What is dementia?
Dementia is not a specific disorder or disease. It is a syndrome (group of symptoms) associated with a progressive loss of memory and other intellectual functions that is serious enough to interfere with performing the tasks of daily life. Dementia can occur to anyone at any age from an injury or from oxygen deprivation, although it is most commonly associated with aging. It is the leading cause of institutionalization of older adults.
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.
Causes and symptoms
Dementia can be caused by nearly forty different diseases and conditions, ranging from dietary deficiencies and metabolic disorders to head injuries and inherited diseases. The possible causes of dementia can be categorized as follows:
- Primary dementia. These dementias are characterized by damage to or wasting away of the brain tissue itself. They include Alzheimer’s disease (AD), frontal lobe dementia (FLD), and Pick’s disease. FLD is dementia caused by a disorder (usually genetic) that affects the front portion of the brain, and Pick’s disease is a rare type of primary dementia that is characterized by a progressive loss of social skills, language, and memory, leading to personality changes and sometimes loss of moral judgment.
- Multi-infarct dementia (MID). Sometimes called vascular dementia , this type is caused by blood clots in the small blood vessels of the brain. When the clots cut off the blood supply to the brain tissue, the brain cells are damaged and may die. (An infarct is an area of dead tissue caused by obstruction of the circulation.)
- Lewy body dementia. Lewy bodies are areas of injury found on damaged nerve cells in certain parts of the brain. They are associated with Alzheimer’s and Parkinson’s disease, but researchers do not yet know whether dementia with Lewy bodies is a distinct type of dementia or a variation of Alzheimer’s or Parkinson’s disease.
- Dementia related to alcoholism or exposure to heavy metals (arsenic, antimony, bismuth).
- Dementia related to infectious diseases. These infections may be caused by viruses (HIV, viral encephalitis); spirochetes (Lyme disease, syphilis); or prions (Creutzfeldt-Jakob disease). Spirochetes are certain kinds of bacteria, and prions are protein particles that lack nucleic acid.
- Dementia related to abnormalities in the structure of the brain. These may include a buildup of spinal fluid in the brain (hydrocephalus); tumors; or blood collecting beneath the membrane that covers the brain (subdural hematoma).
Dementia may also be associated with depression, low levels of thyroid hormone, or niacin or vitamin B 12deficiency. Dementia related to these conditions is often reversible.
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.
The reversible dementias related to thyroid and nutritional disorders can be prevented in many cases by regular physical checkups and proper attention to diet. Dementias related to toxic substances in the workplace may be prevented by careful monitoring of the work environment and by substituting less hazardous materials or substances in manufacturing processes. Dementias caused by infectious diseases are theoretically preventable by avoiding exposure to the prion, spirochete, or other disease agent. Multi-infarct dementia may be preventable in some patients by attention to diet and monitoring of blood pressure. Dementias caused by abnormalities in the structure of the brain are not preventable as of 2002.
With regard to genetic factors, tests are now available for the APOE gene implicated in late-onset Alzheimer’s, but these tests are used primarily in research instead of clinical practice. One reason is that the test results are not conclusive; about 20% of people who eventually develop AD do not carry this gene. Another important reason is the ethical implications of testing for a disease that presently has no cure. These considerations may change, however, if researchers discover better treatments for primary dementia, more effective preventive methods, or more reliable genetic markers.