The global burden of disabling hearing impairment: a call to action
Bolajoko O Olusanya a, Katrin J Neumann b & James E Saunders c
a. Centre for Healthy Start Initiative, 286A Corporation Drive, Dolphin Estate, Ikoyi, Lagos, Nigeria.
b. Department of Phoniatrics and Pediatric Audiology, St Elisabeth Hospital, Bochum, Germany.
c. Dartmouth Hitchcock Medical Center, Lebanon, United States of America.
Correspondence to Bolajoko O Olusanya (e-mail: firstname.lastname@example.org).
(Submitted: 09 August 2013 – Revised version received: 01 December 2013 – Accepted: 22 January 2014 – Published online: 18 February 2014.)
Bulletin of the World Health Organization 2014;92:367-373. doi: http://dx.doi.org/10.2471/BLT.13.128728
In general and irrespective of the age at which it develops, disabling hearing impairment has devastating consequences for interpersonal communication, psychosocial well-being, quality of life and economic independence.1–3 If it develops in the young, such impairment impedes speech and language development and sets the affected children on a trajectory of limited educational and vocational attainment.4–6 Children with hearing impairment may also be at increased risk of physical, social, emotional and sexual abuse and even murder.7,8 In adulthood, disabling hearing impairment can lead to embarrassment, loneliness, social isolation and stigmatization, prejudice, abuse, psychiatric disturbance, depression, difficulties in relationships with partners and children, restricted career choices, occupational stress and relatively low earnings.2,9,10
As part of its core function as the “watchdog” for the well-being of the world’s population, the World Health Organization (WHO) periodically provides estimates of the prevalences of major health conditions to guide policies and programmes in its Member States. This paper highlights the temporal trends seen in WHO’s global estimates of the prevalence of hearing impairment since 1985, against a backdrop of global epidemiologic transition and the United Nations’ emerging agenda for “disability-inclusive” development after 2015.11 We explore the roadmap for the prevention of hearing impairment that was outlined in a resolution of the World Health Assembly in 199512 and suggest prerequisites for an effective and appropriate country-level response – especially for those low- and middle-income countries where the burden posed by such disability is particularly high.
Grades and functional status
For many years, WHO defined “disabling” hearing impairment as a permanent unaided hearing loss – in the better ear and averaged over frequencies of 0.5, 1, 2 and 4 kilohertz (kHz) – of more than 40 decibels (dB) in adults and 30 dB in children.13 Unfortunately, this definition makes comparison with other disabilities or degrees of disability difficult. For example, it excludes people with mild or unilateral hearing impairment and takes no account of any underlying functional impairment in different environmental contexts. In 2001, WHO published the International classification of functioning, disability and health, in which all impairments were assessed in relation to activity limitations and participation restrictions.14 This classification has since provided a uniform framework for evaluating and comparing diverse body dysfunctions.14,15 It recognizes the role of contextual factors – such as environmental noise – in exacerbating functional deficits in people with “mild” or “slight” hearing impairment. It also treats “disabling hearing impairment” – or hearing disability – as a complex phenomenon that embraces bodily functions and structures as well as factors related to activity, participation and context.
