What is dementia?

When loss of mental functions—such as thinking, memory, and reasoning—are severe enough to interfere with a person's independent daily functioning, they are said to be at the stage of dementia. Dementia is not a disease itself, but rather the total impact of symptoms that might accompany certain diseases or conditions on daily function. Symptoms also might include changes in personality, mood, and behavior.

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 numerous other known causes. Most of these causes are very rare.

Dementia is irreversible when caused by degenerative disease or trauma, but might be reversible in some cases when caused by drugs, alcohol, hormone or vitamin imbalances, or depression. 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.

How common is dementia?

Although dementia has always been 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 85 years old develop Alzheimer's disease. Dementia associated with AIDS, which appeared to be increasing in frequency in the 1990s is now much less commonly seen, since the development of highly effect anti-retroviral therapy.

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?

It is convenient to classify most dementias as either of Alzheimer type or non-Alzheimer type. The former are characterized predominantly by memory loss, accompanied by impairment in other cognitive functions or "domain," such as language function (aphasia), skilled motor functions (apraxia), or perception, visual or other (agnosias). Non-Alzheimer dementias include the frontotemporal lobar degenerations, which generally are of two main types. One primarily affects speech, as in the primary progressive aphasia syndromes. The other is characterized primarily by changes in behavior, including apathy, disinhibition, personality change and what is called executive function (e.g., planning ahead and organizational ability). In both of these types, memory loss is relatively mild, if present, until later in the course of the disease. Other forms of dementia, including vascular disorders (multiple strokes), dementia with Lewy bodies, Parkinson's dementia, and normal pressure hydrocephalus would be grouped among the non-Alzheimer disorders.

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 cognitive impairment, 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, frontotemporal lobar degeneration, dementia with Lewy bodies, Parkinson's, and Huntington's diseases
  • Vascular disorders, such as multi-infarct dementia, which is caused 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 within the brain that can result from developmental abnormalities, infections, injury, or brain tumors

Alzheimer's disease accounts for 50 percent to 70 percent of all dementia. However, many patients with Alzheimer's disease also have evidence of co-existing cerebrovascular disease, usually consisting of multiple small areas of ischemic changes (often called "mini-strokes") on MRI and on post-mortem examination of the brain. Thus, many of these patients can be considered to have a "mixed" dementia. Frontotemporal lobar degenerations, of which several types are known, account for a substantial number of dementias, especially among those in their 50s and 60s. Dementia with Lewy bodies has also been diagnosed with increasing frequency in recent years. These patients have clinical signs of Parkinsonism as well as dementia; its relationship to the dementia of Parkinson's disease is still incompletely understood.

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