Introduction

The term health literacy has come to represent a variety of meanings as research in related fields intensified [1]. “The cognitive and social skills that determine the individuals’ motivation and ability to gain access to, understand, and use information in ways that promote and maintain good health” is referred to as health literacy in the second edition of the WHO Health Promotion Glossary [2] published in 1998. In 1999, the American Medical Association’s Ad Hoc Committee on Health Literacy [3], defined health literacy as a “constellation of skills, including the ability to perform basic reading and numerical tasks required to function in the health care environment,” including the “ability to read and comprehend prescription bottles, appointment slips, and other essential health-related materials.” The Institute of Medicine (IOM) [4] decided to use the definition adopted by Healthy People 2010 [5]thus: “the degree to which individuals have the capacity to obtain, process, and understand basic health information and services needed to make appropriate health decisions” [6]. This concept includes many scenarios in which individuals may encounter and interact with health concerns; nonetheless, all the above-mentioned definitions attempt to characterize health literacy as a problem of individual ability and competence [4].

Health literacy is influenced by many factors, including social and personal factors [7], while indirectly impacting health outcomes by changing the status of the relationship between health care providers and patients. Hearing is a factor that influences the level of health literacy, which is important for accessing, understanding, judging, and using health information appropriately. Therefore, it is necessary to objectively explore the association between health literacy and hearing impairment to achieve better health outcomes among people with hearing impairment. Previous reviews looked into the capacity of cancer education to promote health literacy in patients with hearing difficulties [8, 9], but the focus was confined to cancer-related health literacy. In our study, health literacy was not limited to cancer-related but also to other health domains, and we summarized the methods that can improve health literacy except education. In this study, we conducted a comprehensive literature review on the health literacy of people with hearing impairment to understand the level and status of health literacy in persons with hearing loss, outline the challenges and concerns of individuals with hearing disabilities, and propose strategies to improve health information transmission.

Methods

Search strategy and data source

We conducted the systematic review according to the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA). We extracted, analyzed, and integrated research papers found suitable as per the selection criteria. After searching three global search engines, we found there was no literature about both health literacy and hearing disability before 2000, the data search and analysis on research papers published in international journals were conducted from January 2000 to December 2021. We used three global search engines—including PubMed, Cochrane Library, and Embase Emtree—and MeSH (Medical Subject Headings) terms. The applicability of the subject terms for this search is only guaranteed to be valid until July 5, 2022—Additional file 1 describes the specific literature search methods in the three databases.

Inclusion and exclusion criteria

Of the retrieved papers, duplicates were excluded using Endnote 20.3 (Bld 16073). We selected the corresponding literature using key questions prepared in the form of Participants, Interventions, Comparisons, Outcomes, Timing, Settings, Study Design (PICOTS-SD)—that added timing, settings, and study design to the PICO (Participants, Intervention, Comparison, Outcome) framework [10]. The key questions used are listed in Additional file 2. Two authors independently screened papers for relevance to this study by applying the literature selection and exclusion criteria described in Table 1. We selected studies that were relevant both to people with hearing impairment and health literacy and published between 2000 and 2021. Meanwhile, we had five exclusion criteria, including requirements for the topic of the study, population, year of publication, language of publication, and originality. We excluded papers using title and abstract and added manual searches using single specific terms, such as "hearing disability," to find relevant articles.

Table 1 Inclusion and exclusion criteria
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Data extraction

As shown in the Figure, we initially gathered 163 publications after deleting duplicates. After further exclusion by abstract, title, and full text, 17 articles remained, and with the addition of 12 articles from the manual search, 29 articles were finally selected for the study (Fig. 1).

Fig. 1
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PRISMA flow diagram

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Assessment of the risk of bias for the studies

We used the Risk of Bias Assessment Tool for Nonrandomized Studies (RoBANS) to evaluate the quality of the selected studies. RoBANS contains six domains, including selection of participants, confounding variables, measurement of exposure, blinding of the outcome assessments, incomplete outcome data, and selective outcome reporting. For each domain, the bias risk was classified as “low,” “high,” or “unclear” [11]. According to Kim et al. [11], the total bias is evaluated by three relatively more important domains (selection of participants, confounding variables, and incomplete outcome data) out of the six, and if more than two of the three have the same bias—such as “low”—then the total bias is “low.” If the three domains have different biases, then the total bias is “unclear.” Assessment of bias was evaluated by two authors individually, but in case of disagreement, we invited another author to co [12, 13].