Prevalences of disabling impairment
In 1985, when the global prevalence of hearing impairment was first estimated, 42 million people – 0.9% of the world’s population – were thought to having disabling hearing impairment.16 By 1995, the estimated number of people with such impairment had more than doubled, to 120 million – or 2.1% of the world’s population – including about 70 million adults and 8 million younger individuals in developing countries.16,17 Of the 360 million people thought to have disabling hearing impairment in 2011, approximately 32 million were children younger than 15 years and, of these, 7.5 million were younger than 5 years.18 The burden of disabling hearing impairment among both children and the elderly is thought to be greatest in the Asian Pacific area, southern Asia and sub-Saharan Africa.18
The Global Burden of Disease Hearing Loss Expert Group has recently proposed a modified classification of hearing impairment.19 According to this classification, about 538 million people older than 5 years have disabling hearing impairment.19 Although this classification still employs the better-ear hearing threshold, in decibels, averaged over frequencies of 0.5, 1, 2 and 4 kHz, it changes the threshold for disabling hearing impairment to 35 dB for all age groups and equates “unilateral hearing impairment” with “bilateral mild hearing impairment”. It also recalibrates the hearing scale in equal steps of 15 dB in an attempt to reflect crucial shifts in hearing perception more accurately. This new classification is consistent with the International classification of functioning, disability and health and with the increasing evidence that difficulties in language development may arise in children with a hearing loss of less than 35 dB. Approaches to the classification of auditory disorders in general are discussed elsewhere.20
Several factors have contributed to the upward trend seen in estimates of the global prevalence of disabling hearing impairment. One is the increasing prevalence of presbyacusis as mean life expectancy increases in many countries. Another is improvement in the technology available for the early detection and diagnosis of hearing impairment.18 A third reason is the widespread use of ototoxic medications for treating neonatal infections, ear infections, malaria, cancer, human immunodeficiency virus (HIV) infection and drug-resistant tuberculosis.21 Rubella, mumps and measles remain significant causes of hearing impairment in regions with inadequate vaccine coverage.18 Furthermore, rapid and uncontrolled urbanization in many emerging economies – coupled with a common lack of enforceable regulations on environmental and occupational noise – constitutes a growing source of noise-induced hearing impairment.22,23 In general, available estimates of the prevalence of hearing impairment remain crude because many countries struggle to conduct relevant population-based surveys using standardized protocols and classification methods.
Since 2001 the health priorities of donor-dependent countries have largely been driven by the Millennium Development Goals (MDGs). Although interventions based on these Goals have often reduced the incidence of fatal childhood illness and increased longevity, they appear to have had little impact on the global burden of disabling hearing impairment. The effective prevention, recognition and treatment of disabling hearing impairment are probably critical to the attainment of MDG 1 – eradication of global poverty – and MDG 2 – full access to primary education for all children.24 People with HIV infection, tuberculosis or malaria – infections addressed by MDG 6 – are also at high risk of developing hearing impairment, either from their infections or their treatment.25–28
Many individuals with disabling hearing impairment are disadvantaged and vulnerable children, young adults in their prime years or elderly people in dire need of societal support. The challenges of living with such impairment, particularly in poorly-resourced countries, should engender prevention efforts. Recent developments aimed at redressing the omission of disability indicators in the current MDGs offer a useful window of opportunity. Since 2009 several agencies of the United Nations have been trying to develop strategies for mainstreaming disability into the global health agenda.29 In September 2013, for example, a meeting of the United Nations General Assembly at the level of “heads of state and government” was held in New York City to discuss global action and national plans for ensuring a disability-inclusive post-2015 development agenda. The United Nations Children’s Fund has also convened a special forum of stakeholders to articulate action plans for children with disabilities – within the United Nations’ larger post-2015 disability initiative. These efforts are primarily geared towards promoting access and inclusiveness for children and adults with disability. However, they also provide valuable platforms for advocating global support for effective programmes for the prevention of avoidable disabling hearing impairment. Additionally, among global advocates of neonatal and child survival, the need to pursue disability-free survival in the post-2015 agenda is the subject of growing awareness, acknowledgement and interest.30 This is a striking departure from the almost exclusive focus on mortality reduction that has been observed since 2001.
Reducing the burden
Approximately 141 million live births occurred in the world in 2012 and most of them – about 127 million – occurred in developing countries. The estimated incidence of permanent congenital or early onset hearing impairment in developing countries in 2012 – six cases per 1000 live births – was three times higher than in developed countries.32 Although priority must be given to the primary prevention of hearing impairment, especially in low- and middle-income countries, secondary and tertiary prevention via early detection and treatment of hearing impairment, especially in infants and young children, are still needed and should be actively encouraged.33 Routine screening on school entry should be considered, as it can be more readily implemented than universal neonatal screening. The effective rehabilitation of a child with hearing impairment is complex. This is especially the case in countries that are poorly resourced, where assistive technology such as hearing aids or cochlear implants, on-going maintenance and inclusive educational support are not readily available.
