The impact of hearing loss is generally underestimated. The Global Burden of Disease study (1) reveals that hearing loss ranks as the third highest cause of disability globally in terms of years of life with disability, making it the foremost sensory impairment. Moreover, in the United States, hearing loss is the third most prevalent chronic physical condition, surpassing the occurrence of diabetes and cancer (2). Notably, it predominantly affects people aged 55 and older (1). In the United States, age-related lifetime hearing loss is estimated to impose an economic burden of approximately $297,000 per affected individual on society (3). Moreover, hearing loss is associated with cognitive decline (4–15).

Aging (16–18) is also associated with cognitive decline and increasing rates of dementia (19). and sensorineural hearing loss, especially if left untreated, appears to further accelerate cognitive decline. Significant associations were found between hearing thresholds (dB PTA) and cognitive performance [Retrospective: (6, 7, 9–14)]; Prospective: (5, 15, 20); Systematic review: (7, 8).¹ Starting with a subclinical hearing loss < 25 dB, audiometric hearing loss (pure tone average) was associated with cognitive decline [Subclinical hearing loss (10), mild hearing loss (11, 12), moderate and severe hearing loss (5, 21)]. Further findings suggest that with increasing age, there is a progressively greater cognitive decline from older adults with hearing loss (14, 21). However, Croll et al. did not find that hearing loss is accelerating cognitive decline in non demented older adults (15).

Recently published studies have shed light on the significance of both peripheral hearing and central auditory components for cognitive performance. Among older adults, individuals with speech recognition scores in noise below the median exhibited poorer cognitive performance and experienced accelerated cognitive decline over an average follow-up period of 3.5 years compared to those with scores above the median (20). Furthermore, speech recognition in noise in healthy, mostly elderly Chinese was associated with both cognitive performance and parahippocampal cortex (medial temporal lobe) thickness. The parahippocampal cortex was identified as a partial mediator between hearing in noise and cognition, suggesting its potential role between the two (22).

Several articles discuss possible mechanism linking adult hearing loss in older adults and cognitive decline (3, 23–30). Causal mechanisms as well as shared (common) mechanisms have been proposed. An example of a hypothesis addressing common mechanisms (“common cause”) is that age-related functional and structural changes in the brain (spontaneous neural activity and functional brain connectivity, changes in brain areas)² account for changes in auditory processes and cognitive processes in old age (24, 29). An example of a causal hypothesis is that hearing loss is the cause of social isolation that mediates cognitive decline [e.g., (24, 29, 34)]. Rutherford et al., Uchida et al., Slade et al., and Sharma et al. discussed the possible contribution of depression as a consequence of hearing loss to cognitive decline in older adults (25, 27, 28, 30).³

To date, there has been a lack of research investigating the potential mediating role of depression between hearing loss and cognitive performance.

In addition to hearing loss, depression itself appears to independently contribute to cognitive decline. Multiple studies have consistently demonstrated a link between depressive problems and cognitive decline (35–41). Moreover, not only the severity but also the age of onset of depressive symptoms has been found to have a negative impact on cognitive performance (35, 39, 40, 42). Notably, individuals with bipolar disorder tend to experience more cognitive difficulties than those with major depression (43). Executive functions such as attention, working memory, and cognitive flexibility are affected, as well as episodic memory and cognitive processing speed. Depression is also associated with changes in the brain including the limbic system (hippocampus, amygdala, cingulate gyrus, hypothalamus, insula), the prefrontal cortex, and the cerebellum (44–47).

Numerous recent retrospective (6, 48–57) and prospective (58) studies indicate a connection between reduced hearing and depression. Bilateral audiometric hearing loss was found to be associated with depression [Golub et al. ≥ 25 dB HL better ear, Golub et al. beginning with subclinical HL of >15 dB, Shukla et al. ≥ 25 dB HL, better ear; Chern et al. ≥ 25 dB HL, left ear and right ear (53–56)]. Additionally, it seems to increase the risk for depression, also after multivariate comparison (58). However, an older study found no association between audiometric hearing loss and depression (59).

It is important to note that the assessment of depression in various studies may vary, leading to potential confusion. Most authors (6, 48, 51, 53–57) employed validated depression questionnaires, such as the Epidemiological Studies Depression Scale-10 (60). Brewster et al. and Tsimpida et al. utilized a continuum of depressive symptoms, covering both clinical and subclinical manifestations (51, 58). On the other hand, Li et al., Golub et al., Brewster et al., Chern et al., and Shukla focused solely on clinical symptoms (48, 52–56), while Tseng et al. and Kim relied on psychiatric diagnoses of depressive disorders (49, 58).

