Innovative Technology in Hearing Instruments

Abstract

Hearing instrument technology research is almost entirely focused on the projected needs of the consumer market in the developed world. However, two thirds of the world’s population with hearing impairment live in developing countries and this proportion will increase in future, given present demographic trends. In developing regions, amplification and other hearing health needs may differ from those in industrialized nations, for cultural, health, or economic reasons. World Health Organization estimates indicate that at present only a small percentage of individuals in developing countries who are in need of amplification have access to hearing aid provision. New technologies, such as trainable hearing aids, advanced noise reduction algorithms, feedback reduction circuitry, nano coatings for hearing aid components, and innovative power options, may offer considerable potential benefits, both for individuals with hearing impairment in developing countries and for those who provide hearing health care services in these regions. This article considers the possible supporting role of innovative hearing instrument technologies in the provision of affordable hearing health care services in developing countries and highlights the need for research that considers the requirements of the majority of the world population in need of hearing instrument provision.

Introduction

Hearing aid technology has made tremendous advances in recent years. One estimate suggests that there are 22 manufacturers of digital hearing aids marketing products with 40 different model names in the United States (Ricketts, 2011). These hearing instruments currently support a range of sophisticated signal-processing features, and exciting innovations such as “trainable” or “self-adjustable” hearings aids (Dillion et al., 2006; Keidser, Convery, & Dillion, 2007) continue to emerge. Each new generation of digital products comes with high research and development, marketing and training costs. Multinational hearing aid manufacturing companies can support continual product enhancement because of the high purchasing power of consumers in developed economies. All the major multinational hearing instrument manufacturers, with a combined global market share of almost 90% of sales (Brouillette, 2008), are based in developed countries. However, much of their manufacturing is performed in the developing world—China is the top supplier of imported hearing aids to the United States; and Mexico, Vietnam, Thailand, and Djibouti are among the 10 nations that sold the highest value of hearing aids to the United States in 2009 (Workman, 2010). World Health Organization (WHO) figures estimate that hearing impairment affects 278 million people and that hearing impairment ranks third as a cause of years lived with a disability. Two thirds, or possibly more, of these individuals with hearing impairment live in developing countries (Smith, 2008). However, of a projected 35 million hearing aids required each year in developing countries, only one million hearing aids are provided in these regions (Smith, 2007). This dichotomy between the intense focus on hearing aid technology for the richer one third of the global market and the needs of those in developing countries may continue to widen. Out of every 100 persons added to the world population in this decade, 97 will live in developing countries. Population growth is six times faster in less developed economies than in developed economies (Zlotnik, 2005). In addition to this demographic trend, the numbers of aging individuals continues to rise in many developing countries due to improvements in health care. For example, China has about 102 million elderly (aged 65 years or above). This figure is projected to triple from 8% to 24% of the population by 2050, to a total of 322 million persons (Kaneda, 2006). Elderly individuals are at increased risk of hearing impairment, with nearly two thirds of adults aged 70 years and older found to have hearing loss in a recent North American survey (Lin, Thorpe, Gordon-Salant, & Ferrucci, 2011). Overall, hearing impairment is already the 12th most common contributor to the global burden of disease and the third commonest cause of years lived with disability (WHO, 2011). Communication disorders arising from hearing impairment are a significant contributor to poverty among many individuals in developing countries (Olusanya, Ruben, & Parving, 2006).