Although the cost–effectiveness of hearing aids, cochlear implants and other hearing devices has been demonstrated in several studies, albeit predominantly in the developed world, the costs of acquiring and maintaining such a device remain prohibitive for most potential users in low- and middle-income countries.34–37 According to WHO, the target price for an “affordable” hearing aid should be no more than 3% of the per capita of the user’s country.34 When this definition was proposed, 3% of the per capita gross national product of Bangladesh, India, Kenya, Malawi and Nigeria was 26, 46, 26, 10 and 43 United States dollars (US$), respectively. These target prices – which represent small fractions of the corresponding mean price of a hearing aid in a developed country – are too high for many low-income countries. In addition, they exclude the costs of ear moulds, maintenance and the periodic purchase of batteries. The scenario is even more daunting for cochlear implants, which are associated with an estimated lifetime cost of about US$ 90 000 per child with severe to profound hearing impairment.38,39 Even the most advanced of the hearing devices currently available cannot restore “normal” hearing. Outcomes with the same device vary among patients because of a mix of individual biological, physiological and psychological factors.
Many low- and middle-income countries have large shortages of trained professionals who can provide hearing assessments and on-going support services. For most hearing parents, enrolment of a child with a hearing disability in a school for the deaf, where the child will be taught sign language, is the least preferred but most affordable option for the child’s education.
Many of these problems could be resolved, at least partially, by improvements in the primary prevention of hearing impairment. Such prevention, if it is to be effective, requires a thorough understanding of the etiology of, and risk factors for, hearing impairment in a given region or population. Some preventable or treatable causes of hearing impairment – such as mumps, rubella, meningitis, otitis media, use of ototoxic drugs and harmful exposure to prolonged and excessive work place or leisure noise – were listed in 1995 in a resolution of the World Health Assembly.12 This resolution, which outlined a “roadmap” for reducing the burden posed by disabling hearing impairment (Box 1), highlighted the need for routine immunization against the vaccine-preventable causes of such impairment, genetic counselling to reduce inherited hearing impairment – especially in regions with high rates of consanguineous marriage – and legislation for promoting and enforcing hearing conservation activities. Although much of the content of this resolution remains as relevant today as it was almost two decades ago, there is a need to update the resolution’s provisions and reaffirm its guiding principle in the light of important observations made since 1995.31,32,40–42
Box 1. Recommended roadmap for the prevention of hearing loss12
Member States of the World Health Organization are required to:
- prepare national plans for the prevention and control of major causes of avoidable hearing loss and for early detection of such loss in neonates, infants, children and the elderly, within the framework of primary health care;
- take advantage of existing guidelines and regulations or introduce appropriate legislation for the proper management of particularly important causes of deafness and hearing impairment, such as otitis media, use of ototoxic drugs and harmful exposure to noise, including noise in the work environment and loud music;
- ensure the highest possible coverage of childhood immunization against the target diseases of the Expanded Programme on Immunization and against mumps, rubella and meningococcal meningitis whenever possible;
- consider setting up mechanisms for collaboration with nongovernmental or other organizations for support to – and coordination of – action to prevent hearing impairment at the country level, including the detection of hereditary factors and genetic counselling; and
- ensure appropriate public information and education for hearing protection and conservation in particularly vulnerable or exposed population groups.
Since the resolution was published, birth trauma, preterm birth, low birth weight, birth asphyxia and sepsis have been identified as risk factors for diverse developmental sequelae, including hearing impairment.43 Similarly, severe neonatal jaundice – if not treated promptly – can be a contributor to disabling hearing impairment when combined with substantial glucose-6-phosphate dehydrogenase deficiency.43–45 Despite ample evidence of a link between hearing impairment and HIV infection, tuberculosis and malaria,25–28 such an association has rarely been acknowledged in the global fight against these “big three” diseases. Similarly, although childhood malnutrition is of great global health interest, the potential association between malnutrition due to protein–energy or micronutrient deficiency and hearing impairment has also largely remained uncharted.46 The emerging global pressure for a disability-inclusive developmental agenda should serve as an impetus to update and adapt the current roadmap for the reduction of hearing impairment – and incorporate and integrate all known causal pathways for disabling hearing impairment and child survival.
