**5. Implanted hearing devices**

**Figure 8.** Percentage of people in each country who are satisfied with their hearing aids.

cost for these hearings aids is too high.

**Country Amount granted** Denmark, Norway, UK All expenses paid

Switzerland 840 CHF (about 740 Euro)

Germany 840 Euro

158 An Excursus into Hearing Loss

Italy 600 Euro France 120 Euro US Almost none Japan Almost none

**Table 1.** Amount of money granted by governments for hearing aids.

in other countries. This trend is easy to understand after considering how much support people receive to mitigate the cost of hearings aids. **Table 1** shows public financial support for the purchase of hearing aids for each country listed in **Figure 7**. Not surprisingly, the countries showing the highest percentages of people with hearing aids are Denmark, Norway, and the UK, all of which prescribe hearing aids at no cost when a patient is diagnosed with hearing loss. We can thus say that government aid is needed to increase hearing aid distribution or the

**Figure 8** shows the percentage of people in each country who are satisfied with their hearing aids. It is interesting that people in different countries have different views about the performance of their hearing aids, even though the specification criteria are almost the same in all these countries. The countries with established social security systems are ranked in the bottom half, while more than 80% of the people with hearing loss in France, the US, and Although wearable hearing aids come close to the goal desired by people with hearing loss, implanted hearing devices allow room for further improvements. Several types of implanted hearing devices exist, with one being the bone-anchored hearing aid (BAHA), shown in **Figure 9a**. People have known for centuries that we can perceive sound when our skull bones vibrate, and bone conduction hearing was first described in the sixteenth century [11]. Currently, some commercially available hearing aids utilize bone conduction, and the user fixes a vibrator over the mastoid bone using a hair band. However, vibrating the skull through skin and fat tissue effectively is difficult. Therefore, the BAHA implants a titanium anchor on the temporal bone (behind the ear) to work as a bridge for vibration transmission [12, 13]. The other end of the anchor appears on the skin, and the user attaches the vibrator and receiver to the edge. Compared with previous bone

**Figure 9.** Implanted hearing devices: (a) bone-anchored hearing aid, (b) artificial middle ear and (c) cochlear implant.

conduction hearing aids, this technique has better amplification of higher frequency ranges, which improves speech intelligibility [14, 15].

earplugs are not possible for patients with atresia of the external auditory canal or microtia,

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http://dx.doi.org/10.5772/intechopen.73527

The third problem deals with the adaptation for sensorineural hearing loss. Unlike conductive hearing loss that affects sound waves conduction anywhere along the route through the outer ear, tympanic membrane, and middle ear, the root cause of sensorineural hearing loss lies in the inner ear or between the auditory nerves and audio cortex in the brain. Some types of hearing loss, like presbycusis, have mixed causes. Patients with conductive hearing loss are able to achieve nearly 100% accuracy on speech intelligibility tests when the speech is presented at a sufficiently loud volume. However, patients with sensorineural hearing loss including presbycusis can only achieve the maximum peak accuracy between 40 and 80%, even when the speech is presented at an optimal volume. In this condition, patients can hear but cannot identify the syllables. Because the main function of a hearing aid is to amplify sound, it may offer limited benefits for sensorineural and mixed hearing loss when used during conversation.

The following subsections describe the solutions or potential clues for solving these three

Ultrasonic sound waves are those with frequencies greater than 20 kHz, which is the audible limit of human hearing. Although airborne ultrasound cannot be perceived, we can hear ultrasound delivered to the mastoid bone of the skull via a transducer [20]. Importantly, boneconducted ultrasound can even be perceived by people with profound hearing impairment. One study has reported that these individuals are able to identify ultrasound amplitude-modulated speech signals as speech [21]. Additionally, Hosoi used magnetoencephalography and positron-emission tomography to show that bone-conducted ultrasound can activate auditory cortex of people with profound hearing loss [22]. Although the mechanisms underlying this phenomenon are still only hypotheses, the best guess at the moment is that bone-conducted ultrasound stimulates residual inner hair cells in the base of basilar membrane [23, 24].

An accumulation of research has led to the development of several different bone-conducted ultrasonic hearing aids (**Figure 10**). The HD-GU was a test model developed by Nara Medical University in Japan. Connected to a computer, parameters from various digital signal processors (e.g., noise reduction and nonlinear gain) were controlled on a monitor via software. AIST-BCUHA-003 and AIST-BCUHA-005 were developed by the National Institute of Advanced Industrial Science and Technology (AIST) in Japan [25]. AIST-BCUHA-003 was able to control amplitude and carrier frequency (i.e., ultrasound), while the AIST-BCUHA-005 contained digital signal processors and could control the degree of modulation. Another device was HiSonic, a commercial product that controlled sound amplitude for hearing aids and as therapy for suppressing tinnitus [26, 27]. Using these models, Shimokura was able to see vast improvement in speech intelligibility in a woman with profound hearing loss who he advised to try bone-conducted ultrasonic hearing aids. [28]. The results demonstrated significant improvement from the outset of therapy, and her perceived-speech intelligibility reached 60%, as measured by correctly answered questions in a closed-set test of word intelligibility

who do not have enough space to insert it.

problems and introduce the next-generation hearing aids.

**6.1. Bone-conducted ultrasonic hearing aid**

Another implanted device is the artificial middle ear, which is used when vibration from the ear drum does not smoothly travel through the ear ossicles even after tympanoplasty operations (**Figure 9b**). A sound receiver with a built-in transmitting coil is magnetically attached behind the ear to an implanted receiving coil on the temporal bone, and the signal is transmitted through the skin by the reciprocation of the two coils. The receiving coil activates a transducer that touches the round window of the cochlea, which directly stimulates the basilar membrane. The artificial middle ear was originally developed by a Japanese hearing aid maker (RION Co., LTD.) in 1983, and other makers are currently developing a new stimulation approach using artificial ear ossicles [16].

A third device is the cochlear implant, which converts sound into an electrical current and directly stimulates hair cells in the cochlea using an implanted electrode (**Figure 9c**). The first prototype cochlear implant was conducted by William House and John Doyle in 1961 [17], making the history of this method older than either the BAHA or the artificial middle ear. Subsequently, Clark developed a multichannel electrode cochlear implant in 1977 and produced the first commercialized multielectrode device in 1978 [18]. The cochlear implant is adaptive for profound deafness (hearing level > 90 dB) in Japan. The cochlear implant is reported to be especially useful for children who have not acquired language skills for the linguistic development in the future.

Unlike implanted hearing devices, another future option could utilize stem cells, as inner ear stem cells were found in 1999 [19], and studies in regenerative medicine have developed even further since that time. This means that surgical approaches might become mainstream hearing-loss treatments. One irony is that in Japan, medical expenses for implanted hearing devices are more economic for the individual than the cost of hearing aids because they are considered within the scope of the health-care system.
