Background
In 2012, the World Health Organization (WHO) estimated that 360 million people, i.e. 5.3% of the world’s population, were living with disabling hearing loss (HL), while around 15% of the world’s adult population had some degree of HL [
1]. Furthermore, sensory diseases have been estimated to be the world’s second most common group of chronic disability when measured by years lived with disability [
2]. HL increases with age, mostly because of age-related HL, generally referred to as presbyacusis. This term represents the sum of the environmental, sensory, metabolic and neural causes that to various extents are suggested to contribute to age-related physiological hearing loss [
3,
4]. Presbyacusis cause reduced speech understanding in noisy environments, declined processing of acoustic information and impaired localization of sound sources [
4]. Hearing loss is present in nearly two thirds of adults aged 70 years and older in the U.S. population [
5]. Even though most people with HL suffer from presbyacusis, other factors such as other ear diseases [
6], occupational noise exposure [
7] and specific genetic diseases [
8] may cause HL. Thus, HL may affect people at all ages and stages in life [
9].
HL is often characterized by at which sound pressure level pure tones can be detected employing standard audiometric tests [
3]. Presbyacusis typically causes a symmetric bilateral high frequency hearing loss. As human speech is related to relatively high frequencies, even a limited hearing loss at high frequencies may cause impaired speech intelligibility [
10]. HL is often not curable, but hearing aids (HA) and other individual sound amplification devices (ISADs) may improve hearing function [
11].
Patient reported outcome measures (PROMs), such as Quality of life (QoL) questionnaires, should ideally be systematically implemented in health care practices [
12] as there seems to be a need for a more “holistic” approach within a modern view of health care. This calls for the inclusion of both disease-specific and generic QoL outcome measures [
13]. QoL measures constitute important outcome- and state measures [
14,
15], as well as an area of focus for research in its own right [
14,
15]. However, there is no universally accepted definition for the concept of QoL [
16,
17]. Even so, we all have a notion about what QoL is, and most people seem to have an intuitive understanding of their own QoL by referring to their own perception [
16]. Thus, the concept QoL will hold different contents among different people [
16].
WHO defines QoL as “An individual’s perception of their position on life in the context of the culture and value systems in which they live and in relation to their goals, expectations, standards and concerns.” This is a broad-ranging concept related to a person’s physical health, psychological state, level of independence, social relationships, personal beliefs and their relationship to salient features of their own environment. The WHO QoL definition is closely related to the WHO’s definition of health from 1948, which describes health as “physical, mental and social well-being, and not merely the absence of disease or infirmity” [
16]. This is also a wide definition, in which in addition to a physical dimension, the WHO also includes well- being, environmental and psychological factors as part of health. Hence, both generic and disease-specific QoL become relevant as to disease and health [
18].
Many different questionnaires have been developed with the intent of directly measuring the functional consequences of a disease; these may be termed “disease-specific” QoL questionnaires. Thus, QoL instruments intended to study the specific consequences of HL may be considered examples of such instruments [
19]. The effect of HL on hearing function can usually be measured by hearing-specific questionnaires [
20]
, but to what extent HL affects generic QoL is not well agreed upon and constitutes the main aim of this study.