WHO has recently developed a set of excellent and well-illustrated training guides for hearing care at the basic, intermediate and advanced levels.47The main aim of these guides is to equip primary care health workers and communities in developing countries with simple and effective methods to reduce the burden of hearing disorders.47 The typical activities required at each level of prevention – from preconception to adulthood – are summarized in Table 1. Although the guides will eventually have to be tailored to the specific needs of each country, they already provide a useful starting point for the immediate implementation of activities to reduce the incidence of disabling hearing impairment.
Leadership and engagement for national action
Global initiatives are helpful in attracting resources towards tackling particular health problems and generating awareness about them. However, the impetus for bold and effective actions against disabling hearing impairment would appear to rest almost entirely on the advocacy efforts of people with such impairment, their families and those serving their needs. For those with disabling hearing impairments, effective communication with relevant policy-makers is often very difficult. This is particularly true for the most impaired, especially individuals with untreated congenital or early onset profound hearing loss who lack access to any assistive technology.
It is noteworthy that the governments of China, India and Nigeria – countries with some of the highest burdens of disabling hearing impairment globally – have acknowledged the problem by establishing national ear care programmes or agencies. Such programmes and agencies should be nurtured and promoted in many more countries. However, the task of leading a national fight against an invisible and highly stigmatizing chronic condition such as disabling hearing impairment must be approached with great care.48 In such a fight, the consequences of inaction or ineffective leadership are not usually fatal, so the benefits of the fight may not be very obvious to the public, especially in the short-term. Leadership by individuals with academic or professional qualifications is likely to be advantageous but perhaps not sufficient on its own to assure the required outcomes. Leaders must be visionary, self-motivated and held to measurable, time-bound targets within the framework of the 1995 World Health Assembly resolution and any other relevant global agenda. If WHO’s Member States are held accountable to their obligations under the 1995 resolution, including the provision of periodic and independently verifiable reports, then the effectiveness of any relevant national agency is likely to be closely monitored. Since many governments lack the capacity to implement the roadmap in a comprehensive manner, public–private partnerships – including “north–south” humanitarian outreaches – need to be actively encouraged. Such partnerships must be supported with appropriate legislation, in line with the Convention on the Rights of Persons with Disabilities, 2006.29 Additionally, WHO’s regional offices should be willing, able and ready to provide technical assistance where required through established channels of collaboration.
Finally, a revised resolution on the prevention of disabling hearing impairment would need to reflect the on-going dialogue and global action plans for people with disability in the post-2015 developmental agenda of the United Nations and its sister agencies. At least five tasks are worth considering by developing countries. First, such countries could train and equip middle-level health personnel to provide hearing tests and intermediate-level care for individuals with hearing loss, within the framework of existing health-care systems. Second they could develop “tele-health” systems to help fill gaps – created by shortages in the human resources for health – in the delivery of services for the prompt diagnosis, treatment and prevention of disabling hearing impairment. Third, they could facilitate the access of people with disabling hearing impairment to free or affordable hearing devices, possibly through a consortium-buying programme in collaboration with the leading manufacturers. Fourth, they could provide resources for the routine developmental evaluation of those who survive the potentially fatal conditions that are established risk factors for hearing disability. Finally, they could fund population-based research to gain a better understanding of the epidemiological distribution of hearing impairment across different strata within the country and accurately guide the development of interventions against such impairment.
In conclusion, all Member States of WHO – and particularly those in the developing regions of Africa and Asia – should be alerted to the growing prevalence of disabling hearing impairment, which is a silent, invisible and life-long condition. The emerging global and high-level interest in a disability-inclusive developmental agenda offers a unique and timely opportunity, both to address the neglected needs and aspirations of those with disabling hearing impairment and to take the necessary actions to arrest the current upward trend in the burden posed by such impairment. Strong country-level leadership will be required if Member States in the regions most affected are to meet the goals outlined in the 1995 World Health Assembly resolution on the prevention of hearing impairment – and those of any future revision of that resolution.
