Dementia manifests as a set of related symptoms, which usually surface when the brain is damaged by injury or disease.[4] The symptoms involve progressive impairments to memory, thinking, and behavior, which negatively impact a person's ability to function and carry out everyday activities. Aside from memory impairment and a disruption in thought patterns, the most common symptoms include emotional problems, difficulties with language, and decreased motivation. Dementia is not a disorder of consciousness, as that is not usually affected.[2][3] Dementia has a significant effect on the individual, relationships, and caregivers.

Other namesSenility,[1] senile dementia
Image of a man diagnosed with dementia in the 1800s
SpecialtyNeurology, psychiatry
SymptomsDecreased ability to think and remember, emotional problems, problems with language, decreased motivation[2][3]
Usual onsetGradual[2]
DurationLong term[2]
CausesAlzheimer's disease, vascular disease, Lewy body disease and frontotemporal lobar degeneration.[4][3]
Diagnostic methodCognitive testing (Mini-Mental State Examination)[5]
Differential diagnosisDelirium[6] Hypothyroidism
PreventionEarly education, prevent high blood pressure, prevent obesity, no smoking, social engagement[7]
TreatmentSupportive care[2]
MedicationAcetylcholinesterase inhibitors (small benefit)[8]
Frequency50 million (2020)[4]
Deaths2.4 million (2016)[9]

The symptoms may be described as occurring in a continuum over several stages.[10][lower-alpha 1] A diagnosis of dementia requires a change from a person's usual mental functioning and a greater cognitive decline than that due to normal aging.[12] Several diseases and injuries to the brain, such as a stroke, can give rise to dementia. However, the most common cause is Alzheimer's disease, a neurodegenerative disorder. More than 95% of people suffering from Alzheimer's disease have the sporadic form (late onset, 80–90 years of age).[13]

While in DSM-5, dementia has been re-described as a major neurocognitive disorder, with varying degrees of severity, and many causative subtypes, the International Classification of Diseases (ICD-10) differentiates the disease as an organically situated disorder with symptoms found in the mental and behavioral aspects of the patient, similarly, with sub-classifications. Causative subtypes of dementia may be based on a known potential cause such as Parkinson's disease, for Parkinson's disease dementia; Huntington's disease for Huntingtons disease dementia; vascular disease for vascular dementia; brain injury including stroke often results in vascular dementia; or many other medical conditions including HIV infection for HIV dementia; and prion diseases. Subtypes may be based on various symptoms possibly due to a neurodegenerative disorder such as Alzheimer's disease; frontotemporal lobar degeneration for frontotemporal dementia; or Lewy body disease for dementia with Lewy bodies. More than one type of dementia, known as mixed dementia, may exist together. Diagnosis is usually based on history of the illness and cognitive testing with imaging. Blood tests may be taken to rule out other possible causes that may be reversible such as an underactive thyroid, and to determine the subtype. One commonly used cognitive test is the Mini-Mental State Examination. The greatest risk factor for developing dementia is aging, however dementia is not a normal part of aging. Several risk factors for dementia, such as smoking and obesity, are preventable by lifestyle changes. Screening the general population for the disorder is not recommended.

There is no known cure for dementia. Acetylcholinesterase inhibitors such as donepezil are often used and may be beneficial in mild to moderate disorder. The overall benefit, however, may be minor. There are many measures that can improve the quality of life of people with dementia and their caregivers. Cognitive and behavioral interventions may be appropriate. Educating and providing emotional support to the caregiver is important. Exercise programs may be beneficial with respect to activities of daily living and may potentially improve outcomes. Treatment of behavioral problems with antipsychotics is common but not usually recommended, due to the limited benefit and the side effects, including an increased risk of death.

It was estimated in 2020 that dementia affected about 50 million people worldwide. This is an increase on the 2016 estimate of 43.8 million, and more than double the estimated 20.2 million in 1990. The number of cases is increasing by around 10 million every year. About 10% of people develop the disorder at some point in their lives, commonly as a result of aging. About 3% of people between the ages of 65–74 have dementia, 19% between 75 and 84, and nearly half of those over 85 years of age. In 2016 dementia resulted in about 2.4 million deaths, up from 0.8 million in 1990. In 2020 it was reported that dementia was listed as one of the top ten causes of death worldwide. Another report stated that in 2016 it was the fifth leading cause of death. As more people are living longer, dementia is becoming more common. For people of a specific age, however, it may be becoming less frequent in the developed world, due to a decrease in modifiable risk factors made possible by greater financial and educational resources. It is one of the most common causes of disability among the old. However, dementia does not only impact the elderly. Onset of the condition before the age of 65 is less common, and little research as to why it happens has been done. This form of dementia is defined by the term "Early-Onset Dementia" (EOD) or “Presenile Dementia."[14] Less than 1% of patients with Alzheimer's have gene mutations that cause development of the disease much younger, around the age of 45. This is known as Early- Onset Alzheimer's.[15] Worldwide the cost of dementia in 2015 was put at US $818 billion. People with dementia are often physically or chemically restrained to a greater degree than necessary, raising issues of human rights. Social stigma against those affected is common.