Sensory deficits and mental disorders of old age: causal or coincidental associations?

The description of concurrent sensory deficits and mental disorder in old age is complicated by general problems in the estimation of the prevalence of morbidity in old age. First, although there is agreement that the onset of 'old age' is some time in the seventh decade of life (i.e. between 60 and 70 years of age), there is no upper age limit. This has the effect of creating an age group which may span 40 or more years. 'Over 65' prevalence figures will consistently underestimate the prevalence of age-related disorders in the oldest age groups. Secondly, criteria for morbidity are not held constant across age groups. In recognition of normative age changes (i.e. changes which appear to occur as a product of ageing, in the absence of pathogens), the standard for 'normal' as opposed to 'abnormal' or 'disease state' is shifted. Higher thresholds for sensation and lower levels of function are frequently described as relative to age normative standards which assume some decrement. Again, this has the effect of underestimating the prevalence of dysfunction in the oldest age groups. The prevalence of sensory deficit is particularly vulnerable to underestimate, as the incidence of disorder increases with age, and criteria for function in the normal range are lowered. As an example, a hearing loss in the range of 25-35 dB within the speech frequencies, and up to 45-50 dB at high frequencies, may be considered 'normal' for an 80 year old, although these would constitute a clinically significant hearing loss for a 40 year old.