Dementia, now also referred to as ‘major neurocognitive disorder’ in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), is defined by the presence of substantial cognitive decline from a previous level of functioning to the degree that the individual’s ability to live independently is compromised owing to the cognitive deficits. Dementia is a syndrome with many possible causes, with Alzheimer’s disease being the most common in older people. It is generally of gradual onset with a chronic course, although there are exceptions. Dementia must be distinguished from delirium (acute confusional state), which by definition is of acute or recent onset and associated with loss of awareness of surroundings, a global disturbance in cognition, changes in perception and the sleep–wake cycle, and other features.
Mild cognitive impairment
As dementia develops gradually, individuals go through an intermediate stage, generally referred to as mild cognitive impairment (MCI) or mild neurocognitive disorder. This stage involves both subjective and objective evidence of modest cognitive decline but not to a degree that compromises independent functioning. Higher-level function is affected, for which the individual uses compensation strategies.1
Although MCI is often termed ‘pre-dementia’, there is evidence that a significant proportion of patients diagnosed with MCI will not progress or may even revert and no longer meet the diagnostic criteria on follow up. The prevalence of MCI in adults aged 65 years and older is 10 to 20%, and the estimated annual conversion rate to dementia, in particular Alzheimer’s disease, is reportedly 10 to 15% in clinic patients and 6 to 10% in epidemiological studies.2,3
Stress, depression and anxiety
Stress is the body’s response to environmental demands or threats, either physical or psychological. Whereas some stress is normative, stress can become pathological if it is chronic or overwhelming in relation to the person’s coping abilities. This can lead to psychiatric conditions such as depression or anxiety, which can have direct effects on cognition, including inattention, slowed processing and short-term memory impairment.
Anxiety, depression or a combination of the two may also be primary psychiatric conditions, exacerbated by a neurocognitive disorder, or may be related to associated physical disorders. In some instances, depression or anxiety may occur in the prodromal phase of a neurocognitive disorder. It is therefore important to screen patients for symptoms of anxiety and depression as part of the differential diagnosis, and to treat these conditions appropriately.
Cognitive changes of normal ageing
Finally, it must be recognised that normal ageing is associated with some cognitive change, with few people experiencing no cognitive decline. The primary decline is in processing speed, such that with age individuals become slower in reacting to stimuli and performing cognitive tasks. This has effects on the ability to remember information, resulting in ‘on the tip of the tongue’ experiences, where words or names are occasionally forgotten but come to mind later or with a prompt. There can be a slow recall of stored knowledge, although the detail is essentially retained. Memory recall may sometimes be short on detail.
Normal memory changes vary between individuals, but the capacity for immediate recall and long-term memory does not tend to change. The capacity to understand instructions or follow stories on television or in newspapers or books is also usually preserved. These changes do not herald dementia. Differences between forgetfulness associated with healthy brain ageing, stress or depression, MCI and dementia are summarised in Box 1.