In addition, several studies inform about associations between hearing loss, depression, and brain changes in middle-aged and older adults. Tang et al. reported that middle-aged adults with acquired sensorineural hearing loss had higher levels of depression and anxiety and lower functional connectivity between the amygdala and auditory cortex, striatum, multimodal processing areas, and fronto-parietal control areas compared with normal-hearing peers (61). Aoki et al. (62) found that moderate/severe hearing loss was associated with higher depression scores in older adults, hippocampal atrophy and a higher cortisol/dehydroepiandrosterone sulfate ratio, which indicates higher stress levels (62). Brewster et al. examined older people with untreated hearing loss and major depression according to DSM 5 (Diagnostic and Statistical Manual) (63, 64). Significant associations were found between lower speech recognition and cortical thinning in the primary and secondary auditory cortex and lower integrity of the superior longitudinal fasciculus.

There is also evidence that hearing and depression are related to each other via the neurotransmitter serotonin. Impaired serotonergic function in the auditory cortex has been found in normal-hearing adults with major depression (65). Studies in rodents show that hearing damage is associated with changes in the serotonergic system within the auditory system, but also in the hypothalamus, striatum, and frontal cortex” (66–68).

Our clinical experience and research by GBD 19 Hearing Loss Collaborators (1) confirm that hearing loss is associated with depression. Many adults with hearing loss struggle to follow conversations, especially in noisy environments or with multiple people speaking, leading to feelings of depression.

To date, only a limited number of studies have explored the potential link between depression resulting from hearing loss and cognitive decline in adults with hearing loss. One study (69) found that the cognitive decline in older adults with hearing loss, compared to normal hearing, was partially associated with the severity of depressive symptoms. Another study (70) observed that older adults with hearing loss and clinical depressive symptoms experienced greater cognitive decline over a 10-year period compared to those without depression. However, the question of whether there is a direct relationship between depression as a result of hearing loss and cognitive performance, as hypothesized in the models of Rutherford et al., Uchida et al., Sharma et al. (see above), has not yet been investigated (25, 27, 30).

Taken together, the studies published to date support the hypotheses that there are associations between adult hearing loss (peripheral and central hearing) and depression and cognitive performance, and that depression mediates between hearing loss and cognitive performance. However, some limitations of these studies should also be considered. Brewster et al. (51) and Tsimpida et al. (57) used exclusively self-reports single questions to assess hearing loss, for example “Can you hear well enough to carry on a conversation in a crowded room?” (51). Hearing loss is usually defined as a four-frequency PTA of >25 dB HL on the better ear⁴ [see the website of US National Institute of Deafness and other Communication Disorders (71)]. Study results indicate that the exclusive use of self-report single questions is not sufficient to detect such a defined hearing loss (72–75). None of the studies, with the exception of Tseng et al. distinguished between primary and secondary depression, i.e., depression as a presumed consequence of hearing loss (58). In addition, several studies used verbal tasks, which may have confounded the results⁵ (10, 12, 13, 15, 20, 70). In addition, only some of the studies excluded individuals with suspected cognitive impairment or dementia (9, 12, 13, 15, 69, 76). Very few authors (13, 15, 58, 69) provide information on the time of onset of hearing loss (e.g., congenital hearing loss, hearing loss in adulthood). Finally, it is important to remember that hearing loss can affect not only older adults but also younger adults (1). To date, however, the majority of studies have been conducted in older adults, with an average age greater than 65 years [studies conducted with younger adults with a mean age less than 65 years include (5, 9, 48, 49, 58)]. It is not known, whether cognitive decline also occurs in younger adults with hearing loss and whether hearing loss is associated with depression.

The primary objective of our study was to unravel the interrelationships between hearing ability, depressive status (including subclinical and clinical symptoms as potential consequences of hearing loss), cognitive status, and age groups (younger and older adults) in individuals with bilateral sensory onset hearing loss starting in adulthood. Specifically, we investigated whether the severity of depressive symptoms acts as a mediator of cognitive performance. Additionally, we examined potential variations between the younger and older groups regarding secondary depression, cognitive status, and the mediating role of depressive symptoms.