The most commonly used generic QoL questionnaire is the SF- 36, with more than 13,000 “hits” on Pubmed as of 2016. The SF-36 measures functional status and wellbeing [
21]. This questionnaire was first used in a provisional edition in 1988 and in a standard form in 1990 [
22]. Shortened questionnaires have been developed from this original, i.e. the 12-item questionnaire SF-12 [
23]. Another commonly used generic questionnaire is the Euro-QoL instrument (EQ-5D). This is a standardized questionnaire intended to measure generic QoL [
24], and it may be utilized within a wide range of health conditions. The EQ-5D describes five dimensions: mobility, self-care, usual activities, pain/discomfort and anxiety/depression. An index value is calculated for each individual, ranging from 1, which indicates no problems in all five dimensions, to 15, which indicate severe problems in all five dimensions. Other generic questionnaires that may be used are the Health Utility Index (HUI) and the Sickness Impact Profile (SIP) [
25,
26]. General parts of disease-related questionnaires, such as the European Organization for the Research and Treatment of Cancer (EORTC) Quality of Life Questionnaire (QLQ) may also be considered generic QoL instruments [
27]. Disease specific questionnaires may also include some questions about generic QoL. However, generic QoL instruments measure many aspects of QoL, and are often intended for use over a wide range of diseases. Such questionnaires are often also applicable to healthy people. Thus, generic QoL questionnaires allow comparing QoL between patient groups, as well as to data from general populations [
16,
28]. The specific main aim of the present study is to review the existing literature on generic QoL obtained by generic instruments among hearing-impaired patients.
In order to assess generic QoL within a disease context, important modulating factors known to contribute to QoL may be assessed alongside the QoL measure. This may include psychosocial factors [
29], personality [
30,
31] and factors related to activities of daily living [
32]. To study potential modulating conditions in the relationship between HL and QoL has therefore been a supporting aim when reviewing the literature in the present study.
QoL as a construct seems to be closely associated with distress, anxiety, and mood, when measured primarily in generic, but also to some extent in disease-specific QoL questionnaires [
20,
33‐
35]. Hence, it should be of interest to study the impact of HL on distress, mood and depression. Anxiety and depression can be defined using standardized classification manuals such as the ICD-10 [
36] or DSM-5 [
37], while distress seems to have no such clear and universal definition. However, one may understand psychological distress as a unique discomforting, emotional state experienced by an individual that results in harm to the person, either temporarily or permanently [
38]. In psychological research, distress is often quantified as the sum of anxiety and lowered mood [
39]. Distress may also be utilized as an indicator of mental disease [
39]. Thus, as QoL, distress, mood and anxiety are closely related concepts [
40], we have conducted a search for the major publications on associations between HL and distress, anxiety and mood in order to present a more complete picture of the associations between HL and generic QoL.
Aim of this paper
So far, no empirically based consensus about if, and in case to what extent, HL patients have reduced generic QoL has been reached. The main aim of this study was to review studies on the relationship between HL and generic QoL published in the period 2000 to present day. As a supporting aim we have also determined noted psychological explaining factors reported in the above-identified publications. As an additional investigational tool, we have reviewed papers from the same period that study HL and distress, anxiety and mood. This was done because level of distress, anxiety and mood seems closely associated to generic QoL.
Discussion
The literature included in this review yield equivocal findings regarding the association between generic QoL and HL. Some authors argue that there are strong associations [
44,
54], while others find less strong [
47,
50‐
53,
56] or no relationships at all [
45,
46,
48,
49,
55]. All the included studies on associations between distress and HL give firmly support to such a conclusion, in particular concerning depression among younger individuals [
58,
59,
61].
One of the two studies with the highest number of subjects, supported an association between generic QoL and HL and focused on older adults [
44]. These subjects showed more severe HL the older they were. The association between increased age and severity of the HL in this study makes it difficult to conclude whether the age or the HL caused the change in generic QoL. Furthermore, when studying older adults by the use of self- reported questionnaires like a QoL questionnaire, it is important to ensure that the informants have the cognitive capacity needed to understand and complete the questionnaire. We have found no report concerning this matter in any of the published studies included in this survey. This should be a matter of future improvement of the investigational design.
Age is an example of a demographic variable that may influence generic QoL [
32]. Therefore, such variables should be reported, and analyses carried out in order to estimate the relative importance of these variables. Furthermore, one should preferably adjust the QoL scores by these variables as additional analyses. This has to some extent been reported within the included papers, but no exhaustive study on this matter has been presented. Most of the included studies, however, do not lend any substantial support to the claim that demographic variables are of high importance concerning generic QoL and HL.
