What is dementia?
Dementia is the loss of mental functions—such as thinking, memory, and reasoning—that is severe enough to interfere with a person’s daily functioning. Dementia is not a disease itself, but rather a group of symptoms that might accompany certain diseases or conditions. Symptoms also might include changes in personality, mood, and behavior. Dementia is irreversible when caused by disease or injury, but might be reversible when caused by drugs, alcohol, hormone or vitamin imbalances, or depression.

Dementia develops when the parts of the brain that are involved with learning, memory, decision-making, and language are affected by any of various infections or diseases. The most common cause of dementia is Alzheimer’s disease, but there are as many as 50 other known causes. Most of these causes are very rare.

Some of the disorders that cause dementia might be reversible, although unfortunately most types of dementia do not respond to treatment. Therefore, it is very important to evaluate dementia symptoms comprehensively, so as not to miss potentially treatable conditions. The frequency of "treatable" causes of dementia is believed to be about 20 percent.

What are some of the other causes of dementia?
There are many causes of dementia, including neurological disorders such as Alzheimer's disease, blood flow-related (vascular) disorders such as multi-infarct disease, inherited disorders such as Huntington's disease, and infections such as HIV. The most common causes of dementia include:

• Degenerative neurological diseases, such as Alzheimer's, dementia with Lewy bodies, Parkinson's, and Huntington's
• Vascular disorders, such as multiple-infarct dementia, which is causes by multiple strokes in the brain
• Infections that affect the central nervous system, such as HIV dementia complex and Creutzfeldt-Jakob disease
• Chronic drug use
• Depression
• Certain types of hydrocephalus, an accumulation of fluid in the brain that can result from developmental abnormalities, infections, injury, or brain tumors

Alzheimer's disease causes 50 percent to 70 percent of all dementia. However, researchers are finding that some of what was previously considered Alzheimer's disease is really one of two other degenerative diseases: Lewy body disease and Pick's disease. There also are a number of other important disorders that can lead to dementia. Some of these are potentially reversible, at least partially, and should be considered before a diagnosis of Alzheimer's disease is made. 

How common is dementia?
Although dementia has always been somewhat common, it has become even more common among the elderly in recent history. It is not clear if this increased frequency of dementia reflects a greater awareness of the symptoms, or if people simply are living longer and thus are more likely to develop dementia in their older age.

Dementia caused by neurological degenerative disease, especially Alzheimer's disease, is increasing in frequency more than most other types of dementia. Some researchers suspect that as many as half of all people over 80 years old develop Alzheimer's disease. Also, the increased incidence of AIDS dementia complex, which results from HIV infection, helps account for the increased dementia in recent history. 

Who gets dementia?
Dementia is considered a late-life disease because it tends to develop mostly in elderly people. About 5 percent to 8 percent of all people over the age of 65 have some form of dementia, and this number doubles every five years above that age. It is estimated that as many as half of people 85 or older suffer from dementia. 

What are the types of dementia?
Some doctors and researchers split dementia into two broad categories — the cortical dementias and the subcortical dementias — based on which part of the brain is affected.

• Cortical dementias arise from a disorder affecting the cerebral cortex, the outer layers of the brain that play a critical role in cognitive processes such as memory and language. Alzheimer's and Creutzfeldt-Jakob disease are two forms of cortical dementia. Cortical dementia patients typically show severe memory impairment and aphasia, the inability to recall words and understand common language.
• Subcortical dementias result from dysfunction in the parts of the brain that are beneath the cortex. Usually, the memory loss and language difficulties that are characteristic of cortical dementias are not present. Rather, people with subcortical dementias, such as Huntington's disease, Parkinson's Disease, and AIDS dementia complex, tend to show changes in their personality and attention span, and their thinking slows down.

There are cases, such as with multi-infarct dementia, where both parts of the brain tend to be affected. 

Is dementia treatable?
Treatable causes of dementia are reversible disorders that can be cured completely, or at least partially, by treating the underlying disorder. Because some types of dementia are treatable or partially treatable, it is important not to assume that a person who is showing any symptoms of dementia is suffering from Alzheimer's disease or another incurable disease. For example, dementia caused by any of the following are often at least partially treatable:

• Chronic drug abuse
• Tumors that can be removed
• Subdural hematoma, an accumulation of blood beneath the outer covering of the brain that results from a broken blood vessel, usually as a result of a head injury (which can be minor and even unrecognized)
• Normal pressure hydrocephalus
• Metabolic disorders, such as a vitamin B12 deficiency
• Hypothyroidism, a condition that results from low levels of thyroid secretion
• Hypoglycemia, a condition that results from low blood sugar

Non-treatable causes of dementia include:

• Alzheimer’s disease
• Multi-infarct dementia
• Dementias associated with Parkinson’s disease and similar disorders
• AIDS dementia complex
• Creutzfeldt-Jakob disease (CJD), a quickly progressing and fatal disease that is characterized by dementia and myoclonus — muscle twitching and spasm

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This information is provided by the Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition. For additional written health information, please contact the Health Information Center at the Cleveland Clinic (216) 444-3771 or toll-free (800) 223-2273 extension 43771 or visit www.clevelandclinic.org/health/.

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