The present study was designed to assess depressive symptoms as a potential mediator of cognitive impairing effects of hearing loss. In otherwise healthy adults with bilateral hearing loss, all of whom were candidates for cochlear implantation, we found no evidence of a mediating quality of secondary depressive symptoms between hearing loss and cognitive performance. We found a significant association between subjective hearing ability in noise (APHAB) and severity of depressive symptoms, but no significant associations with cognitive performance. Hearing threshold (PTA) was not associated with depression or cognitive performance (Figure 4). There was no association between depressive symptom severity and any cognitive performance. Unexpectedly, there were also no significant differences between the younger and older groups in either depressive symptom severity or cognitive performance.

To our knowledge, the present study was the first to examine the mediator quality of depression in both older people and younger people with hearing loss.

As expected, subjective APHAB hearing in noise was significantly related to the severity of secondary depressive symptoms, result of BDI II.

Speech understanding performance in noise depends on complex central processing and cognitive functions (105–110). Studies on adults without hearing loss suggest that depression may be associated with hearing in noise. For example, Xie et al. found that speech recognition in noise (talker masker) was associated with more errors and higher speech recognition accuracy in young normal hearing adults with major depressive disorder (111). The authors hypothesized that distractibility would be increased in individuals with depression due to speech interference in background noise. These findings were supported by behavioral neuroimaging studies that used both auditory and visual stimuli for distraction (112–115). In all studies, the authors found a higher distractibility in individuals with depressive disorders. Other studies addressed possible associations between speech recognition in noise—audiological test scores—and depressive symptom severity. Carvalho et al. found significant associations between speech perception performance in noise and depressive status in older hearing aid users (116). Heinze-Köhler et al. identified significant associations between short- and long-term speech perception and depressive status in middle-aged and older patients after cochlear implantation (117). Please note that all these studies, including the present study, are correlational studies. Therefore, the question of whether hearing disorders are the cause of depression or whether depression is the cause of hearing disorders cannot be answered.

Unexpectedly, peripheral hearing ability (PTA, the better ear) was not associated with the severity of depressive symptoms. Our results are consistent with those of Gobinath et al. (59) but not consistent with Li et al., Brewster et al., Golub et al., Golub et al., Shukla et al., Chern et al., and Tsimpida et al. (10, 48, 51–57, 118). It should be noted, however, that the nonconsistency refers to the significance of the results and not to the effect sizes. We obtained a small effect size, and this was also the case in these previous studies, with odd ratios ranging from 1.26 (52) to 1.64 (59). These previous studies, with the exception of Gobinath et al. used very large samples (ranging in size from n = 1,204 to 8,529) in which even small effect sizes became significant. With such a large sample, an │r│ of 0.08 as in the present study would most likely have become significant as well.

Furthermore, contrary to our expectations, we did not observe any significant correlation between four frequency hearing thresholds and cognitive performance. Instead, beyond the planned analyses, significant correlations were found between subjective hearing at quiet (APHAB “Ease of communication”) and cognitive performance (Figure 4, for details on the specific cognitive functions, see the discussion of our exploratory analyses below). We attribute this disparity to the fact that the APHAB “Ease of communication” captures different hearing aspects than an audiometric procedure. Hearing thresholds are determined through controlled tests in laboratory-like conditions, evaluating pure tone hearing and hearing with hearing aids without hearing aids (evaluating air conduction, bone conduction, loudness and frequency, best hearing ear)⁷ whereas subjective hearing in everyday situations is determined by a questionnaire. On the other hand, the APHAB subscale “Ease of communication” assesses hearing abilities in real-life verbal communication situations through a questionnaire-based approach, offering a more comprehensive representation of hearing in everyday settings. Hearing in verbal communication situations is assumed to be more cognitively demanding than hearing in audiometric tests of hearing thresholds, we believe it is not only reasonable but also clinically relevant to incorporate hearing variables that closely relate to verbal communication in daily life when investigating potential links between hearing loss and cognitive decline.

Another potentially interesting auditory variable in this context is aversion or misperception when listening to certain environmental sounds (APHAB AV subscale) (82). Beyond the planned analyses, we found significant associations between the APHAB AV and cognitive performance, which is a novel finding (Figure 4). AV differs from the EC and RV subscales in that it is not correlated with audiometric data [hearing thresholds, speech recognition (85–88)]. It also differs from EC, BN, and RV in that it does not correlate with the SSQ [Speech, Spatial, and Qualities of Hearing Scale (84)]. Possible relationships with other hearing variables for example hyperacusis⁸ and noise sensitivity⁹ has not yet been investigated.