Despite the obvious needs for provision of hearing instruments in developing countries, there are many barriers to an equitable distribution of hearing health care resources. As mentioned above, multinational manufacturers typically concentrate on products that can be marketed in developed economies at a premium price point. In the United States, the average price for all hearing aids sold in 2009 was US$1942, with sophisticated mini-BTE hearing aids averaging US$2957 each (Kirkwood, 2010). A WHO guideline for an “affordable” hearing aid fitting is that it should be no more than 3% of gross national product (GNP) per capita per hearing aid (Brouillette, 2008). Using current World Bank figures, this would deem a hearing aid fitting affordable if it cost US$1,390 in the United States, US$1,313 in Australia, US$110 in China, US$35 in India, and US$10 in Ethiopia (World Bank, 2011). In developed countries such as the United States, there is a range of available hearing instruments that meet this cost criterion (Kirkwood, 2010). In low-income developing nations—those with a GNP per capita of US$995 or less—the guideline is much harder to meet with available hearing instruments. The cost of the least expensive hearing aid in northern Nigeria has been reported as US$222 (Adoga, Nimkur, & Silas, 2010) whereas WHO guidelines suggest affordability at a US$36 price point. The cost barrier can be exacerbated by the high import taxes and informal charges levied on medical appliances such as hearing aids in many developing countries (Bate, Tren, & Urbach, 2006). Total cost of ownership also includes earmolds, maintenance expenses, and periodic purchase of hearing aid batteries. These costs in themselves may prohibit hearing aid use in many developing countries. Annual costs for hearing aid cells may be greater than the yearly income of an African subsistence farming family (Lasisi, Ayodele, & Ijaduola, 2006). Noninstrument-related factors are also very important. Limited access to private health insurance or consumer loans in many developing countries inhibits purchase options (Tarabichi et al., 2008). Many developing countries have a chronic shortage of professionals who can provide hearing assessment and rehabilitative services (Goulios & Patuzzi, 2008), with many of those who have been trained working in developed nations. Health professionals migrate in great numbers to developed countries in search of improved opportunities (Pang, Lansang, & Haines, 2002). Audiologists are among those who travel from the developing world. For example, it is estimated that more than 50% of Indian audiologists have emigrated to more developed nations (Basavaraj, 2008).

Although these barriers to hearing instrument provision exist, there are real opportunities to improve services in many developing countries. Recently, several studies have considered the economics of hearing aid fitting in rural China (Baltussen, 2009b) and India (Baltussen, 2009a). These studies suggest alternative approaches to conventional hearing aid fitting and are in line with trends toward critically examining hearing health care provision pathways in developed countries such as the United Kingdom (National Health Service, 2011). Telemedicine applications in hearing health provision are beginning to be trialed in developed (Wesendahl, 2003) and developing nations (Swanepoel, Koekemoer, & Clark, 2010b). Telehealth technology has the potential to allow professional input in remote and disadvantaged regions (Swanepoel et al., 2010a). To assist in the identification of individuals with hearing impairment low-cost audiometers for use in developing countries have been, and continue to be, trialed (McPherson, Law, & Wong, 2010; McPherson & Knox, 1992; Swanepoel, Mngemane, Molemong, Mkwanazi, & Tutshini, 2010c).

Low-cost hearing aids—both analog and digital—are being produced by local manufacturers in a number of developing countries, including India (Basavaraj, 2008) and China (Brouillette, 2008). Body level analog hearing aid prices start at US$10 in India and digital products sell for as little as US$50. Organizations based in developing countries have also sourced multinational manufacturers who are willing to fulfill large orders of entry-level digital hearing aids at comparable costs per hearing aid. One international, not-for-profit, purchasing consortium for organizations working exclusively in developing nations has offered a basic digital BTE hearing instrument to consortium members for US$52 to US$58 (R. Brouillette, personal communication, April, 2011). A Brazilian team (Bento & Penteado, 2010) has recently designed a low-cost digital programmable BTE hearing aid using off-the-shelf components that closely adheres to WHO guidelines on the minimum performance requirements for hearing aids in developing countries (McPherson & Amedofu, in press; WHO, 2004). Several organizations have developed relatively low-cost solar battery charging systems that enable rechargeable nickel–metal hydride hearing aid cells to be reused many times (Gòmez Estancona et al., 1994; McPherson & Brouillette, 2004; Parving & Christensen, 2004). Once hearing instruments are provided in developing countries, the reported rehabilitation outcomes are similar to those noted in developed nations (Olusanya, 2004; Pienaar, Stearn, & Swanepoel, 2010) and quality of life is significantly enhanced.