We thank our colleagues at the Coalition for Global Hearing Health and the GBD 2010 Hearing Loss Expert Group for inspiring this work and Bradley McPherson for his valuable comments on Table 1.
- Mason A, Mason M. Psychologic impact of deafness on the child and adolescent. Prim Care 2007; 34: 407-26 http://dx.doi.org/10.1016/j.pop.2007.04.003 pmid: 17666235.
- Shield B. Evaluation of the social and economic costs of hearing impairment: a report for Hear-it. London: London South Bank University; 2006. Available from: http://www.hear-it.org/multimedia/Hear_It_Report_October_2006.pdf [accessed 7 October 2014].
- Kotby MN, Tawfik S, Aziz A, Taha H. Public health impact of hearing impairment and disability. Folia Phoniatr Logop 2008; 60: 58-63 http://dx.doi.org/10.1159/000114646 pmid: 18235192.
- Karchmer MA, Allen TE. The functional assessment of deaf and hard of hearing students. Am Ann Deaf 1999; 144: 68-77 http://dx.doi.org/10.1353/aad.2012.0468 pmid: 10369003.
- Venail F, Vieu A, Artieres F, Mondain M, Uziel A. Educational and employment achievements in prelingually deaf children who receive cochlear implants. Arch Otolaryngol Head Neck Surg 2010; 136: 366-72 http://dx.doi.org/10.1001/archoto.2010.31 pmid: 20403853.
- Schroeder L, Petrou S, Kennedy C, McCann D, Law C, Watkin PM, et al., et al. The economic costs of congenital bilateral permanent childhood hearing impairment. Pediatrics 2006; 117: 1101-12 http://dx.doi.org/10.1542/peds.2005-1335 pmid: 16585304.
- Jones L, Bellis MA, Wood S, Hughes K, McCoy E, Eckley L, et al., et al. Prevalence and risk of violence against children with disabilities: a systematic review and meta-analysis of observational studies. Lancet 2012; 380: 899-907 http://dx.doi.org/10.1016/S0140-6736(12)60692-8 pmid: 22795511.
- Report of the independent expert for the United Nations study on violence against children. New York: United Nations Children’s Fund; 2005. Available from: http://www.unviolencestudy.org/ [accessed 14 February 2014].
- Mohr PE, Feldman JJ, Dunbar JL, McConkey-Robbins A, Niparko JK, Rittenhouse RK, et al., et al. The societal costs of severe to profound hearing loss in the United States. Int J Technol Assess Health Care 2000; 16: 1120-35 http://dx.doi.org/10.1017/S0266462300103162 pmid: 11155832.
- Ruben RJ. Redefining the survival of the fittest: communication disorders in the 21st century. Laryngoscope 2000; 110: 241-5 http://dx.doi.org/10.1097/00005537-200002010-00010 pmid: 10680923.
- The way forward: a disability-inclusive development agenda towards 2015 and beyond. Report of the Secretary-General. New York: United Nations; 2013. Available from: http://www.un.org/disabilities/default.asp?id=36 [accessed 25 November 2013].
- Resolution WHA48.9. Prevention of hearing impairment. In: Forty-eighth World Health Assembly, Geneva, 12 May 1995. Twelfth plenary meeting, Committee A. Geneva: World Health Organization; 1995. Available from: http://www.who.int/pbd/publications/wha_eb/wha48_9/en/ [accessed 25 November 2013].
- Prevention of blindness and deafness. Grades of hearing impairment [Internet]. Geneva: World Health Organization; 2013. Available from: http://www.who.int/pbd/deafness/hearing_impairment_grades/en/ [accessed 25 November 2013].
- International classification of functioning, disability and health. Geneva: World Health Organization; 2001.
- International classification of functioning, disability and health for children and youth. Geneva: World Health Organization; 2007.
- Smith AW. Preventing deafness—an achievable challenge. The WHO perspective. Int Congr Ser 2003; 1240: 183-91 http://dx.doi.org/10.1016/S0531-5131(03)00960-9.
- Smith AW. WHO activities for prevention of deafness and hearing impairment in children. Scand Audiol Suppl 2001; 53: 93-100 http://dx.doi.org/10.1080/010503901750166808 pmid: 11409786.
