Dementia is the loss of cognitive functioning—thinking, remembering, and reasoning—and behavioral abilities to such an extent that it interferes with a person’s daily life and activities. Dementia ranges in severity from the mildest stage, when it is just beginning to affect a person’s functioning, to the most severe stage, when the person must depend completely on others for basic activities of daily living.
Many conditions and diseases cause dementia. The most common cause of dementia in older people is Alzheimer’s disease. Other causes include different kinds of brain changes that lead to vascular dementia, Lewy body dementia, and frontotemporal disorders.
In addition, some people have mixed dementia—a combination of two or more disorders, at least one of which is dementia. A number of combinations are possible. For example, some people have Alzheimer’s disease and vascular dementia at the same time.
Other causes of dementia include Huntington’s disease, Creutzfeldt-Jakob disease, head injury, and HIV. In addition, some conditions that cause dementia, such as normal pressure hydrocephalus, thyroid problems, and vitamin B deficiency, can be reversed with appropriate treatment.