Matching Technology With Needs in the Developing World

Digital hearing instrument technology has now become the universal fitting option in the United States (Kochkin, 2010) and in other developed economies. Digital signal processing (DSP) offers a number of actual and potential advantages over analog sound processing in hearing aids. Although digital technology does not change the basic configuration of a hearing aid (Kim & Barrs, 2006), DSP does offer specific advantages for individuals with hearing impairment who reside in developing countries and for the professionals who provide hearing aid services in these regions. Some of these actual advantages include:

The wide range dynamic compression features found in digital hearing aids allow discrete frequency ranges throughout the listener’s auditory spectrum to be reduced in amplitude and the discomfort arising from loud intensity sounds to be minimized. This reduces the perceived intensity of environmental sounds. Individuals residing in urban areas of developing countries are often exposed to high levels of ambient noise (Evans & Kantrowitz, 2002; Kjellstrom et al., 2007) and compression features are particularly valuable in these difficult listening environments. Students with hearing impairment in developing countries often also face increased environmental noise burdens, leading to reduced signal-to-noise ratios in the classroom. High levels of ambient noise may be due to within-school factors, such as very large student numbers (Benbow, Mizrachi, Oliver, & Said-Moshiro, 2007; Martins, Tavares, Lima Neto, & Fioravanti, 2007) and/or outside school factors, such as urban noise pollution (Choi & McPherson, 2005).

Aligned with compression facilities are the many noise reduction algorithms used in digital hearing instruments. These protocols selectively reduce low frequency background noise while attempting to maintain optimal amplification for speech frequencies. DSP allows for a high number of frequency bands to be monitored for noise and hence more effectively controlled (Kim & Barrs, 2006), particularly if the noise is relatively constant. Studies report mixed findings for the effects of noise reduction protocols on speech intelligibility in noise but indicate that these algorithms provide improved ease of listening in adverse acoustic environments (Bentler, Wu, Kettel, & Hurtig, 2008; Mueller, Weber, & Hornsby, 2006). Digital speech enhancement technology may, in future, also improve ease of listening in noisy situations although the technology is as yet largely unproven (Edwards, 2007; Franck, Boymans, & Dreschler, 2007). The advantages of directional hearing aid microphones for listening in noise are now well-known (Ricketts, 2005) and DSP allows for strategies that provide for automatic directional and omni-directional microphone usage (Banerjee, 2011).

Earmold facilities are often scarce in developing countries and perhaps the majority of hearing aid users wear alternatives to custom-made earmolds (Brouillette, 2008). Using locally sourced materials, earmold costs can be low but often the retail price in developing countries makes them unaffordable to the majority of those with hearing impairment (Basavaraj, 2008) and there may also be issues related to substandard product quality (Brouillette, 2008). For these reasons, noncustom earmold alternatives are advantageous in many developing countries. DSP hearing instruments can be equipped with feedback reduction options. These systems monitor for acoustic feedback and, when this occurs, produce a counter-phase signal that has noise-canceling effects (Chung, 2004). Feedback reduction allows the greater use of nonoccluding, noncustom earmolds for individuals with mild to moderate hearing loss. This technology also allows the successful fitting of custom earmolds that are not completely occluding in regions where quality control is an issue.

The scarcity of professional hearing health care workers in developing nations creates a pressing need for hearing instruments that are relatively easy to fit and require minimal follow-up resources. Self-adjustable or trainable hearing aids may offer particular advantages in developing countries. Once basic amplification parameters are set, the DSP hearing instrument can be adjusted by the wearer to his or her own listening choices. Over time, the trainable instrument “learns” the wearer’s preferred gain and frequency response patterns for a range of everyday acoustic environments and then can automatically optimize the user’s listening experience (Keidser et al., 2007). In theory, such hearing aids should reduce the number of appointments needed to fine tune a hearing aid and better ensure that amplification is appropriate in the adverse acoustic environments found in many developing countries. A trainable hearing aid that covers a wide range of amplification needs and has a simple initial fitting interface could be a major advance for individuals with hearing impairment in developing nations. However, the actual acceptability of such devices with clients is as yet unknown and requires field trials in both developed and developing locations. It is also important to remember that initial amplification parameters may affect later client preferences with a trainable hearing aid (Mueller et al., 2008)—an evidence-based prescriptive approach would still be the foundation of hearing instrument fitting.

