Hearing Aids and Implantable Devices

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What are the major components of a digital hearing aid and how does each contribute to the function of the device?

Broadly speaking, the digital hearing aid has five major components: microphone, analog-to-digital converter, digital signal processor (microprocessor), digital-to-analog converter, and receiver. The microphone on the outside of the hearing aid picks up sound and converts sound pressure into an electrical signal. This signal is then passed through an analog-to-digital converter and sent to the microprocessor, which is essentially a tiny computer chip. The microchip filters the signal into frequency channels and amplifies the sound according to the programming of the device (according to the user’s hearing loss). The manipulated signal is converted back to an analog electrical signal through the digital-to-analog converter. The analog electrical signal is sent to the receiver where it is converted back to an acoustic signal that the patient hears.

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When should a patient be referred to audiology for a hearing aid consultation?

A hearing aid consultation should be recommended for patients who exhibit hearing loss and report a disruption in communication with others. With the improved hearing aid technology available today, hearing aids can enhance the quality of life for almost any patient with hearing loss, regardless of the type, severity, or configuration. Emerging data over the last decade have demonstrated that hearing loss is associated with a higher risk for the development of dementia, with some evidence that auditory rehabilitation with hearing aids can return this risk level to baseline. As such, hearing loss is emerging as one of the most modifiable risk factors for the development of dementia. Hearing aids can be fitted on children of any age and are commonly used for children with congenital hearing losses as early as 3 months of age.

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Name the most common styles of hearing aids ( Fig. 37.1 ).

• Behind the ear (BTE)

• Receiver in the canal (RIC)

• In the ear (ITE)

• In the canal (ITC)

• Completely in the canal (CIC)

• Invisible in the canal (IIC)

• CROS and BiCROS

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What is acoustic feedback and what causes it?

Acoustic feedback occurs when the acoustic signal leaks out from the receiver of the hearing aid and is picked up again by the microphone and amplified. This amplification loop results in an unpleasant, high-pitched squealing sound. Feedback occurs most often in high-power hearing aids and hearing aids used in conjunction with a vented earmold or open-fit configuration. Feedback is most typically an indication that the patient’s earmolds are not inserted properly or are a poor fit or the patient has outgrown the molds. Feedback can also be caused by cerumen impaction.

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What is reduced dynamic range and how does this impact the fitting of hearing aids?

Patients with sensorineural hearing loss (SNHL) experience a reduced dynamic range compared to individuals with normal hearing sensitivity. With SNHL, patients lose the perceived soft sounds at a higher absolute level than individuals with normal hearing, although the loudest tolerable sound is similar between the groups. Some with SNHL experience “recruitment” of loudness, where the growth rate of perceived loudness increases abnormally rapidly with increasing sound level. One theory of recruitment is that as the hair cells in the cochlea become damaged, normal adjacent hair cells are “recruited” to help hear the frequency of the damaged hair cell in addition to their own frequency, especially for high-level inputs. This increases the signal from the good hair cell and perceived loudness at the brain rapidly increases, causing discomfort. Clinically, dynamic range can be assessed by calculating the difference between the maximum tolerable loudness for speech (uncomfortable loudness level or UCL) and the speech reception threshold (SRT). Patients with SNHL commonly complain of recruitment and often describe this as “I have difficulty understanding soft speech, but it feels like loud sounds are uncomfortable.”

One way to address recruitment in the hearing aid fitting is through the use of wide dynamic range compression (WDRC). WDRC increases the audibility of soft sounds and reduces discomfort of loud sounds by applying more gain to low-level inputs and less gain to high-level inputs. Additionally, the maximum power output of hearing aids can be limited for patients who experience recruitment.

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