Hearing is one of the essential senses for human communication. Hearing impairment at any stage of life can compromise the communication process and influence an individual’s quality of life (Gondim et al., 2012). Hearing impairment in childhood can cause delays in the development of speech, language, and cognition which may later lead to educational disadvantage, social isolation and ultimately economic disadvantage (Finitzo, Albright & O’Neal, 1998; Stevens et al., 2011; Yoshinaga-Itano, 2003). In adults, untreated hearing impairment has been linked to depression, anxiety and other psychological disorders (Kochkin & Rogin, 2000), as well as an increased risk of dementia (Lin et al., 2011a).

Hearing impairment is a highly prevalent societal problem and it is one of the biggest contributors to the burden of disabilities in the world (Agrawal, Platz & Niparko, 2008). Hearing impairment was ranked first in the category of health conditions associated with disability and among the leading causes of the global burden of disability in the World Disability Report of 2011 (World Health Organization [WHO], 2011). The WHO estimates that approximately 360 million persons live with disabling hearing impairment worldwide and that the majority of these people live in developing countries, specifically South Asia, Asia Pacific and Sub-Saharan Africa (WHO, 2012).

Various risk factors, also linked to specific stages of life, are associated with hearing impairment. In children, risk factors for hearing impairment include, but are not restricted to: low birth weight, craniofacial anomalies, neonatal infections such as cytomegalovirus, herpes hyperbilirubinemia requiring exchange transfusion, respiratory distress, prolonged mechanical ventilation, meningitis, a family history of hearing loss and low Apgar score (Saunders et al., 2007; Todd, 1994). For adults, the main risk factors for hearing impairment are advanced age and prolonged exposure to excessive noise (Isaacson & Vora, 2003). Research also links some non-communicable diseases, e.g. cardiovascular disease and/or diabetes, and lifestyle factors such as smoking with increased risk of developing hearing impairment during adulthood (Fransen et al., 2008). Finally, exposure to ototoxic medications during the treatment of some health conditions such as cancer and drug-resistant tuberculosis have been shown to increase the risk of acquiring hearing impairment amongst both children and adults (Harris et al., 2011; Saunders et al., 2007; Whitehorn et al., 2014).

Population-based surveys that estimate the magnitude of hearing impairment are generally few, despite the fact that hearing impairment is considered to be a leading contributor to the burden of disability (Stevens et al., 2011). The lack of good estimates of the magnitude of hearing impairment makes it difficult to plan adequately for interventions and services aimed at individuals with hearing impairment. Currently, information on the prevalence of hearing impairment in South Africa is based primarily on data from the national population census (Statistics South Africa, 2003). Prevalence of hearing impairment estimates based on census data is typically based on self-report (or proxies’ report) of hearing impairment by individuals interviewed, and this tends to underestimate the magnitude of the problem (Nondahl et al., 1998; Sindhusake et al., 2001). This study therefore aims to estimate the prevalence of hearing impairment in the Cape Town Metropolitan area using a survey protocol that includes actual assessment of hearing status.

The research personnel comprised four teams of two senior Audiology students of the University of Cape Town per team, and two qualified audiologists with a total of 18 years of clinical experience between them. All of the students were senior Audiology students and therefore had adequate clinical skills required for the assessments required in this study. Furthermore, all the students were given additional training specific to the testing protocol used in this study to ensure consistency in assessment procedures and in the reporting of results.

Key stakeholders in the communities selected for this study were approached by the researchers after ethical approval to conduct the study was granted. This served to inform the stakeholders about the study and to request their support in encouraging community members to participate in this study. Key stakeholders included: Managers of health facilities, representatives of community forums as well as representatives of community health forums in the selected sub-districts. Community members were additionally informed about the study through posters placed at various points in the community and through flyers placed in mail boxes.

Hearing impairment was classified according to the WHO grading of hearing impairment (refer to Table 1-A2, Appendix 2) criteria. Disabling hearing impairment was defined as a permanent unaided hearing threshold level in the better ear of ≥ 41 dB HL (for adults) and permanent unaided threshold level in the better ear of ≥31 dB or (children younger than 15 years old) (WHO, 2014). Hearing impairment was further classified according to the type (conductive, mixed or sensorineural hearing loss) for all participants ≥ 4 years old.

