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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

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:

  • Creutzfeldt-Jakob disease

  • Traumatic head injury

  • Acquired immunodeficiency syndrome

  • Alcohol abuse

  • Degenerative diseases, such as Huntington's disease and Pick's disease

  • Brain abscess

  • Multiple sclerosis

  • 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:

  • Depression

  • 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.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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