15 Hearing Aids • A hearing aid (HA) is any device that amplifies sound or assists the hearing-impaired individual (generally considered electroacoustic devices) • Consider when hearing loss (HL) ~≥30 dB, though may be suitable for minimal losses with tinnitus, or for listening in specific circumstances • For asymmetric HL generally fit poorer-hearing ear, although binaural fitting usually preferable • Glasgow benefit scale and Belfast rule of thumb can be considered for HA application as well as surgery • Microphone: converts acoustic energy to electrical energy • Amplifier: boosts the electrical signal • Receiver: transforms electrical signal back to acoustic signal and broadcasts it into the ear • Behind the ear (Fig. 15.1): most commonly available on NHS; best for more severe losses as less issues of feedback compared with in-the-ear aids • In the ear (Fig. 15.1): fills concha; suitable for mild, severe loss • In the canal: more difficult to insert (relevant if patient has arthritis, for example) • Completely in the canal • Bone conductor: suitable when no ear canal or pinna, or chronic discharge; body-worn and bone-anchored types • CROS: contralateral routing of signal • Implantable devices: middle ear transducers, cochlear implants, and auditory brainstem implants • Gain: amplification of sound sufficient for it to be heard, though not uncomfortable (acoustic gain = input–output dB) • Frequency response: can be varied in most aids to have low- or high-frequency emphasis • Compression: so that the output does not exceed the comfortable listening levels of the individual • Telecoil: allows aid to pick up sound by electromagnetic induction (e.g., telephones, cinema), excluding environmental sounds • Molds: – Feedback occurs when amplified sound leaks from receiver back into microphone • Binaural amplification: benefits include: • Acclimatization: it takes several weeks for central auditory compensation to allow patient to acclimatize to the new amplification of an HA; adjustments are often required after initial fitting, but can be automatically programmed • Data logging: some aids can be set to record timing and context of use • Suitable for patients who are hearing impaired but cannot wear conventional aids, with conductive or mixed loss:
15.1 Principles
15.2 Components
15.3 Types of HAs
Open fit: avoids problems of occlusion caused by conventional aids, helping with low-frequency noise appreciation, quick fitting (no mold required), and allows better ear canal ventilation; generally good for high-tone losses (>1 kHz); better cosmetically
15.4 Principles
Looser, more comfortable, and prevents occlusion effect, which makes patient’s voice sound louder because of the conductive loss a tight mold gives; however, greater potential for feedback; venting may help tighter molds
A mold blocks the external auditory canal, which traps low-frequency energy, making low-frequency internal sound (speaking, chewing) unpleasantly loud, especially if the HL is not >40 dB; vented molds help reduce this; even more if patient has “open fit” aid
Improved word identification and sound localization
Sense of balanced hearing and elimination of head shadow effect
Need for less gain
15.5 Bone-Anchored HAs
15.5.1 Indications
< div class='tao-gold-member'>
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
