Insertion Gain Measures with Type C Tympanogram in Children

Question

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.