From Normal to Pathological Ageing - The Spectrum of Mild Cognitive Impairment and Alzheimer's Disease
From Normal to Pathological Ageing - The Spectrum of Mild Cognitive Impairment and Alzheimer's Disease
Introduction
Human ageing is joined by various physical, social and cognitive-mnestic changes, which can differ considerably both inter- and intra-individually. Cognitive-mnestic changes are caused by processes in the central nervous system, but they are also determined by genetic factors, education, profession, lifestyle, intellectual and physical activity and, especially, general physical condition. Reliable norms about cognitive and mnestic functions in older people are usually lacking.
Many tests just indicate norms for a group older than 60 years. In geronto-psychological literature, a differentiation is sometimes made between ‘young olds’ (65–75 years), ‘old olds’ (75–85 years) and ‘eldest olds’ (greater than 85 years) in orderto define the clinical heterogeneity of ageing in the elderly.
Study Design
There are also problems in study design. In crosssectional studies, age-dependent impairments are often overestimated, while in longitudinal studies they are underestimated. Since the neurons of the central nervous system lose their ability to replicate, irreversible destructive processes cumulate over the course of the years.1 After the age of 30, the human brain begins to lose weight; this can be demonstrated in computed tomography (CT) and magnetic resonance imaging (MRI). The enlargement of cortical sulci and ventricles increases 20% per decade after the age of 50 in men and after the age of 60 in women. Not only shrinkage and loss of neurons, but also changes in the energy metabolism of the neurons can restrict cell function. A reduction of density of the synapses and the ‘dying back’ of axonal branching, increases of plaques and tangles and changes in the cholinergic and dopaminergic transmitter systems have all been described. There is little experience in the agesensitivity of the approximately 50 different transmitter systems. Cognitive-mnestic changes are described in the elderly.2 Intelligence deficits can be ascertained in distinguishing between crystalline and fluid intelligence.3 Components of the crystalline intelligence can remain preserved or even increase until late age, while fluid intelligence decreases.4 This means that older people act successfully in routine situations and that their knowledge and vocabulary remains stable. In contrast, there is a successive loss in the processing speed of new information. The flexible adaptation to new situations and problem-solving can become difficult. Older people frequently complain about memory deficits. The extent of mnestic problems largely depends on age, the material to learn and the task to solve. Memory performances are hampered more strongly in abstract material than in everyday familiar material. There are also problems in free recall. Memory performance in recognition tasks worsens later, but to a small extent only. Attention task performance, especially processes in selective and divided attention, is impaired. Verbal and communicative abilities may pass as being relatively stable in age, but communication can be disturbed by sensory deficiencies like hearing deficits. A decline in abstraction ability and cognitive flexibility, as well as an increased susceptibility to interference can be found in older people. Deceleration of reaction time in cognitive information processing is a universal phenomenon in older people and is one of the main causes of cognitive dysfunction. Reaction time in 60-yearolds compared to 30-year-olds is thought to be reduced by 20%. Except for some subtypes of vascular dementia, cognitive decline in dementia starts slowly and sneakily, and the leap from normal cognitive decline in age and the beginning of dementia is often very difficult to define. In different classification schemes, the diagnostic uncertainty is described in terms of ‘age-related cognitive decline’, ‘mild neuro-cognitive disorder’, ‘ageassociated memory impairment’ and ‘mild cognitive impairment’ (MCI).
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