chronic, usually progressive deterioration of intellectual capacity associated with the widespread loss of nerve cells and the shrinkage of brain tissue. Dementia is most commonly seen in the elderly (senile dementia), though it is not part of the normal aging process and can affect persons of any age.

The most common irreversible dementia is Alzheimer disease. This condition begins with memory loss, which may first appear to be simple absentmindedness or forgetfulness. As dementia progresses, the loss of memory broadens in scope until the individual can no longer remember basic social and survival skills or function independently. Language, spatial or temporal orientation, judgment, or other cognitive capacities may decline, and personality changes may also occur. Dementia is also present in other degenerative brain diseases including Pick disease and Parkinson disease (parkinsonism).

The second most common cause of dementia is hypertension (high blood pressure) or other vascular conditions. This type of dementia, called multi-infarct, or vascular, dementia results from a series of small strokes (stroke) that progressively destroy the brain. Dementia can also be caused by Huntington disease, syphilis, multiple sclerosis, acquired immune deficiency syndrome (AIDS) (AIDS), and some types of encephalitis. Treatable dementias occur in hypothyroidism, other metabolic diseases, and some malignant tumours (tumour). Treatment of the underlying disease in these cases may inhibit the progress of dementia but usually does not reverse it.