We found no significant association between subclinical and clinical severity of depressive symptoms and cognitive performance. The results also do not support the models of Rutherford et al., Uchida et al., and Sharma et al., which hypothesize a direct relationship between depression and cognition of individuals with a hearing loss, see introduction (25, 27, 30). In addition, our findings on secondary depression in people with hearing loss are not consistent with the findings on primary depression in people without hearing loss, see introduction. Depression can be described as a result of genetic predisposition and environmental factors [e.g., (124–126)]. We hypothesize that individuals with secondary depression caused by the critical life event of hearing loss represent a specific group that is distinct from other groups with depression.

There is a well-established consensus that both hearing loss and cognitive decline tend to increase with age. Notably, hearing loss accelerates cognitive decline in older adults, a phenomenon that also has been observed in individuals with normal hearing (14, 21). Given this relationship, one might anticipate that a younger group with hearing loss would outperform an older group with hearing loss on cognitive tests, even after accounting for age differences. However, our study’s findings did not support this hypothesis. To investigate this, we employed age norm scores, such as T scores and percentile ranks, and surprisingly, no significant difference was observed between the younger and older groups with hearing loss. These results challenge the assumption that cognitive decline solely affects older individuals with hearing loss. Instead, our findings lend support to the notion that cognitive decline can also manifest in younger individuals who experience hearing loss. Support for the validity of this hypothesis can be found in a study conducted by Kocabay et al. who compared a small group of younger adults with high-frequency hearing loss (mean age 39 years) with normal-hearing individuals (127). Moreover, it is usually assumed that older individuals are considered to be at increased risk for depression (see the website of US-National Institute on Aging). Unexpectedly, we found no significant differences between young and older groups in depressive status. Our results justify the hypothesis that depression scores are not only elevated in older people with hearing loss (69) but also in younger people.

In addition, there seems to be a widespread assumption that the longer the hearing loss, the more severe the depressive status. In our exploratory analyses, we found no significant association between the years of hearing loss and the severity of depressive symptoms. To our knowledge, there are no other studies on this topic yet. That depressive moods can also occur at any age in people with hearing loss, the use of depression questionnaires such as the BDI are recommended in clinical work.

In summary, our hypothesized model that depression mediates between hearing loss and cognition (see Figure 5A) was not supported by our results, neither in the overall group nor in the older or younger groups.

Our results also seem to contrast with a recent study, finding that “depressive symptoms partially mediated the association between hearing loss and cognitive function [standardized regression B coefficient (β) = −0.114; 95% confidence interval (CI): (−0.158, −0.076)] (128). Despite this apparent inconsistency, we believe that there is basically no contradiction between the two studies. The sample of Cao’s studies was large enough (n = 8,094) to allow even small effects to become significant.¹⁰ Our studies would have needed a sample of (n = 120) to achieve an effect size of small to medium size and an even larger sample for small effect sizes.¹¹ Therefore, the small effects found in Cao et al. (128) could not be detected in our study. Independent from this we have concerns about the validity of the measurement tools used in Cao et al. (128). The presence of hearing loss was only assessed using a single question to study participants (“Do you feel that you have difficulty hearing?”). As mentioned in the introduction, a self-report single question is insufficiently valid for assessing clinical hearing loss (72–75). Furthermore, single question data do not inform about the better and the worse ear or about unilateral deafness. No differentiation can be made between the difference between hearing in quiet and hearing in noise. Moreover, a single test, namely the MMSE (Mini Mental States Examination, Folstein et al.), was used to assess cognitive status (129). Because a substantial part of this test consists of verbal tasks, it is not a sufficiently valid instrument for assessing cognitive performance in individuals with hearing loss (see Introduction).

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by Hannover (protocol number: 8419_BO_S_2019) and Salzburg (protocol number: 415-E/22489/2-2019). The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

MH: Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. LR: Investigation, Project administration, Writing – original draft, Writing – review & editing. LW: Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing. BP: Conceptualization, Formal Analysis, Methodology, Software, Validation, Writing – review & editing. SR: Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing. AI: Funding acquisition, Supervision, Visualization, Writing – original draft, Writing – review & editing.