Disorders that cause dementia include conditions that impair the vascular (blood vessels) or neurologic (nerve) structures of the brain. A minority of causes of dementia are treatable. These include normal pressure hydrocephalus, brain tumors, and dementia due to metabolic causes and infections. Unfortunately, most of the disorders associated with dementia are progressive, irreversible, degenerative conditions.

The two major degenerative causes of dementia are Alzheimer's disease, which is a progressive loss of nerve cells without a known cause or cure and vascular dementia, which is loss of brain function due to a series of small strokes. Vascular dementia may or may not play a role in the progression of Alzheimer's disease: the conditions often occur together and neither can be diagnosed definitively except until autopsy. In those with the genetic and environmental susceptibility to develop Alzheimer's disease, the concomitant presence of small infarcts (lacunar strokes) speeds up the onset of Alzheimer's disease to an earlier age than if it were to occur alone (i.e., without small infarcts).

Dementia may be diagnosed when there is impairment of two or more brain functions, including language, memory, visual-spatial perception, emotional behavior or personality, and cognitive skills (such as calculation, abstract thinking, or judgment). Dementia usually appears first as forgetfulness. Other symptoms may be apparent only on neurologic examination or cognitive testing. Loss of functioning progresses slowly from decreased problem solving and language skills to difficulty with ordinary daily activities to severe memory loss and complete disorientation with withdrawal from social interaction.