According to DSM-IV, dementia is characterised by multiple cognitive deficits that are severe enough to cause significant impairment in social or occupational functioning. These deficits must show evidence of decline from previous levels of functioning, include memory impairment, and at least one other cognitive disturbance (aphasia, apraxia, agnosia, or a disturbance in executive functioning). The rate of occurrence is higher in later life; the highest prevalence is above age 85. Dementia is classified as being progressive, static, or remitting. The subtypes of dementia are listed below.

• Dementia of the Alzheimer's type (DAT)—deterioration of higher cortical function (this is the most common form)
• Vascular dementia—one form is multi-infarct dementia (MID), which is secondary to atherosclerosis
• Dementia due to other general medical conditions, including infection with the human immunodeficiency virus (HIV), traumatic brain injury, Parkinson's disease, Huntington's disease, Pick's disease, Creutzfeldt-Jakob disease, normal-pressure hydrocephalus, hypothyroidism, brain tumour, and vitamin B deficiencies
• Substance-induced persisting dementia due to drug or alcohol abuse, medication, or toxin exposure