- WHO global estimates on prevalence of hearing loss. [Internet]. Geneva: World Health Organization; 2012. Available from: http://www.who.int/pbd/deafness/estimates [accessed 25 November 2013].
- Stevens G, Flaxman S, Brunskill E, Mascarenhas M, Mathers CD, Finucane M, Global Burden of Disease Hearing Loss Expert Group. Global and regional hearing impairment prevalence: an analysis of 42 studies in 29 countries. Eur J Public Health 2013; 23: 146-52 http://dx.doi.org/10.1093/eurpub/ckr176 pmid: 22197756.
- Neumann K, Stephens D. Definitions of types of hearing impairment: a discussion paper. Folia Phoniatr Logop 2011; 63: 43-8 http://dx.doi.org/10.1159/000316412 pmid: 20699618.
- Arslan E, Orzan E, Santarelli R. Global problem of drug-induced hearing loss. Ann N Y Acad Sci 1999; 884: 1-14 pmid: 10842579.
- Basner M, Babisch W, Davis A, Brink M, Clark C, Janssen S, et al., et al. Auditory and non-auditory effects of noise on health. Lancet 2013 pmid: 10842579.
- Fuente A, Hickson L. Noise-induced hearing loss in Asia. Int J Audiol 2011; 50: S3-10 http://dx.doi.org/10.3109/14992027.2010.540584 pmid: 21288065.
- Olusanya BO, Ruben RJ, Parving A. Reducing the burden of communication disorders in the developing world: an opportunity for the millennium development project. JAMA 2006; 296: 441-4 http://dx.doi.org/10.1001/jama.296.4.441 pmid: 16868302.
- Zhao SZ, Mackenzie IJ. Deafness: malaria as a forgotten cause. Ann Trop Paediatr 2011; 31: 1-10 http://dx.doi.org/10.1179/146532811X12925735813724 pmid: 21262104.
- Assuiti LF, Lanzoni GM, Santos FC, Erdmann AL, Meirelles BH. Hearing loss in people with HIV/AIDS and associated factors: an integrative review. Braz J Otorhinolaryngol 2013; 79: 248-55 http://dx.doi.org/10.5935/1808-8694.20130042 pmid: 23670333.
- van der Westhuizen Y, Swanepoel W, Heinze B, Hofmeyr LM. Auditory and otological manifestations in adults with HIV/AIDS. Int J Audiol 2013; 52: 37-43 http://dx.doi.org/10.3109/14992027.2012.721935 pmid: 23043519.
- Seddon JA, Godfrey-Faussett P, Jacobs K, Ebrahim A, Hesseling AC, Schaaf HS. Hearing loss in patients on treatment for drug-resistant tuberculosis. Eur Respir J 2012; 40: 1277-86 http://dx.doi.org/10.1183/09031936.00044812 pmid: 22700838.
- United Nations Enable [Internet]. New York: United Nations; 2013. Available from: http://www.un.org/disabilities/default.asp?navid=3&pid=17 [accessed 25 November 2013].
- Lawn JE, Blencowe H, Darmstadt GL, Bhutta ZA. Beyond newborn survival: the world you are born into determines your risk of disability-free survival. Pediatr Res 2013; 74: 1-3 http://dx.doi.org/10.1038/pr.2013.202 pmid: 24240732.
- Primary care and training resource: advanced level. Geneva: World Health Organization; 2006.
- Olusanya BO. Neonatal hearing screening and intervention in resource-limited settings: an overview. Arch Dis Child 2012; 97: 654-9 http://dx.doi.org/10.1136/archdischild-2012-301786 pmid: 22611062.
- Newborn and infant hearing screening: current issues and guiding principles for action. Outcome of a WHO informal consultation held at WHO headquarters, Geneva, Switzerland, 9–10 November, 2009. Geneva: World Health Organization; 2010. Available from: http://www.who.int/blindness/publications/Newborn_and_Infant_Hearing_Screening_Report.pdf [accessed 25 November 2013].