To ensure consistency and reliability of data collected, all members of the research team were trained on the study protocol and procedures prior to commencement of the study. Furthermore, the data collected was verified by re-visiting 10.7% of the selected households to confirm the following information: Head of the household (or the person who allowed the participants to enter the household), whether otoscopy, tympanometry and audiometry were done. The verification process showed good agreement (Kappa = 98%) between data collected by different research teams and therefore the data was considered was reliable. Finally, all decisions regarding the diagnosis of hearing loss were made by a qualified audiologist and all decisions involving a diagnosis of hearing loss were forwarded to the second qualified audiologist for cross-checking. There was also good agreement between the two audiologists (kappa = 0.96). All of the equipment used in this study were calibrated prior to the start of this study and also underwent a daily biological calibration check prior to data collection sessions. Test procedures used in this study were routine audiological tests with established validity.

Data was captured on the hard copy version of the WHO/PDH Ear and Hearing Disorders Examination Form (Version 7.1A), then transferred into an excel spread sheet by four research assistants. To ensure consistency when entering the data into an excel spread sheet, at least 10% of the data entered by each one of the research assistants was double checked by a different person to ensure accurate capturing of the data.

Data analysis was performed using the STATA Data Analysis and Statistical Software package (Stata Corp LP, 2014) and both descriptive and inferential statistical methods were used. Tables and histograms were used to summarise and describe the results of the study. An independent t-test (p = 0.05) was used to compare hearing thresholds obtained under different ambient noise levels, whilst binomial logistic regression was used to model the relationship between hearing impairment and several explanatory variables; age, gender, level of education, a family history of hearing impairment, a history of stroke, a self-reported history of hypertension, a self-reported history of diabetes, smoking, a history of respiratory difficulties and a history of head and neck trauma. Binomial logistic regression was also done to estimate the probability (odds) of occurrence of hearing as a function of these explanatory variables.

The primary aim of this study was to estimate the prevalence of hearing impairment in selected health sub-districts within the Cape Town Metropolitan area. Using the WHO grading of hearing impairment classification (WHO, 2014), it was estimated that overall prevalence of hearing impairment in this study was 12.35% (95% CI: 11.06% – 13.64%). Age, gender, a family history of hearing impairment, a self-reported history of hypertension, and a history of prior head and neck trauma were main factors associated with hearing impairment.

While the estimates of the prevalence of hearing impairment in this study were consistent with that of Stevens et al., (2011), it was noted that it was slightly lower than prevalence rates reported in previous studies which used the WHO Ear and Hearing Disorders Survey protocol (WHO, 1999). For instance, a study conducted in Brazil (Béria et al., 2007) reported prevalence rates of 26.1% (overall hearing impairment) and 6.8% (disabling hearing impairment). An Egyptian study by Abdel-Hamid, Khatib, Aly, Morad, and Kamel (2007) reported prevalence rates of 16.0% (overall hearing impairment) and 2.9% for disabling hearing impairment, whilst a study by Westerberg et al. (2008) done in Uganda also reported much higher prevalence rates for disabling hearing impairment than reported in the current study: 11.7% in adults and 10.2% in children respectively.

It is generally difficult to compare prevalence estimates across different studies even when using similar protocols and definitions of hearing impairment. The difficulty in such comparisons stems from differences in demographic characteristics of participant cohorts and study contexts (Lin, Thorpe, Gordon-Salant and Ferrucci (2011b). Therefore, it was expected that prevalence estimates in this study would be somewhat different from those of other studies. However, part of the reasons for differences in the prevalence rates reported in previous studies and the current study may also have to do with the criteria used to ascertain the degree of hearing impairment in children younger than four years old. It was not clear how some of these studies confirmed the degree and type of hearing impairment in children. For instance, the Brazilian study only used pure tone audiometry (and did not use tympanometry) and therefore would have had difficulties in confirming hearing impairment in participants in this age category. The Egyptian study (Abdel-Hamid et al., 2007) on the other hand, deviated from the recommended WHO Ear and Hearing Disorders protocol and used only otoacoustic emission and tympanometry to assess hearing impairment, which makes it extremely difficult to ascertain degree and type of hearing impairment across all age categories.

