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Chapter II
Global and Regional Hearing Impairment Prevalence Gretchen A. Stevens , Seth Flaxman, Maya Mascarenhas, Katrina Davis, Emma Brunskill, Adrian Davis, Andrew W. Smith, Howard J. Hoffman, and Colin D. Mathers Abstract Hearing impairment is a leading cause of disease burden, yet studies of its prevalence are rare. We present estimates of the global prevalence of hearing impairment generated for the Global Burden of Disease 2010 study. We estimated that 1.2% (0.8%1.8%) of children under age 15, 9.8% (7.7%-13.2%) of females over age 15, and 12.2% (9.7%-16.2%) of males over age 15 experienced moderate or worse hearing impairment in 2008 (defined as a hearing level of 35 decibels or more). We found very high prevalences of adult hearing impairment in low-income regions, especially in subSaharan Africa and in South and Southeast Asia; childhood onset of hearing impairment was also higher in these regions than other regions. While hearing aid use reduces the burden of hearing impairment in high-income regions, there is little evidence of their use in the developing countries where the burden of hearing impairment is greatest.
Introduction Hearing impairment is a leading cause of disability worldwide [1]. Childhood hearing impairment can result in reduced ability to communicate, inability to interpret speech sounds leading to poor language acquisition, economic and educational disadvantage, and social isolation. While childhood hearing impairment has more serious implications due to its potential for interfering with language acquisition, it is far less common than adult onset hearing impairment. Most people will experience some hearing loss in their lives. Despite its high prevalence and burden, hearing impairment receives little attention. This fact, combined with the significant logistical hurdles involved in collection of hearing impairment data, such
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as the need for a quiet setting for testing (preferably a soundproof booth), means that measured hearing impairment data are very sparse, and few studies are nationally representative. The Global Burden of Disease (GBD) project aims to produce cause-specific estimates of global mortality, disease burden and risk factors for fatal and nonfatal conditions such as hearing impairment. A key principle of the GBD framework is to make the best possible estimates for every condition and population, producing estimates and corresponding uncertainty intervals even when data are sparse [2]. These estimates are updated periodically. The current update is the GBD 2010 study, which has generated estimates of burden of disease and risk factors in 1990, 2005, and 2010. In this chapter, we describe our estimates of the prevalence and causes of hearing impairment, which were carried out as part of the GBD 2010 study [2]. These estimates were carried out under the guidance of an international Expert Group on Hearing Loss convened for the GBD project. To evaluate the prevalence of hearing impairment, we synthesized available data from population-based surveys of hearing impairment [3]. We accessed papers from a published literature review [4] and obtained additional detailed data tabulations from investigators. We fit a Bayesian hierarchical model, making estimates by region, sex, age and hearing level (severity of hearing impairment). We used Bayesian methods because they are well-suited to accurately reflect the uncertainty associated with estimates, particularly when data are sparse. We also estimated the proportion of individuals with hearing impairment who had access to a hearing aid. An analytic challenge in this work is that hearing impairment is defined using different audiometric thresholds in the literature. Under the guidance of the Expert Group, we have used new hearing impairment categories which fully describe the range of hearing impairment, from mild to total. An important feature of our modeling approach is its inclusion of the level of hearing loss, in dB HL, as a term. As a result, we are able to model data reported using various definitions and we are able to quantify the relationship we observed between hearing impairment prevalence and hearing loss threshold.
Defining Hearing Impairment Many models of classification of hearing impairment based on the audiogram have been devised to suit the needs of researchers and clinicians. The classification published by the World Health Organization is one of the most commonly employed models to date, particularly by those publishing information about developing countries [5,6]. WHO guidelines suggest different categorizations for hearing impairment in children and adults. In the USA, UK and other parts of Europe different definitions and models have been used [7, 8]. The expert group convened to advise on estimates for the GBD project did not find the current categories, and particularly the different categorization for children and adults, compelling in understanding the needs of the hearing impaired population. The expert group has proposed a unified classification for adults and children. They proposed to use the average threshold in the better ear across the 0.5, 1, 2 and 4 kHz frequencies (as in the WHO classification). Additionally, the GBD classification uses the four frequency pure-tone hearing
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average, better ear thresholds in equally spaced (15 dB HL intervals) to classify hearing as “not impaired”, viz., excellent hearing (10 to