HL may be unilateral or bilateral. Standard procedure would be to report hearing levels from the least affected ear [
64]. Nevertheless, to differentiate between the two conditions should be of importance and this was done in one investigation [
50]. It should be of interest to study subjects with unilateral HL more extensively in order to acquire knowledge of any impaired QoL in this group.
Many of the studies yielding the highest CCAT-scores employed SF-36 as QoL measure, which only to some extent represents a generic HRQoL instrument. The SF-36 does not cover the full range of QoL. General symptoms are not covered [
49]. More specifically health related QoL generic questionnaires could additionally be utilized in order to study whether HL affects a broader array of symptoms in persons with HL [
44,
49,
53].
The associations between HL and distress, anxiety and depression are better documented than the general relationship between QoL and HL. Many factors may explain this relationship. HL may be the causative factor secondary to the social isolation caused by HL. Furthermore present comorbidity may explain both. This needs to be studied further. Distressed persons are expected to have lowered generic QoL [
40]. Therefore, solely based on this association, generic QoL is suggested to be lowered among HL patients.
Regarding justifying HL treatment, improvements in both generic and disease-specific QoLs are important outcome measures, both clinically and for researchers [
20]. To what extent individuals with untreated HL have lower generic QoL [
49] is therefore interesting to study. A low generic QoL baseline subsequently improved after treatment constitutes an excellent HA treatment argument. A low baseline QoL among HL patients would also lend support to offering a larger range of treatments to this group beyond fitting a hearing aid [
65]. The studies where generic QoL were measured following HA fitting after 3 months [
46,
48,
53] or 6 months [
45,
56] show equivocal findings. Some of these studies suggest increased generic QoL caused by the use of a HA, while other studies explain HA use as one of several possible factors that leads to better generic QoL. In conclusion, future generic QoL studies should be encouraged since a firm conclusion about HL and generic QoL has not yet been reached.
Despite the fact that HL may cause poorer generic QoL, and that using a HA may improve generic QoL, some studies suggest that many who are fitted with HAs, used their HA only to a limited degree [
66]. This may be caused by the patients not receiving sufficient help and follow-up to master the HA [
67]. Other studies on treatment show that HAs are an important contributor to increased QoL in HL patients [
65]. Some studies suggest that using HAs over time seems to reverse the adverse effects of HL on QoL [
62]. The process of HA fitting may also carry a placebo- effect. If so, this could also indicate that, as previously suggested [
33,
68‐
70] concerning other diseases, generic QoL to a large extent mainly originates from the personality and thus stays more or less stable, regardless of the severity of HL.
We suggest a need for including both PROMs and physical measures in all hearing assessments [
50]. Many modern HAs have the capability to log the actual use of the HAs in addition to the patient’s self-reported use. By collecting both physical and QoL data repeatedly, more robust data would be available to evaluate the strength of the relationship between the actual use of HAs and eventual improvements in QoL. By including control groups within research, one could in addition obtain more conclusive answers as to whether an improved QoL following HA fitting may be considered a Hawthorne effect [
71], i.e. if the QoL improvement during HA fitting is due to the attention in this period.
For researchers, it also seems reasonable to measure additional potentially explaining variables, at several time points, when trying to determine what affects the QoL in persons with HL. Such screening would provide the opportunity to unravel why and to what extent patients with HL has lowered QoL, or even psychiatric disease. This could provide important clues on how to better help these patients. Systematic studies of HL treatment, with this perspective included, could likely provide evidence on how to better the health care services for patients with HL.
Data were collected using a literature review design with the aim to identify relevant literature published from the timespan 2000–2016 concerning patients with HL and the evaluation of their generic QoL. When using a limited time span there will always be a risk of missing important publications. This represents a possible weakness in our study that could have been overcome by extending the timespan to include previous years. Furthermore, we did not systematically search the reference list of the included papers for additional papers. This may have provided additional relevant papers and this represents a weakness in our design. Also, differences in sample sizes, age of subjects, hearing loss configurations and methodological presentations between studies complicated the comparison of results between studies.