Dementia is defined as an impairment of memory and thinking that is of such severity that the affected person is no longer able to be independent and carry out normal activities of daily living. Alzheimer's disease is the commonest cause of dementia. It is a neurodegenerative disorder which begins to damage the brain years before the onset of dementia. As the disease progresses, people will develop initial symptoms - typically increasing forgetfulness and getting lost - but will still maintain functional independence. At this stage they may be given the diagnosis of "mild cognitive impairment". As Alzheimer's disease progresses, other symptoms can occur including changes in speech and language, impaired decision-making and alterations in behaviour. As these symptoms worsen, people may become dependent on others and be given the diagnosis of dementia.
Dementia may be caused by conditions other than Alzheimer's Disease. These include other neurodegenerative disorders such as Lewy body dementia and Parkinson's disease associated dementia - both characterised by mobility problems, sleep disturbance and hallucinations - and frontotemporal dementia, which can occur in younger people and is associated with personality change and disturbances of speech and language. The commonest non-degenerative cause of dementia is vascular dementia, in which slowing of thought is a prominent feature and in which mobility may be affected. Sometimes dementia may be caused by conditions that are potentially treatable or reversible, such as encephalitis (inflammation of the brain), thyroid disease, excess alcohol intake, nutritional or vitamin deficiencies.
A detailed assessment of symptoms and examination is required to help distinguish between these various disorders. This will typically be accompanied by an assessment of memory and thinking (cognitive assessment) and brain scanning using state of the art MRI.