Part 1: Hearing Amplification
Microphone picks up sound and converts it into electrical signals.
Amplifier/signal processor is the heart of a digital hearing aid. A variety of digital signal processing (DSP) techniques can be used to process sound, identify and selectively amplify speech, recognize and suppress ambient noise, etc.
Receiver converts the processes electrical signals back to acoustic energy and delivers sound to the ear, acting as a loudspeaker.
Power source is usually a zinc-air disposable battery, or more recently a rechargeable nickel metal hydride or silver-zinc battery.
Hearing aid components are housed in a custom-made acrylic shell. Hearing aids can be analog, digital, or hybrid (analog-digital combinations). Analog and hybrid models are obsolete. All hearing aids dispensed today are digital.
Hearing aid is characterized by three parameters: frequency response, gain, and OSPL-90.
Frequency response of a typical hearing aid extends up to 3 to 4 kHz; however, some hearing aids have an extended high-frequency response.
Gain is the ratio of output power to input power. Gain depends on the input frequency and input intensity.
OSPL-90 (SSPL-90)—output or saturated sound pressure level. Describes how much sound energy is generated by the hearing aid when input level is high at 90 dB.
Linear processing—hearing aid amplifies by same factor regardless of input level. This technique is obsolete.
Nonlinear processing (compression)—hearing aid amplifies softer sounds more and louder sounds less. This method is utilized in digital hearing aids and allows for more comfortable listening in ears with sensorineural hearing loss with much recruitment (narrowed dynamic range).
Directional microphones enhance the signal-to-noise ratio and allow better understanding of speech in noisy environments. Many hearing aids allow the user to switch between directional and omnidirectional microphones.
Some digital hearing aids use DSP techniques to recognize different listening environments and optimize their performance in real time. These devices can suppress feedback and amplify speech selectively while suppressing ambient noise.
Two major parameters predict patients’ success with hearing aids. These are word recognition scores and dynamic range. Patients with word recognition scores of at least 50% and a wide dynamic range are usually successful hearing aid users.
Dynamic range is the difference between the uncomfortable loudness level (UCL) and the speech reception threshold (SRT).
Dynamic range = UCL − SRT.
MCL (most comfortable level) approximately bisects the dynamic range.
The narrower the dynamic range of an ear, the more difficult to fit a comfortable hearing aid.
The four basic styles of hearing aids are as follows:
BTE (behind the ear)
ITE (in the ear)
ITC (in the canal)
CIC (completely in the canal)
Several variations of these basic styles are offered by various manufacturers, such as mini-BTE, invisible in the canal (IIT), half-shell (HS), which is a smaller version of the ITE, and others.
Each of these styles incorporates some degree of venting. Large venting makes a hearing aid more comfortable by eliminating the occlusion effect and enhancing hearing in noise. Occlusion effect results from reverberation of low frequencies in a closed ear canal. Large venting allows low frequencies to escape the ear canal. Large venting unfortunately is contraindicated in ears with significant low-frequency hearing loss (Table 19-1).
Open-fit hearing aid is essentially a BTE style aid with a modified earmold that is open, soft, not custom fitted, and eliminates the occlusion effect, making it a comfortable first choice for new hearing aid wearers. Because of the large venting, it is not suitable for patients with significant low-frequency hearing loss.
RITE (receiver in the ear) is a BTE style aid with the receiver (speaker) positioned through the acoustic tube into the ear canal. This design takes advantage of the general principle that sound is perceived as more natural if delivered closer to the tympanic membrane. This feature is common today and can be recognized by the narrow acoustic tube with a wire inside.
Extended-wear hearing aids. These CIC style hearing aids are placed in the ear canal by an audiologist and worn continuously for a few months. When the battery needs to be replaced, the device is removed and replaced by the audiologist.
Other helpful hearing aid features may include built-in programmable tinnitus masking technology, Bluetooth connectivity to other electronic devices, and connectivity to FM systems in lecture halls and other public spaces.
Implantable hearing systems have been used primarily for rehabilitation of moderate to severe sensorineural hearing loss, but also for certain types of conductive and mixed hearing loss. These systems can be partially or fully implantable. Two technologies are currently available: electromagnetic and piezoelectric.
In electromagnetic implantable hearing devices, a floating mass transducer is surgically attached to the ossicular chain, usually incus. Examples are the partially implantable Vibrant Soundbridge (Med-El, GmbH, Austria) FDA approved in 2000, and Maxum (Ototronix, St. Paul, MN) FDA approved in 2001. The fully implantable Carina (Cochlear Ltd, Australia) is not FDA approved.
In piezoelectric implantable hearing devices, a driver is implanted to mechanically vibrate the ossicular chain. A piezoelectric crystal changes shape (bends) in response to a changing electric field. A driver containing a piezoelectric crystal can convert electrical signals into mechanical vibrations of the ossicular chain. Example is the fully implantable Esteem (Envoy Medical Corp, White Bear Lake, MN), which requires removal of the incus and was FDA approved in 2010.
Most patients perceive a benefit in sound fidelity of implantable hearing devices over conventional hearing aids. Occlusion effect is eliminated, and feedback problems are significantly reduced. Hearing while bathing, swimming, or sleeping carries a clear advantage.
Disadvantages of implantable hearing systems are the complexity of the surgical procedure for device placement, high cost, lack of insurance coverage, need for surgical battery replacement, and limited MRI compatibility. Some systems require an iatrogenic ossicular discontinuity to reduce feedback problems.
The osseointegrated bone conduction hearing device (Ponto by Oticon Medical, Somerset, NJ, and Baha by Cochlear Ltd, Australia) is indicated for rehabilitation of mixed and conductive hearing loss. It can also be used for unilateral deafness which is discussed separately. The titanium implant/abutment complex is surgically placed in the bone behind the affected ear, above the temporal line, and is allowed to osseointegrate for 3 months. The abutment protrudes through an opening in the skin. The processor is then attached to the abutment. The system collects sound as a hearing aid, processes the sound electronically, and delivers it to the cochlea via bone conduction (Figure 19-1). The advantage of this system over conventional hearing aids is that any conductive hearing loss component is effectively bypassed while amplification is provided only for the sensorineural component. The greater the conductive component of hearing loss, the greater the advantage of this bone conduction system over conventional hearing aids. A bone conduction system is a good hearing rehabilitation method for mixed hearing losses with a significant conductive component, for ears with mastoid cavities which may be draining, and for ears with conductive losses which are not amenable to surgical correction (ie, congenital atresia with severe ossicular malformations).
Magnetic bone conduction hearing system (Baha Attract by Cochlear Ltd, Australia, and Sophono by Medtronic, Minneapolis, MN) has the same indications as a direct connect osseointegrated bone conduction system. The internal portion consists of a titanium osseointegrated implant and magnet, all placed under an intact skin flap. The external processor has a magnet as well, attaching to the internal components similar to a cochlear implant. The internal magnet is MRI safe up to 1.5 T for the Baha Attract and 3 T for the Sophono.