Central Auditory Processing in Presbycusis: an Epidemiologic Perspective

Presbycusis (literally elder hearing) or age-related hearing loss is the inevitable deterioration in hearing that occurs as people get older. While loss of the highest frequencies can be detected in young adulthood, it is not until the 6th decade and beyond that clinically significant hearing loss is evident. Presbycusis is a multifactorial process that affects people in their senior years in degrees ranging from mild to profound. It is conceptually useful to consider presbycusis as a mixture of acquired auditory stresses, trauma, and otologic diseases that affect hearing over time superimposed upon an intrinsic, genetically controlled, aging process.1 Separating aging effects from age effects and aging changes from age-related diseases complicates the study of presbycusis. Presbycusis has both peripheral and central components. Although the latter is the primary focus of this review, peripheral conditions that may confound central auditory processing are included. The review is based primarily on my epidemiologic investigations of the Framingham Heart Study cohort and the Adult Change of Thought Cohort (Seattle). These investigations provide a more controlled and representative overview of auditory aging than is seen in clinic populations. Unlike clinical studies, however, trends in groups can only be described with a broad brush. The clinical picture thus obtained focuses on central tendencies, which provide valuable insight (but not proof) into possible processes involved. By addressing the effects of brain dysfunction (dementia, e.g.) on central auditory processing in these people, a fuller picture of the central presbycusis may be obtained. The high prevalence of presbycusis, which is a consequence of our aging population, leads to hearing difficulty as a common social and health problem. Overall, 10% of the population has a hearing loss great enough to impair communication, and this rate increases to 40% in the population over 65 years.2 Eighty percent of hearing loss cases occur in the elderly.3 Although hearing worsens increasingly with age, the magnitude of the hearing problem at any given age varies greatly. It is rare to find a person over 70 years of age who has no hearing impairment or whose hearing sensitivity has not declined from youthful levels. Cardiovascular disease as well as cardiovascular disease risk-factors affect hearing to some extent. Stroke, myocardial infarction, claudication, hypertension, hyperlipidemia, and diabetes mellitus have all been associated with excessive hearing loss. Therefore, it is logical that maintenance of good general health and fitness would minimize the risk of hearing loss due to systemic disease. High lipid diets are associated with poorer hearing.4 There is inconclusive evidence that low-caloric diets, which clearly prolong life in laboratory animals, have an effect on presbycusis.5 While free radical accumulation is presumed to be involved in presbycusis, research into anti-oxidant agents is have not been shown to counter auditory aging. Dividing the auditory system into peripheral and central components is useful for didactic purposes but one must bear in mind the integrated nature of system and the overlapping attributes of peripheral and central auditory function. For example, deficits in speech perception may occur at any level of the system. Therefore, peripheral changes must be taken into account when central problems are being considered.