Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Dementia

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.