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.

A person with dementia loses mental abilities. Memory loss usually comes first. Gradually the person becomes unable to perform basic mental and physical tasks. Typically, dementia develops slowly over months or years. The first symptoms are often subtle. Eventually, people with dementia can have significant memory loss. They also may lose their ability to communicate effectively, recognize other people, perform complex tasks or think critically.

Most commonly, dementia occurs when nerve cells (neurons) in the brain break down (degenerate) 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

About 5 million people in the United States have dementia. Approximately 15% of people older than 65 are thought to have dementia. Alzheimer's disease, the most common cause of dementia, will affect more than 12 million Americans in the next 20 years.