Prevalence of hearing impairment and associated factors in school-aged children and adolescents: a systematic review

Introduction Hearing impairment is one of the communication disorders of the 21st century, constituting a public health issue as it affects communication, academic success, and life quality of students. Most cases of hearing loss before 15 years of age are avoidable, and early detection can help prevent academic delays and minimize other consequences. Objective This study researched scientific literature for the prevalence of hearing impairment in school-aged children and adolescents, with its associated factors. This was accomplished by asking the defining question: “What is the prevalence of hearing impairment and its associated factors in school-aged children and adolescents?” Methods Research included the databases PubMed/MEDLINE, LILACS, Web of Science, Scopus and SciELO, and was carried out by two researchers, independently. The selected papers were analyzed on the basis of the checklist provided by the report Strengthening the Reporting of Observational Studies in Epidemiology. Results From the 463 papers analyzed, 26 fulfilled the criteria and were included in the review presented herein. The detection methods, as well as prevalence and associated factors, varied across studies. The prevalence reported by the studies varied between 0.88% and 46.70%. Otologic and non-otologic factors were associated with hearing impairment, such as middle ear and air passage infections, neo- and post-natal icterus, accumulation of cerumen, family history, suspicion of parents, use of earphones, age and income. Conclusion There is heterogeneity regarding methodology, normality criteria, and prevalence and risk factors of studies about hearing loss in adolescents and school-aged children. Nevertheless, the relevance of the subject and the necessity of early interventions are unanimous across studies.


Introduction
In the 21st century, communication disorders (which include hearing impairment, HI) constitute a serious concern within public health; if not treated, there are negative effects on the economic well-being of a society in the era of communication. 1 The problem deserves to be highlighted, as the sense of hearing is essential for the development of speech, language and learning, 2 and the higher the degree of hearing impairment, the greater the difficulties in perceiving and distinguishing speech, including language deficits. 3 In children under the age of 15, 60% of hearing loss occur due to avoidable causes, 4 and estimates indicate that 1.1 billion people around the world could be at risk for hearing impairment due to unsafe hearing practices, such as the use of individual audio devices. 5 Adolescents deserve close attention, as they are exposed to high levels of non-occupational noise. 5,6 Some factors associated with hearing impairment include infections of the superior air passages 7 and middle ear, 8---10 in addition to the presence of cerumen obstructing the external acoustic meatus, 9---11 as these can interfere in the transmission of the hearing stimulus. However, despite the fact that the causes of HI can be identified in children and adolescents, data are limited regarding possible risk factors for acquired HI. 8 Early detection of HI can help prevent academic delays, 10 besides being a determinant for productivity and life quality of the potential bearer of HI. 12 Auditory tests are indicated for the early detection of hearing disorders. 7 Therefore the need or deeper knowledge on the prevalence and associated factors for hearing impairment in school-aged individuals is evident. Prevention and intervention actions could then be carried out to minimize the negative consequences of HI in the life of individuals. The objective of this study is to carry out a systematic review in the scientific literature on the prevalence of hearing impairment and its associated factors in school-aged individuals.

Methods
A systematic literature review was carried out, guided by the question: ''What is the prevalence of hearing impairment and its associated factors in school-aged children and adolescents?'' The databases consulted were PubMed/MEDLINE, LILACS, Web of Science, Scopus and SciELO. The main descriptors related to the investigated subject were crossed: ''prevalence'', ''epidemiology'', ''cross-sectional studies ''hearing'', ''hearing loss'', ''hearing disorders'', ''school health services'', ''school health'', ''child'', and ''adolescent'', as shown by the strategies depicted in Table 1.
The review included only the studies that were cross-sectional and presented the prevalence of hearing impairment in children and/or adolescents. Other types of studies or formats were excluded as well as cross-sectional studies that included children and/or adolescents but did not present a specific prevalence for this population. Bibliographic data compilation occurred on April 10, 2018, based on the aforementioned inclusion criteria. The first phase of the selection of papers was the exclusion of duplicate studies, followed by the reading and analysis of titles and abstracts of all identified papers. The next step was the complete reading of the selected studies, which led to the exclusion of papers that were not aligned with the review proposal. The bibliographies of the papers identified were analyzed to identify possible additional studies that could be added to the review presented herein.
The selected papers underwent methodological assessment in accordance with the checklist provided by Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) 13 for cross-sectional studies, receiving the value 1 when the item was contemplated, 0 when not contemplated and 0.5 when partially contemplated. All phases were carried out by the two first authors/researchers, independently. The study presented herein only included the papers that reached at least 60% of the score determined by the STROBE checklist, with a cutoff point established to ensure good methodological quality. Papers that did not meet the cutoff threshold were excluded. All procedures of the review presented herein were conducted in accordance with the checklist of the Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA).

