Hearing impairment among adults--extent of the problem and scientific evidence on the outcome of hearing aid rehabilitation.

Scientific surveys on current and estimated prevalence of hearing impairment (HI) in adult populations (> or = 18 years of age) in Denmark, Finland, Norway, Sweden and the United Kingdom, and scientific reports on the outcome of hearing aid (HA) rehabilitation worldwide were reviewed. Only a few of the studies meet strict scientific criteria, and many locally clinically relevant studies cannot be generalized to larger populations. Population-based studies indicate an increase in prevalence of HI with age, but because of differences in study populations and available national population statistics, the studies do not allow reliable comparisons between countries or estimation of future prevalence of HI. Studies on HA prescription or outcomes do not provide uniform data in favour of non-linear amplification, but they do show some subject preference for the newer technology. No conclusions can be drawn regarding the degree of HI and the effects of amplification. The literature review alone gives only limited information regarding the extent of the problem of HI in adult populations in the target countries. Similarly, only a few studies on HA outcome meet strict scientific criteria and even fewer studies correlate rehabilitation outcome with the degree of HI, disability or handicap.