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The first step in diagnosing dementia is to show that the person's ability to think and learn has in fact declined from its earlier level. His or her current ability in different spheres of mental activity can be measured by any of a variety of mental status tests. The difficulty comes in comparing these current ability levels with those at earlier times. A patient's own reports cannot be relied upon, since memory loss is typically part of dementia. Frequently, however, family members' descriptions of what the person once could do will establish that a decline has occurred. In other cases, comparison with what a person has accomplished throughout his or her life is enough to show that a decline has occurred. If neither source of information provides a clear answer, it may be necessary to readminister the mental status test several months later and compare the two results.

Is any decline, no matter how small, sufficient to establish a diagnosis of dementia? The answer is not entirely clear. Research has shown that most older people suffer a small but measurable decrease in their mental abilities. For example, one recent study followed 5,000 people, some for as many as 35 years. This study found that scores on tests of mental abilities did not change between ages 25 and 60, but declined about 10% between ages 60 and 70. More significantly, people in their late eighties had scores more than 25% below those seen earlier.

Since none of the people tested were considered demented, one might assume that these declines are normal. It is still possible, however, that some tested individuals were in the early stages of dementia; these people's results may then have pulled down the average scores for the group as a whole and created a false impression of a sizable "normal" drop in IQ. This ambiguity is particularly unfortunate because it has significant implications at the individual level: No one knows whether, if an older person's mental sharpness starts to decline, this a normal part of aging or a possible signal of approaching dementia.

Once the existence of dementia has been established, the next question is: What is causing the condition? Alzheimer's disease is by far the most common cause of dementia, especially in older adults. One recent study found that it directly caused 54% of dementias in people over 65, and may have been partially responsible for up to 12% more.

Unfortunately, there is no direct way to diagnose Alzheimer's disease in a living person; only microscopic examination of the brain after death can conclusively establish that a person had this disorder. The same is true for the second most common cause, multi-infarct dementia. Both diagnoses are made by excluding other causes of dementia.

It is particularly crucial to exclude causes for which appropriate treatment might prove helpful. Among the most common and important of these are side effects of medications an individual may be taking—for example, sleeping pills, antidepressants, certain types of high blood pressure medications, or others to which a person may be particularly sensitive. Medications are particularly likely to be responsible when the affected person is not only confused and forgetful, but also is not alert to what is going on around him or her.

Older individuals—the group most likely to suffer dementia from other causes—are particularly likely to be taking multiple drugs for their various disorders. Sometimes these drugs interact, producing side effects such as dementia that would not occur with any single drug at the same dosage. Drug side effects, including dementia, may also be more common in older people because their body's ability to eliminate the drug often declines with age. Reduced speed of elimination calls for a corresponding reduction in dosage that does not always occur.

Another common, but treatable, cause of dementia, or of what looks like dementia, is depression. Some psychiatrists refer to the slowed thinking and confusion sometimes seen in people with depression as pseudodementia because of its psychological origin. Others believe the distinction does not reflect a meaningful difference. In any case, effective treatment of the depression will relieve the dementia it has produced.

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Science EncyclopediaScience & Philosophy: Cyanohydrins to Departments of philosophy:Dementia - Diagnosis, Causes