Digital hearing aids have the capability of self-generating acoustic signals. This facility may be of great benefit to individuals with hearing impairment in developing countries. For hearing instruments that have in-situ auditory threshold measurement ability, it may be possible to design assessment and fitting protocols that use the hearing instrument as a platform. Such protocols could require less expenditure on audiometric equipment and be feasible in the less than ideal acoustic environments often available in developing country hearing clinics. DSP technology also typically allows a wide range of hearing loss configurations (in terms of both severity and shape) to be aided with a single hearing instrument type. This is a benefit in developing regions, as it leads to lower inventory costs and reduces expenditure associated with training hearing health care workers.

Many developing countries are located in tropical regions with climatic conditions that are challenging for long-term hearing aid use, whether DSP or analog products are used. Hearing aid breakdown due to high humidity is a frequent client complaint in these regions, adding an additional burden to the individual and/or the hearing health care service (Brouillette, 2008). Technicians who can repair hearing aids are often unavailable, particularly in rural areas (McPherson & Holborow, 1985). Dry-aid kits are sometimes promoted as an accessory to accompany hearing aid fitting in developing countries but their effectiveness is limited (Hall & Croutch, 2010) and their use creates another recurrent expense for the wearer. “Tropicalizing” hearing instruments involves spray or dip-applied coatings of water-repellent materials that reduce the potential for humidity-related damage. However, these solutions often have a limited useable life and may adversely affect component performance (Coulson, 2010). Recently, several reports have described liquid repellent nano coatings that can be applied to all hearing aid components (Coulson, 2010; ReSound, 2011). Nano coatings bond with components at a molecular level and should create instruments that are highly water (and oil and wax) resistant. If initial trials are successfully replicated and technology costs are appropriate, nano-coated hearing aids would be very advantageous in developing countries.

As mentioned previously, hearing aid batteries are an unavoidable recurrent cost that can make hearing instrument ownership prohibitive in developing countries. Several not-for-profit organizations in developing countries have considered ways to reduce this barrier, primarily through the use of rechargeable nickel–metal hydride or lithium-ion batteries (Passerini, Owens, & Coustier, 2000) paired with a solar-powered recharging device (Gòmez Estancona et al., 1994; McPherson & Brouillette, 2004). A range of rechargeable hearing aid batteries are now available, including common size 675 button cells. This innovation considerably reduces recurrent costs of ownership, as Ni-MH cells may be recharged up to 300 times over a 2-year life (Brouillette, 2008). An additional initial cost is added and this strategy will therefore only be successful if recharger units are affordable. One such unit sells for approximately US$10 and enables the user to recharge two size 13 or 675 cells (IMPACT Foundation, 2011). Another caveat is that hearing aids need to be laboratory and field trialed with rechargeable cells to ensure that their electroacoustic characteristics are not altered, that performance over time is stable, and that discharge rate and overall battery life are reasonable. In some regions, hearing aid batteries of any type are virtually unobtainable in local markets. Alternative, more affordable batteries have been trialed in some clinics, such as substituting a readily available LH44 watch battery for a 675 hearing aid cell. This substitute battery provided very reasonable battery life but slightly altered the frequency response and output of one DSP hearing aid used in this manner (R. Brouillette, personal communication, April, 2011).

Conclusions

Access to assistive technology such as hearing aids is a basic right for persons with hearing impairment, recognized by the Convention on the Rights of Persons with Disabilities (Borg, Lindström, & Larsson, 2009), a United Nations human rights treaty signed by 147 countries to date. This basic right has yet to be accorded to the majority of individuals with hearing impairment. The present overview has highlighted some current and emerging hearing instrument technologies that may reduce barriers to access and promote efficient use of amplification in developing countries. However, there has been little research conducted to date on how to match amplification technology with the need in developing countries for affordable and appropriate hearing instruments. The WHO Guidelines for Hearing Aids and Services for Developing Countries (2004) implicitly recognize this deficit and call for a research agenda—in the form of comprehensive pilot projects—to be linked with hearing aid provision in developing nations. The WHO Guidelines also make the very important point that hearing instrument provision should be part of a much wider effort throughout a society to raise awareness of hearing impairment and how it can be prevented. Given the long-term global demographic trends, there is an increasingly urgent need for research that considers appropriate hearing instrument technologies for use in developing countries and how affordable hearing aid provision can be effectively integrated into existing community health care and social welfare services.