Results
A total of 463 papers were identified, which approached the prevalence of hearing impairment in school-aged children and/or adolescents. After all the methodological steps, 26 papers were included (Fig. 1), with a description of the methodological quality shown in Table 2. The papers investigated different populations, age groups, hearing impairment diagnosis criteria and methods, revealing heterogeneity in the results.
The studies evaluated different age groups, and eight papers included age groups beyond children and adolescents. 11,12,14---19 There was variation in the diagnostic methods and normality criteria across the selected studies. Some studies utilized the auditory threshold as screening procedure, 9,11,12,15,16,18,19---28 automated auditory threshold, 8,17,29,30 audiometric screening, 14,31 and audiometric diagnosis at some point. 10,32,33 Regarding normality criteria, there were differences even among those that utilized the same technique, either auditory threshold or scanning, and some studies presented a set of procedures to indicate test normality. Due to these differences, there was variation in the prevalence values encountered. Most studies did not provide the respective confidence intervals (CI) ( Table 3), and some studies analyzed prevalence through different criteria and/or assessed a wider age group that what was included herein, presenting CI for some criteria.
Similarly, the study of associated factors was not homogeneous. Seven studies did not include analysis of associated factors besides prevalence of hearing impairment, 16,22,24,26,27,29,33 and seven studies included analysis, but it was not specific for the age group of children and/or adolescents. 11,12,14,15,17---19 Due to the low number of studies that evaluated associated factors, the causes established by the studies were indicated as associated factors in Table 3.

Discussion
Twenty-six papers were selected for systematic review, but there was significant variation in the identification method for hearing impairment, normality criteria and investigated age groups, which consequently led to variability in the prevalence and its associated factors.
The lowest prevalence encountered was 0.88% 21 and the highest was 46.7%. 33 While some studies included diagnosis assessment, 7,10,32 others considered incapacitating hearing loss. 11,12,15,16,18 Some studies applied questionnaires, 9,26,32,33 but with different objectives. Questionnaires were applied with parents 9,26,32 and school-aged individuals, to investigate potential causes of hearing changes 26 and risk factors for HI 32 such as health history, 9 possible presence of buzzing and learning difficulties. 26 However, one of the studies had the objective of developing a questionnaire as a low-cost tool for auditory screening. 33 The prevalences found in the studies varied according to method, age group and normality criterion established by the authors and population under study; there was also variability in the study of risk factors associated with HI. Considering the studies that focused on evaluating children and/or adolescents, and considering the age group ''children'' limited to 12 years of age, it was verified that the same number of studies considered children, 21,25,26,28,29,32,33 and both age groups (children and adolescents), 7,9,10,20,22,27,31 with limited specific research on adolescents. 8,23,24,30 It must be highlighted that the age ranges within the age groups were not the same, nor were the sampling criteria for each study.
Some studies mixed preschoolers with school-aged individuals, 10,12,15,19,22,31,33 and within these studies the most common causes for hearing impairment were impacted cerumen 10,31 and infections 31 such as otitis media. 10,31 In these studies, prevalence varied between 1.75% 10 and Table 1 Search strategy for the selected databases.