Consistent with the findings of previous studies (Lin et al., 2011b; McMahon, Kifley, Rochtchina, Newall & Mitchell, 2008; Saunders et al., 2007), this study found that increasing age was associated with hearing impairment. In one American study, Lin et al., (2011b) reported that about two thirds of adults aged 70 years and older had some degree of hearing impairment. Regarding association between gender and hearing impairment, previous research studies (Caban, Lee, Gómez-Marín, Lam & Zheng, 2005; Smeeth et al., 2002) have reported a higher prevalence of hearing impairment in males than females, and the same trend was also observed in this study. It is worth noting though that gender differences in the prevalence of hearing impairment has been reported even in populations with no evidence of noise-induced hearing loss (Pearson et al., 1995) therefore this difference cannot simply be attributed the fact that men generally tend to be exposed to noisy activities.

A family history of hearing impairment and a history of head and neck trauma were other factors that were found to be associated with hearing impairment in this study, which is also consistent with the findings of previous studies (Fitzgerald, 1996; McMahon et al., 2008; Ottaviano et al., 2009). In an Australian study involving 2669 individuals aged 50 years and older, McMahon et al., (2008) reported that a family history of hearing impairment (especially a maternal family history of hearing impairment) was strongly associated with moderate to severe age-related hearing impairment. With respect to a history of head and neck trauma, hearing loss following trauma to the head and neck areas has been documented in literature (Fitzgerald, 1996; Ottaviano et al., 2009).

Results indicate that participants who reported a history of hypertension, in this study, were twice as likely to have hearing impairment as participants who did not report hypertension. This finding was consistent with those of previous studies (Chang et al., 2011; Chao, 2004; Fransen et al., 2008; Kakarlapudi, Sawyer & Staecker 2003). An unexpected finding in this study was the lack of statistically significant association between a history of diabetes and hearing impairment. This is in spite of the fact that there are now several studies that have reported a plausible association between diabetes and hearing impairment (Jáuregui-Renaud, Sánchez, Ibarra Olmos & González-Barcena, 2009; Pemmaiah & Srinivas, 2011; Thimmasettaiah & Shankar, 2012) Possible reasons for the apparent lack of association between diabetes and hearing impairment may have to do with the small number of participants who reported a history of diabetes in this study. Some of the participants may not have been aware that they are diabetic or may have simply chosen not to disclose their diabetes status.

The findings of this study must be interpreted with caution given its methodological limitations. These included unfavourable testing environments as audiological assessment done outside of audiometric booths is bound to be challenging. Specific to this study, the fact that the test environment varied from one household to the next, introduced a further challenge to this study. However, the study protocol was very stringent regarding ambient noise levels that were considered acceptable for a hearing assessment to be conducted. For instance, if the environment was found to be too noisy (i.e. ambient noise >55 dB) then a hearing assessment was not done at all or was terminated. Furthermore, an analysis of the results also revealed that there was no association between a participant being diagnosed with hearing impairment or having a particular degree of hearing impairment and the level of noise in the room.

Another limitation of this study was the small sample size which compromised the representativeness of the study sample. The calculated sample for this study was 5924 participants. However, only 42% (2494 participants) of this sample was achieved during this study. Data collection for this study had to be terminated prematurely before achieving the required sample size due to a flare-up of gang-related violence in the areas selected for this survey. This may have led to the resultant study sample being biased in favour of female participants and participants under the age of 20 years old, and this may have potentially introduced a bias that could have led to an underestimation of the prevalence of hearing impairment in this study. However, despite a possibly biased sample, the trends revealed by the data in this study were consistent with those reported in previous studies, i.e. more hearing impairment in males than females, as well as an association between hearing impairment and advanced age.

This study showed that the prevalence of hearing impairment in the communities surveyed is much higher than current estimates obtained through national census. Main factors associated with hearing impairment were also identified. Therefore, in spite of its limitations, this study represents one of the few population-based surveys of hearing impairment done in South Africa. The findings of this study could therefore be useful in planning for prevention and interventions services for hearing impairment, especially in the Cape Town Metropolitan area. It is also hoped that the findings of this study will stimulate interest among the South African research community to conduct similar research studies in other parts of the country (especially in rural areas) to obtain a more complete picture of the magnitude of hearing impairment on a national scale. This will yield information that could be used to lobby for resources that are required for prevention and intervention services for hearing impairment.