- McPherson B. Innovative technology in hearing instruments: matching needs in the developing world. Trends Amplif 2011; 15: 209-14 http://dx.doi.org/10.1177/1084713811424887 pmid: 22068223.
- Bond M, Mealing S, Anderson R, Elston J, Weiner G, Taylor RS, et al., et al. The effectiveness and cost-effectiveness of cochlear implants for severe to profound deafness in children and adults: a systematic review and economic model. Health Technol Assess 2009; 13: 1-330 pmid: 19799825.
- Peñaranda A, Mendieta JC, Perdomo JA, Aparicio ML, Marín LM, García JM, et al., et al. Beneficios económicos del implante coclear para la hipoacusia sensorineural profunda. Rev Panam Salud Publica 2012; 31: 325-31 http://dx.doi.org/10.1590/S1020-49892012000400009 pmid: 22652973.
- Baltussen R, Li J, Wu LD, Ge XH, Teng BY, Sun XB, et al., et al. Costs of screening children for hearing disorders and delivery of hearing aids in China. BMC Health Serv Res 2009; 9: 64 http://dx.doi.org/10.1186/1472-6963-9-64 pmid: 19371419.
- Saunders J, Barrs D. Cochlear implantation in developing countries as humanitarian service: physician attitudes and recommendations for best practice. Otolaryngol Head Neck Surg 2011; 145: 74-9 http://dx.doi.org/10.1177/0194599811401343 pmid: 21493294.
- Kral A, O’Donoghue GM. Profound deafness in childhood. N Engl J Med 2010; 363: 1438-50 http://dx.doi.org/10.1056/NEJMra0911225 pmid: 20925546.
- Baltussen R, Smith A. Cost effectiveness of strategies to combat vision and hearing loss in sub-Saharan Africa and South East Asia: mathematical modelling study. BMJ 2012; 344: e615 http://dx.doi.org/10.1136/bmj.e615 pmid: 22389341.
- Tucci D, Merson MH, Wilson BS. A summary of the literature on global hearing impairment: current status and priorities for action. Otol Neurotol 2010; 31: 31-41 http://dx.doi.org/10.1097/MAO.0b013e3181c0eaec pmid: 20050266.
- Saunders JE, Vaz S, Greinwald JH, Lai J, Morin L, Mojica K. Prevalence and etiology of hearing loss in rural Nicaraguan children. Laryngoscope 2007; 117: 387-98 http://dx.doi.org/10.1097/MLG.0b013e31802e3726 pmid: 17334299.
- Mwaniki MK, Atieno M, Lawn JE, Newton CR. Long-term neurodevelopmental outcomes after intrauterine and neonatal insults: a systematic review. Lancet 2012; 379: 445-52 http://dx.doi.org/10.1016/S0140-6736(11)61577-8 pmid: 22244654.
- Wickremasinghe AC, Kuzniewicz MW, Newman TB. Black race is not protective against hazardous bilirubin levels. J Pediatr 2013; 162: 1068-9 http://dx.doi.org/10.1016/j.jpeds.2012.12.092 pmid: 23403251.
- Howes RE, Piel FB, Patil AP, Nyangiri OA, Gething PW, Dewi M, et al., et al. G6PD deficiency prevalence and estimates of affected populations in malaria endemic countries: a geostatistical model-based map. PLoS Med 2012; 9: e1001339 http://dx.doi.org/10.1371/journal.pmed.1001339 pmid: 23152723.
- Olusanya BO. Undernutrition and hearing impairment: In: Watson RR, Grimble G, Preedy VR, Zibadi S, editors. Nutrition in infancy. Volume 2. New York: Springer; 2013. pp. 189-204.
- World Health Organization [Internet]. Primary ear and hearing care. Geneva: WHO; 2014. Available from: http://www.who.int/pbd/deafness/activities/hearing_care [accessed 25 November 2013].
- Olusanya BO. Hearing impairment prevention in developing countries: making things happen. Int J Pediatr Otorhinolaryngol 2000; 55: 167-71 http://dx.doi.org/10.1016/S0165-5876(00)00392-X pmid: 11035172.