Pubmed
((((prevalence and epidemiology)) AND cross-sectional studies) AND (hearing loss or hearing)) AND (child or adolescent) (school health services or school health) Web of science (TS = (prevalence) AND TS = (Hearing loss or hearing) AND TS = (cross-sectional studies) AND TS = (child or adolescent)) Scopus ALL(prevalence) AND ALL(''cross-sectional studies'') AND ALL(''hearing loss'' OR ''hearing disorders'') AND ALL(''school health services'' OR ''school health'') AND ALL (  46.7%. 33 These higher values could be explained by the diagnosis criterion utilized, which besides audiometry, also considered Type A tympanogram and the presence of acoustic reflexes. Also, there were groups of children with higher prevalence of conductive alterations, such as diagnosis of conductive loss in 84.4% 10 of the children with HI. However, the study that compared two age groups within the same population found similar prevalence: 1.3% for the age group 4---9 years old, and 1.4% for the age group 10---19 years old, from the analysis of the best ear. 16 The normality criterion employed, the number of schoolaged individuals included and/or the selected population could have caused such discrepancies, as the main causes of HI for younger individuals are conductive factors ---otitis media with effusion (age group 4---8 years old), 10 otitis media with effusion, associated with auditory tube dysfunction and adenoid dysplasia (age group 4---10 years old). 11 The study that encountered the lowest prevalence evaluated a specific group of school-aged individuals, with the objective of establishing HI prevalence in those who underwent neonatal auditory screening. For this reason, those that did not undergo screening or those already diagnosed with HI were excluded. 23 The studies did not present deep discussions on the etiology, possibly because the results originate from prevalence studies and not from diagnostic investigation. It is important to study not only the factors that lead to hearing impairment, but also the genetic causes.
The risk factors for HI in children and adolescents can be otologic or non-otologic. 9 The consulted studies revealed different factors associated with HI such as suspicion of parents, 32 poorer short term phonologic memory, 21 use of personal electronic devices, 23 middle ear infections, 8---11,18,31 infections such as measles, meningitis, mumps and maternal German measles, 31 tube dysfunction, 7,9 cerumen, 9---11,14,18,20 tympanic membrane abnormalities, 9,10 neonatal 9 and postnatal 32 icterus, convulsions, and hospitalization. 9 On the day of the evaluation, self-reported associated signs were also included, such as sinusitis, cold, earache and use of ventilation tube. 20 Low socioeconomic level, 18,32 income, 8,9,15 education level 12,15 and low maternal education level 28 were associated with HI. Untreated middle ear infections, in the Table 2 Methodological quality of the studies included, in accordance with the STROBE checklist.    case of limited access to pediatric care, constitute an important risk factor for HI. Variation in the prevalence among adolescents was verified herein, depending on the normality criterion utilized, as some studies analyzed incapacitating hearing loss, 11,12,15,16,18 while others included frequencies over 4 kHz in the normal hearing criterion, 7---9,17,19,24,27,30 evidencing the importance of evaluating high frequencies in this group. The four studies that focused on adolescents as main investigated subjects were carried out within the last decade, and the prevalence found varied between 2.2% 30 and 34.88%. 24 The highest prevalence can be explained by the inclusion of frequencies over 8 kHz and evoked optoacoustic emissions. It is possible that this occurred due to noise exposure when using personal devices, 17,23 as the use of ear-and head-phones is common, without concerns regarding the exposure levels or duration. 6 The use of media technologies must be highlighted, as well as the habit of listening to music with ear-and headphones, which occurs progressively earlier in life, 34 and therefore it is common to be precociously exposed to high levels of noise. A study involving school-aged individuals, aged between 6 and 14 years old in Poland, investigated lowered thresholds in high frequencies ---6---8 kHz, altered in 17.8% of the sample, being the influence of noise the most probable factor for such change. 7 It is important to mention that the classifications for hearing loss generally do not include high frequencies, such as the classification proposed by the WHO and employed in some of the included studies. 9,12,15 Some of the screenings carried out did not include high frequencies, and therefore might not have evidence the beginning of noise-induced hearing loss, which surely presents high incidence in this specific population, as revealed by the increase in HI prevalence in adolescents over a time interval of almost ten years. 8 There was an association between the use of ear-and head-phones and academic issues, 9 highlighting the importance of auditory health interventions.
Overall, it is difficult to compare the prevalences encountered in different studies, 19 as demonstrated in the results presented herein. Besides the heterogeneity of the methods employed to detect and classify HI in school-aged children and adolescents, the life context and the health of this population is diverse, and so are the auditory changes experienced by younger and older children. 15 These factors interfere with HI prevalence, constituting the main limitation of the study presented herein. Despite the heterogeneity of methods, prevalence and its associated factors, HI is an important factor that compromises the academic development and performance of children and adolescents.

Conclusion
There is heterogeneity regarding methodology, normality criteria, and consequently, regarding prevalence and its associated factors. Nevertheless, the relevance of the subject and the necessity of early interventions are unanimous across studies. More studies are required, locally and globally, to investigate the correlation between the associated factors and hearing impairment in this population, so that auditory health interventions and public policies are progressively more assertive and directed to the new necessities of this generation.