Our systematic review of epidemiological data on prevalence of ARHL in Europe revealed more information gaps than information that would allow gaining a meaningful picture. Neither geographic distributions nor developments over time could be extracted to a reasonable degree. Most studies reported prevalence rates over age and frequency intervals; others reported data for single ages and frequencies. Because of this heterogeneity of the data, we were unable to establish an integrative quantitative overview of prevalence rates over age and hearing loss. Nevertheless, the studies do reflect the well-known patterns of a nonlinear increase of hearing loss with age affecting men more than women. If the data are crudely averaged and interpolated, roughly 30% of men and 20% of women in Europe have a hearing loss of 30 dB HL or more at age 70 years, and 55% of men and 45% of women at age 80 years.
These data confirm the known fact that ARHL is a major health concern in the aging population of Europe, but because of important information gaps, it is difficult to devise any recommendations. They demonstrate the need for standardized collection of epidemiological data on hearing loss.
Difficulties in data evaluation: classification of HL
An evident and primary difficulty in comparing reported prevalence data are the different measures and cut-offs for hearing impairment [15, 17]. For example, the sample of the study in Denmark, Sweden and Finland consisted of 75-year-old subjects, and results were subdivided in categories such as ‘with minor difficulties’ or ‘with considerable difficulties’ of hearing . Other studies did not consistently report sample sizes , had small sample sizes , or summarized results over all age groups . Many studies reported data with cut-offs of 25 dB and 35 dB HL. One study  proposed an adaptive threshold model that uses different thresholds as a function of frequency , but without applying these thresholds in the reported data.
Prevalence is relatively easy to establish in clearly defined states, such as whether a person is alive or dead, or if a pathohistological diagnosis of cancer exists. Hearing loss is a gradual condition, and the limit between normal and not normal must be defined along a continuous scale. Obviously, the lack of a common definition prevents the availability of comparable prevalence data of HL. Standards such as ISO-7029  define the distribution of normal hearing as function of age, but they do not define the limit of not normal hearing loss. Moreover, ISO-7029 is based on a linear model for ages 18–70 years not including the older ages with their nonlinear increase of hearing loss.
For the sake of acceptable homogeneity and to obtain an overview, we focused on prevalence data for the better ear and a range of cut-offs for the definition of hearing loss between 25 and 45 dB HL in the audiometric frequency range of 0.5–4 kHz. Tables 3 and 4 reveal that such limits cannot be regarded as a standard. In fact, it may be difficult to define any standard when evaluating the literature considered in this review. Even international classification systems such as those of the EU or the WHO differ considerably (Table 2). Because any definition of HL has unavoidably arbitrary elements, the acceptance and use of a standard definition of HL should be easy. It may be that the greater number of multitude of systems in use and the lack of knowledge of systems such as the WHO classification that hinder their regular use. It seems to us that the use of the WHO classification in the collection of epidemiological data is a logical step that could help to improve such data collection in the future. Universal acceptance and propagation of the WHO classification system is recommended.
Difficulties in data evaluation: selection of populations
Similar arbitrary limits occur when defining the “age” for prevalence in ARHL. Again, Tables 3 and 4 reveal that a standard age limit may be even more difficult to extract than the definition of HL. Even gender as a known significant influence on ARHL was not always clear. Only a portion of the studies explicitly reported prevalence rates for men and women, age groups, and hearing impairment categories [19, 21, 26, 28, 32]. Some studies either did not separate age and gender [12, 13, 18, 25, 29, 30, 33], did not calculate row and column prevalence for hearing impairments among other impairments , did not report sub-sample sizes for age groups , had overlapping age class limits , or had inconsistent hearing loss criteria between age groups . Three studies did not report prevalence data even though it seemed possible judging from the description of the data [22–24, 27]. In the study of Smits et al.  we derived approximate prevalence rates from a figure.
While international systems of HL classification exist, we are not aware of recommendations about the age limit defining ARHL. Even though the definition of such a limit may be biologically difficult if not impossible, it is highly desirable or even necessary to do so for epidemiological data. Again, for the sake of homogeneity this review set a lower age limit of 60 years.
Connections between aetiology and epidemiology
Epidemiology and aetiology are intimately related in that population-based endogenous factors will be exposed to geographically varying exogenous factors. One of our initial goals was to search for evidence linking epidemiology and aetiology.
As demonstrated in the previous sections, we have only a crude and patchy picture of the overall prevalence of ARHL in Europe. We know even less about the interaction of the different aetiological factors leading to what is termed ARHL. A separate overview of the literature not reported here in detail revealed that the most relevant factors in the aetiology of ARHL seem to be heredity (that may contribute up to 50%), noise, history of chronic middle ear inflammation, and cardiovascular factors including diabetes, smoking and hypertension; additional relevant factors are hormones (including gender), exposure to ototoxic medication or chemicals and co-morbidities. As an example, ARHL seems to be more prevalent in patients with rheumatologic disease [39, 40].
Given the vast gaps in epidemiological knowledge, it is not surprising that we were not able to establish meaningful connections between epidemiology and aetiological factors. Besides the complex biological background, it is also the lack of clinical methods able to differentiate and measure the contributions of the different pathophysiological mechanisms that limits our ability to gain meaningful data on epidemiology and aetiology. The differentiation between peripheral and central hearing disturbance is often difficult to assess [41, 42]. Patients with impairment of the central nervous system typically also exhibit deficits in other cognitive functions that may affect daily life additionally or more severely than hearing loss Thus, unresolved questions about the reciprocal influence between hearing loss and cognition or isolation and central nervous function arise.
Geographic distribution in prevalence
Most studies observing the demographic pattern of hearing loss were made in the northern and western part of Europe, and there were only two studies in southern Europe [18, 19]. The findings of these two latter studies did not differ clearly from those of the other studies. Some additional data were found from Poland, the Netherlands, and Germany, but they were either inapplicable or not population based.
No geographic-related pattern could be derived from these data. The relative absence of studies in the middle, southern and eastern parts of Europe may reflect more the differences in public health systems than attaching less importance to ARHL in these regions. Nevertheless, given the fact that vowels have a distinct perceptual advantage over consonants in determining speech intelligibility [43, 44], the different languages spoken in Europe may also influence epidemiological data of ARHL with respect to prevalence of self-reported hearing impairment.
Screening for ARHL?
Ideally, a comprehensive strategy to screen, assess, and improve ARHL would be based not only on crude prevalence data and projections of its longitudinal development, but also on the prevalence of aetiologically different subsets. It seems likely that ARHL with a primary aetiological background of vascular or metabolic factors will need different preventive measures than an ARHL primarily connected to factors such as noise or due to inflammatory middle ear disease. Given the biological variability of ARHL, it may be difficult to devise a universal approach for improving ARHL. Rehabilitation strategies may also have to consider the contribution of peripheral and central auditory aspects [42, 45]. Testing these components selectively is not well established and no experience with screening for them exists.