal. When the sound from the hearing aid hits a normal eardrum some of the sound bounces back but most is transmitted through the system. In ears with Type C tympanograms the eardrum is retracted so it is stiff. Since the impedance is greater, some of the sound is not transmitted through the system so the SPL in the ear canal goes up. In Type Adeep tympanograms the opposite applies, impedance goes down so the SPL at the eardrum goes down too. You know what a drum sounds like when you hit it. Think about how that would sound if the drum head was loose instead of tight. For a more detailed discussion of individual variations in probe mic measures, read Robert De Jonge's chapter in Hearing Aids: Standards, Options and Limitations edited by Michael Valente (1996).
Important things to keep in mind: If you match your target while a child has severe negative middle ear pressure once the pressure resolves, the impedance of the middle ear will change and so will the frequency response measured in the ear canal. It might or might not be a big change depending on the gain, venting, frequency response and other individual variables. That is why we use probe microphones, to measure those individual variations. For type Adeep tympanograms you probably don't have to worry. If you have verified your desired gain/output with a hyper mobile TM, that condition is not likely to change so your response will remain stable. If you fit a kid and 2 months later he takes a header off a skateboard and the ossicles separate, then you might want to re-measure and reprogram as needed.
Steve Huart has been fitting hearing aids for over 20 years. He is a clinical and dispensing audiologist at Mayo Clinic in Scottsdale, AZ. He is also an Instructor in Audiology at Mayo Clinic College of Medicine and Adjunct Audiology Faculty at Arizona School of Health Science.
Steve Huart, AuD
help dispensing professionals identify and refer those patients who no longer benefit from amplification
Steven A. Huart, Au.D., has over 20 years experience in audiology and hearing aid dispensing. Most of his career has been spent in the clinic seeing patients and fitting hearing aids. Huart was an Instructor in Audiology at Mayo Clinic Arizona and Adjunct Faculty at Arizona School of Health Sciences until he became the Director of Professional Development for AmplifonUSA. He joined Cochlear Americas in 2007 where his goal is to help dispensing professionals identify and refer those patients who no longer benefit from amplification.
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