Analysis method

Due to the diversity of the literature in terms of populations, settings, interventions, and outcomes, it was not possible to consolidate the results quantitatively. Alternatively, we collated the main components of the literature and analyzed the key themes between the selected papers, thus establishing commonalities and differences. The papers were collated in terms of countries, characteristics of subjects, study aims, methodology, main findings, and tools for measure health literacy. The results on health literacy status were separately collated for the characteristics of health literacy problems, difference in health literacy levels among people with hearing disabilities, barriers against health literacy, and the ways of improving health literacy.

The final collected papers were categorized into quantitative and qualitative research based on the research method used. Taking the unknown into consideration, the research was classified as quantitative if objective generalized results could be obtained using health literacy tests. If observation and interview methods were conducted to investigate the phenomenon, the research was classified as qualitative.

This study collated the results of the research based on the following. First, the characteristics of the included studies were examined. Second, what is the level of health literacy for people with hearing impairment compared to those without such impairment? Third, what are the barriers to health literacy for people with hearing impairment? Fourth, how does the level of health literacy for the hearing-impaired lead to health management? Fifth, what are the ways to improve the health literacy of people with hearing impairment?

Results

Characteristics of included studies and results of bias assessment

In general, most of our selected papers had low bias and were of high quality. As shown in Additional file 3, the risk of bias was rated as “low” for 25 of the 29 papers, “high” for 2 papers, and “uncertain” for 2 papers. Twenty of the 26 papers were from the United States, one involved both Kuwait and Saudi Arabia, one was from both South Africa and China, and the other seven from Canada, South Africa, Germany, Turkey, Nigeria, Australia, and Swaziland. The sample size varied from 7 to 19,223 people, with 24 studies having fewer than 1,000 and 4 papers having more than 10,000. There were 22 quantitative studies and 7 qualitative studies. Four of the studies involved teenagers, while the others were conducted on adults. Four of the studies included just female hearing-impaired participants, while none involved only male hearing-impaired participants. Twenty-two studies were conducted by survey, three by interview, and four by focus groups (Table 2).

Table 2 Characteristics of study included
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Tools for measuring health literacy

The most used assessment tools in the articles we searched were the Rapid Estimate of Adult Literacy in Medicine (REALM), Test of Functional Health Literacy in Adults (TOFHLA), Newest Vital Sign (NVS), Health Literacy Skills Instrument (HLSI), European Health Literacy Survey (HLS-EU-Q47), and Brief Health Literacy Screen (BHLS). Some studies also used their own developed questionnaires. Table 3 shows some of the sources of health literacy measurement tools. All the tools used for measuring the health literacy of people with hearing impairment classify the level of health literacy into two to four degrees, and all have been tested for validity.

Table 3 Tools for measuring health literacy
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As follows, we have summarized the findings of each article according to those listed in Table 4. Although each article does not show the same content due to differences in the purpose of the study, we can still summarize based on what is available.

Table 4 Current status of health literacy among people with hearing impairment and improvement methods
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Health literacy levels of people with hearing impairment

The prevalence of inadequate health literacy among people with hearing impairment is high. Mckee et al. [27] conducted a study with 166 participants with hearing loss and 239 hearing participants without, and showed that 48% of participants with hearing loss have inadequate health literacy and were 6.9 times more likely than hearing people to have poor health literacy. Pollard et al. [19] investigated health-related vocabulary knowledge in a sample of adults through a modified REALM and found that 32% of people with hearing loss received scores comparable to low health literacy scores; all of their study participants were people with hearing disabilities and 80.8% obtained a college degree, implying that even people with hearing disabilities with high or average levels of education may be at risk of lower health literacy. Tolisano et al. [38] found ratings of participants’ hearing were categorized into four grades, and patients with poorer hearing grades C and D had lower health literacy scores measured with the BHLS than those with grades A and B (BHLS score in the range of 11.6–13.6). Wells et al. [39] divided 19,223 older adults into five groups based on their self-reported hearing disability level, including those with severe unaided hearing loss, severe with hearing aid use, mild with assistance, mild without assistance, and no hearing loss. Of these, the unaided mild, aided severe and unaided severe hearing loss groups showed lower health literacy than the other groups, although this connection diminished with the use of hearing aids.

The barriers to health literacy for people with hearing impairment

Among the people with hearing impairment, there is a lack of knowledge and misconceptions about diseases, such as cervical cancer [18, 23], ovarian cancer [25], breast cancer [24], HIV/AIDS [17], COVID-19 [41], and diabetes [35]. People with hearing impairment also had limited access to health information and encounter difficulties in seeking health information [49]. Several studies were conducted on a hearing-impaired group and a control group to compare their knowledge on cervical, ovarian, and breast cancers after listening to a graphic-rich video in American Sign Language with English subtitles regarding cancer [18, 23, 25]. The studies obtained similar results; women in the control group performed better before watching the video, while following the intervention, both groups’ knowledge on relevant tumors improved significantly [18, 23,24,25]. These findings imply that the health literacy of people with hearing impairment can be improved if there are more effective ways to expose them to health information and provide more access to it.

