Neurologic disorders are much less common as causes of death than cardiovascular disease and cancer, but account for a tremendous share of morbidity, disability, and dependence on nursing care in the elderly. This particularly applies to cognitive decline and dementia, the epidemiology of which we will focus on in this section.
The prevalence of dementia has been found to increase exponentially with age, with prevalence doubling every five years. Overall prevalence has been estimated to be about 2% between ages 65 and 74 and reaching 30% for ages 85 and over (Borenstein Graves, 2004). Age-specific overall prevalences of dementia appear to be quite similar in different countries.
Two major forms of dementia are commonly distinguished: Alzheimer's disease (AD) and vascular dementia. In most studies, the majority of cases of dementia have been classified as AD, but there have been substantial variation in estimates of the relative frequency of both disorders between studies from different populations. Furthermore, there is considerable overlap between both disorders: Between 18% and 46% of dementia cases share both Alzheimer and vascular lesions, and this overlap increases with age. Recent epidemiologic studies have consistently found cardiovascular risk factors to be related to both forms of dementia, suggesting they may share a common etiology (Launer, 2002).
Although analytical epidemiologic studies on dementia are facing particular logistic challenges (e.g., the need to rely on proxy respondents in case-control studies), a number of risk factors have now been quite consistently established in more recent large-scale prospective cohort studies. Genetic factors undoubtedly play some role, particularly among early onset cases, with the ApoE-fi4 alleles being the best established single markers. The ApoE-fi4 allele is also an established risk factor for cardiovascular disease (CVD). Other risk factors of CVD linked to the occurrence of dementia include diabetes, hypertension, hyperlipedemia, and high levels of plasma homocysteine. As is the case for CVD, moderate alcohol consumption as well as intake of nonsteroidal anti-inflammatory drugs seem to be protective (Borenstein Graves, 2004).
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