Dementia is a pattern of mental decline caused by different diseases or conditions. Most commonly, dementia occurs when brain nerve cells (neurons) die, and connections between neurons are interrupted. These disruptions have a variety of causes and usually cannot be reversed.
Among the causes of dementia:
Alzheimer's disease causes about 40% to 45% of all dementias.
Vascular disease, such as stroke, causes about 20%.
Lewy body disease, which causes neurons in the brain to degenerate, causes another 20% of dementias.
Other conditions that can cause dementia include:
Traumatic head injury
Acquired immunodeficiency syndrome
Degenerative diseases, such as Huntington's disease and Pick's disease
More than 50 other rare degenerative conditions
In rare cases, dementia is caused by a treatable condition, and it may be partially or entirely reversed if the condition is diagnosed and treated early:
Adverse reactions to drugs
Infections, such as syphilis or fungal meningitis
Metabolic conditions, such as deficiencies of vitamin B12, folate or thyroid hormone
In the developed nations, about 15% of people older than 65 are thought to have dementia.