People with hearing impairment tend to visit their doctors more often than those with good hearing, but they can have more difficulty communicating with professionals [50], which is consistent with the findings of a survey of American Sign Language (ASL) interpreters [51]. In Stevens et al.’s study [36], more than 90% of people with hearing impairment reported problems with poorly communicated information and communication difficulties when their name was called with the presenter’s back being turned, as well as communication over the telephone, which can affect the quality of patient care, satisfaction, and health outcomes. According to Steinberg et al. [15], adults with hearing impairment who used ASL were distrustful, fearful, and frustrated with medical care and believed that if doctors could communicate with them in sign language or with a live interpreter, it would help them establish good communication with the doctor and improve their satisfaction with the medical service. Similar communication challenges were observed among teenagers with hearing disabilities [37], with 55.7% preferring written prescriptions or care procedures and 87.5% preferring sign language communication. A study [28] has shown that when pharmacists lack patience or understanding of the real needs of people with hearing impairment when they seek medication care, the bond between them can be weakened and, in turn, can affect the safe administration of medication to patients. All of these findings illustrate the importance of effective communication between patients and doctors about medication, treatment, and health outcomes.

The impact of health literacy levels on health management among people with hearing impairment

People with both hearing impairment and limited health literacy were more likely to have higher medical costs [39]. In addition, people with hearing loss have very limited awareness of health insurance and have little access to the related information [40]. Willink et al. [40] discovered that Medicare participants who had a little or a lot of hearing impairment were 18% and 25%, respectively, more likely to report having difficulty understanding Medicare than those who did not have hearing problems. Approximately 20% of Medicare enrollees with hearing loss reported that their hearing disability made it hard to find information about Medicare. This also implies that methods to assist people with hearing loss in understanding Medicare should be consistently developed.

Methods for improving health literacy

Online education can help enhance the health knowledge of people with hearing impairment as well as their capacity to seek health information on the internet [14, 18, 23, 25, 33]. Palmer et al. [33] reported that the bilingual modality via both the signer and closed captioning provided better access to cancer genetics information for less-educated ASL users compared to the monolingual modality. This study supports that materials prepared with sign language and extra captioning and graphics improve the sign language user’s understanding and satisfaction. Short Message Service (SMS) can help people with hearing loss become more aware of hypertension and healthy living. However, because of the special demands and preferences of sign language users, SMS services must be investigated further to fulfill the needs of the hearing impaired. Haricharan et al. [32] suggested numerous strategies to improve SMS campaigns for people with hearing disabilities, including the use of images, combination of SMS with signed drama, use of 'signed’ SMS, and linkage of SMS campaigns to interactive communication services.

Additionally, communication issues between people with hearing impairment and health care providers need to be addressed. Most people that are hard-of-hearing in the survey mentioned difficulties in communicating with doctors, the obscurity of some terminology, and even misunderstandings [36]. They hoped to communicate with the doctor in sign language or have a sign language interpreter on-site to help them speak with their doctor, which would increase their satisfaction with health services, strengthen the doctor-patient relationship, and improve their health outcomes [15, 36, 37]. To facilitate communication between patients with hearing impairments and healthcare professionals, it is essential to pre-educate healthcare professionals about people with hearing loss [20]. In Hoang et al.’s study, medical students were divided into two groups based on whether they had undertaken a community education program on hearing impairment. When knowledge of the overall “deaf culture “ in the healthcare setting was assessed, the summary scores varied widely—26.9 for those who had completed the education program, 17.1 for the medical school faculty, and 13.8 for those who had not completed the education program [20]. Overall, the faculty members scored similarly to medical students who did not complete the educational program on issues particularly relevant to interactions with hearing loss patients. This research proved that primary education is crucial in interacting with deaf patients, regardless of clinical expertise [20].

Discussion

This study assessed the existing literature relating to the health literacy of people with hearing impairment. There are some common observations showing that people with hearing impairment have low health literacy and have difficulties communicating with health providers and understanding—and often misunderstanding—when seeking medical services [15]. They also have limited sources of relevant health information, incomplete knowledge of diseases, health insurance, and even misunderstandings of the same [35, 40, 41, 49]. In addition, participants were under-informed about the palliative care services available to them, and health providers were unknown about the deaf culture [22].

