section name header

Information

Editors

JukkaKokkonen

Technical Rehabilitation of Hearing Impairment

Estimated need for hearing rehabilitation

  • It has been estimated that in the Nordic countries and in Great Britain 20% of the population have a hearing defect of more than 25 dB and 5% have a defect of more than 45 dB over the frequencies 0.5 to 4 kHz.
  • According to the prevalence of hearing defects, 5% of the population would be in need of hearing rehabilitation even if rather strict criteria were applied. In Finland, not more than approximately 1% of the population have a hearing aid, whereas in other Nordic countries and in Great Britain the respective figure is 2.3-3.5%.
  • Along with the aging of the population the need will increase.

Hearing aids

Principles of operation

  • Types of hearing aids
    • Behind-the-ear hearing aids (or hearing spectacles)
    • In-the-ear hearing aids may be used in mild or moderate hearing impairment Interpretation of an Audiogram and Impaired Hearing. Flexible fingers and a good visual acuity are required.
    • Body-worn hearing aids may be used in severe hearing impairment and in patients who are not able to handle the smaller instruments. This type of hearing aid is used ever less frequently.
  • The majority of hearing aids in current use are based on digital signal processing. The device is individually adjusted according to the hearing of the patient, and some of the devices are adaptive, i.e. their action is automatically altered according to the ambient auditory environment.
  • Usually the sound is conducted into the ear by air conduction through an earpiece or, in case of an in-the-ear device, through the shell of the device. If the patient has good low-frequency hearing, the earpiece may be of open type which increases wearing comfort. If the patient has ear canal atresia or chronic middle ear infection with discharge from the ear, a bone conduction vibrator or a bone-anchored hearing aid may be used.
  • Using a hearing device always requires practice. As many as 10-25% of patients do not use the hearing aid because of poor motivation or lack of skills in handling the instrument. Adjusting to the new auditory world conveyed by the device may often take several months.
  • Hearing aids need regular inspection and maintenance. The batteries must be changed as required. All primary health care units should be able to help patients with their hearing aids.

Common causes of hearing aid dysfunction

  • Displacement of the earpiece or the in-the-ear hearing aid: causes whistling (acoustic feedback). Correct the position of the instrument.
  • The earpiece is loose: causes whistling. Replace the earpiece.
  • The sound channel of the ear mold (or the shell of the in-the-ear hearing aid) is obstructed by ear wax: the hearing aid is mute. Clean the channel. The ear mold may be cleaned with mild soap or special detergent. Always remember to dry the mold as waterdrops may also block it. The sound channel of an in-the-ear hearing aid is cleaned with a special instrument and/or the wax shield is replaced with a new shield.
  • The battery is empty or incorrectly inserted: the hearing aid is mute. Replace the battery correctly. The battery may discharge itself over time, so even a newly unpacked fresh battery may be empty.
  • The battery is unsuitable. Zinc-air batteries may cause a humming noise in some hearing aids. Try to replace the battery with another type of battery (or consult a hearing centre).
  • The tube of a behind-the-ear hearing aid is stiff or broken: causes whistling. Wrong length of the tube may result in the same problem. Change the tube.
  • M-T switch is in the wrong position: the hearing aid is mute. Correct the position: M for microphone, T for induction (telephone) coil of the hearing aid. In some instruments, both functions may be used at the same time (MT position of the switch).
  • Some devices do not have a mechanical switch but a push button to change the hearing program; the chosen program is not visible to the outside.

Testing the functioning of a hearing aid

  • Use the checklist above.
  • The quality of the sound may be tested by listening to the hearing aid with an ear mold fitted for tester's ear canal. Another possibility is to use special stethoscope earpieces. Note that the sound of a hearing aid is far from linear high fidelity output.

Assistive listening devices

Communication amplifiers

  • Suited for the elderly for whom ordinary hearing aids are motorically too challenging or too complicated.
  • Used instead of a hearing aid to amplify speech and environmental sounds.

Alarm equipment

  • High volume doorbells
  • Additional telephone alarms
  • Vibration alarm clocks
  • Light indicators for telephone and doorbell
  • Low-frequency smoke alarms
  • Cry alarms

Aids for telephone communication

  • Telephone amplifiers
  • Portable amplifying telephone adapter (most adapters may also be used to change the acoustic signal from the telephone to an inductive signal which is received by the hearing aid switched into the T-position).
  • Induction loops for mobile phones (to be used with the T-switch on).
  • Text telephone (in severe hearing impairment). The short message function of cellular phones, or fax and e-mail may also be used for communication.

Entertainment electronics, inductive listening

  • Accessory head or earphones (wireless infrared earphones also exist; some infrared devices produce an acoustic signal and some an inductive signal to be received with the person's hearing aid in the T-position).
  • A portable induction loop to be attached to an accessory output of a radio or a television, or a loop with its own amplifier installed in the room (to be used with the hearing aid switch in the T-position). Many hearing aids can be wirelessly connected to accessory devices (e.g. through Bluetooth).
  • Induction loops are often available in public buildings (churches, meeting rooms, theatres) and offices. They should also be installed in health care institutions. If a fixed induction loop is installed in a public space there should be a visible sign to inform the visitors about it.

Group listening devices

  • A separate microphone, nowadays usually using radiofrequency transmission. The microphone is brought closer to the speaker so as to improve signal-noise ratio.
  • So-called FM devices in schools: the teacher has a microphone that employs radio frequencies to transmit the teacher's speech to a receiver that is connected to the hearing aid of the pupil.

Cochlear and brainstem implants

  • Cochlear implants are hearing instruments for profoundly impaired hearing. The instrument transmits an electric signal directly to the cochlear nerve by-passing the nonfunctioning inner ear.
  • Cochlear implants may be considered for both postlingually deaf children and adults as well as for pre- or perilingually deaf children. Prelingually deaf adult subjects are not suited for implant candidacy.
  • After implantation, the device has to be programmed individually and hearing must be trained. Pre- and perilingually deaf children need the same kind of hearing rehabilitation as other children with profound early childhood hearing impairment.
  • The implantees need follow-up for the rest of their lives.
  • If the cochlear nerve is not functioning (due to e.g. bilateral acoustic schwannoma or injury), the electric signal may be transmitted to the brain stem auditory nuclei (brainstem implant).

Indications for hearing aid assessment

  • Technical hearing rehabilitation should be considered within specialized care if the better ear hearing level, measured in a sound-proof testing booth, over the frequencies 0.5-4 kHz (BEHL0.5-4kHz) is 30 dB or worse (current Finnish guidelines). In children, the corresponding limit is 20 dB. The goal in hearing rehabilitation with hearing aids is binaural listening, i.e., both ears should be fitted with hearing aids as a rule.
  • The motivation of the patient and his/her social needs are of great importance. Note that hearing impairment in an elderly person living alone may result in social isolation.
  • Assessment of the need and selection of the hearing aid and assistive listening devices may require several home or workplace visits.
  • The rehabilitation plan includes assessment of the hearing impairment, other medical problems, and the social situation.

Indications for referral for hearing aid assessment

  • Summary
    • In a child, the better ear hearing level over the frequencies 0.5-4 kHz (BEHL0.5-4kHz) is worse than 20 dB.
    • If a child is suspected to have a hearing impairment but reliable hearing test is not feasible, he/she has to be referred to specialized care for investigations.
    • In an adult, the better ear hearing level over the frequencies 0.5-4 kHz (BEHL0.5-4kHz) is 30 dB or worse.
  • Ask about the patient's problems and record them in the referral.
  • Before drawing up the referral, make sure that the patient has own motivation for hearing aid experimentation.