Dementia is a chronic disturbance involving multiple cognitive deficits, including memory impairment. Dementia is characterized by chronicity and deterioration of selective mental functions. Onset is insidious over months to years in most cases. Dementia is usually progressive, more common in the elderly, and rarely reversible even if underlying disease can be corrected. Dementia can be classified as cortical or subcortical.
There are three types of cortical dementia:

* Primary degenerative dementia (eg, Alzheimer dementia), accounting for about 50–60% of cases.
* atherosclerotic (multi-infarct) dementia, 15–20% of cases (this figure is probably low because of the tendency to overuse the diagnosis of Alzheimer dementia)
* Mixtures of the first two types or dementia due to miscellaneous causes, 15–20% of cases . Examples of primary degenerative dementia are Alzheimer dementia (most common) and Pick, Creutzfeldt-Jakob, and Huntington dementias (less common).