Of our selected literature, 25 were evaluated as having a “low” risk of bias because most studies used instruments to measure health literacy or conducted case–control group studies. Two were deemed to be at high risk of bias, owing to some confounding variables and missing data in the research, which influenced the results. The other two articles were judged to be having an “unclear” risk of bias due to the complete inconsistency of the three main domains. This showed that for the people with hearing impairment, high-quality research has been conducted as enough comprehensive factors were considered in the study to minimize possible bias.

Few studies have investigated health information sources for people with hearing impairment. Smith et al. (2015) interviewed hearing loss adolescents who used sign language and identified five main sources of cardiovascular health information, including family, health education teachers, health care providers, print, and informal sources. They suggested that communal funds be used to conduct appropriate health surveys and identify health education programs, enhance interpreter education, and distribute information using social media. All three authors of this literature [29] are also people with hearing disabilities; thus, they researched a group to which they belong and understand, which is one way that future research on people with disabilities could be conducted—by researchers who know enough about particular groups.

Increased health literacy through conversational means is crucial, and because of the influence of independence from their families, college students who lose their hearing often rely on friends to get health information and education [34]. Mutual sign language education with friends, discussion, and participation in these early social discussions can improve overall health knowledge. Recent improvements in online information technology, social networks that enable the gathering of sign language users, and frequent gatherings through online virtual meetings can provide them a space for dialogues, potentially sharing their most recent health information [34]. Another option is to employ SMS functionality, which in 2015 had a nationwide penetration rate of 85.67% in South Korea; this value will reach 97.4% by 2025 [52]. We also anticipate that text messaging will become increasingly crucial as more cellphone apps and technology are developed for individuals with disabilities [53, 54]. Haricharan et al. [32] previously investigated sending regular text messages to boost health information and hypertension awareness among people with impairments. The content contained 20 well-designed hypertension messages, 57 food habits, methods to avoid hypertension through healthy living—such as exercise—and four activities [32]. Although some literature recommend the use of written materials [31, 36, 37], we must recognize that people with hearing impairment who speak sign language as their first language frequently struggle to interpret written texts and lack literacy in this area [15, 55]. This is especially true for people who are more likely to have a low level of education [33]. In Korea, the hearing handicapped commonly employ Korean Sign Language, lip-synthesis, and notes, among other means [55]. According to the 2017 Korean Sign Language Use Survey, 69.3% of people with hearing loss use sign language as their primary form of communication, but only 30.5% fully understand it [56]. Hence, using textual material to spread health messages in Korea may be a good option.

In summary, our literature review indicate several currently feasible ways to improve the health literacy of people with hearing impairments: health literacy education for people with hearing impairments, hearing disability-related education for healthcare professionals to help understand better the needs of people with hearing impairments, and popularization of the use of smartphones (e.g., SMS). In addition, if we can analyze subgroups of people with hearing impairment by conditions such as whether they use hearing aids or not and whether they know sign language or not in subsequent studies, that would help to target people with hearing impairment to improve their health literacy.

There are several limitations to this study. First, it is hard to generalize the selected literature as there were variations by countries in selection criteria for people with hearing impairments, age distribution and number of participants, and methods and tools used. Second, it was not possible to compare the selected foreign studies with the Korean results as there are no national studies on health literacy among people with hearing impairment. Third, although the literature we collected was published over a wide time period, from 2000 to 2021, there are not many studies on health literacy among people with hearing impairment. Each paper defined low health literacy differently and, to our knowledge, there is not a clear standard at present. Paul et al. [57] summarized 43 different health literacy instruments and illustrated that the quality of these instruments varied considerably according to their psychometric properties. Regardless, overall, the literature is of high quality and contains information on the low health literacy of people with hearing impairment, dilemmas faced in the healthcare setting, limited sources of medical information, and some feasible ways to improve it. These existing relevant articles can inform about the current state of health literacy among people with hearing loss and ways of improving the health literacy of people with hearing impairment.

Conclusions

People with hearing impairment have shown a willingness to seek out and learn about health information. However, because of the complex terminology, a lack of easily available health information resources and illustrations by handicap type has been discovered. The recent continuous development of technology for people with disabilities and easy access to smartphones have correspondingly provided more and varied options for people with disabilities to receive health information. This suggests that developing new technology and policies for people with hearing impairment is necessary not to mention promoting provision of information via sign language. For instance, enforcement policies would be required for the existing app technologies in the uploading of health information for various chronic diseases, such that deafness characteristics are taken into consideration and people subscribe only to the information they require.