Dementia is a decline in a person's ability to think and learn. To distinguish true dementia from more limited difficulties due to localized brain damage, the strict medical definition requires that this decline affect at least two distinct spheres of mental activity; examples of such spheres include memory, verbal fluency, calculating ability, and understanding of time and location.
Some definitions of dementia also require that it interfere with a person's work and social life. However, this may be difficult to show when a person's work and social life is already limited, either by choice or by another mental or physical disorder. As a result, the most recent and most authoritative definition (that developed jointly by the National Institute for Neurological and Communicative Disorders and Stroke—part of the National Institutes of Health—and the Alzheimer's Disease and Related Disorders Association) does not include this criterion. The NINCDS-ADRDA definition focuses strictly on a decline from a previously higher level of mental function.
The term dementia goes back to antiquity, but was originally used in the general sense of being "out of one's mind." Identification specifically with difficulties in thinking and learning occurred in the late eighteenth and early nineteenth centuries. Even then, however, the term was used for almost any sort of thinking, learning, or memory problem, whether temporary or permanent and without regard to cause. The most typical picture was of a young adult suffering from insanity or a disease affecting the brain.
This picture changed later in the nineteenth century, as psychiatrists (then called alienists) sought to group disorders in ways that would help reveal their causes. Temporary stupor, dementia associated with insanity, and memory problems resulting from damage to a specific area of the brain were all reclassified. The central core of what was left was then senile dementia: the substantial, progressive loss of mental function sometimes seen in older people and now recognized as resulting from one or more specific, identifiable diseases. Current definitions still recognize the existence of dementia in younger people, however.
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.
Dementia can result from a wide variety of disorders and conditions. Some are quite rare, while others are moderately common. In some cases (measles, for example) dementia may be a rare complication of an otherwise common disease; in other cases, such as infection with Human Immunodeficiency Virus (HIV), an impact on mental function well known to medical specialists may not be widely recognized by the general public.
Non-Alzheimer degenerative dementias
In addition to Alzheimer disease, dementia may result from several other conditions characterized by progressive degeneration of the brain. The three most common of these are Pick's disease, Parkinson disease, and Huntington disease (Huntington's chorea).
Like Alzheimer disease, Pick's disease affects the brain's cortex—that is, the outer part where most of the higher mental functions take place. In other respects, however, the disorders are quite different. In Pick's disease, for example, microscopic examination of the brain reveals dense inclusions (Pick bodies) within the nerve cells, while the cells themselves are inflated like blown-up balloons. This does not at all resemble the neurofibrillary tangles and beta-amyloid plaques seen in Alzheimer disease. However, since microscopic examination of a living person's brain is rare, symptoms are used to distinguish the two diseases in practice.
Typically, Pick's disease affects different parts of the cortex than does Alzheimer disease. This influences the order in which symptoms appear. The earliest symptoms of Pick's disease include personality changes such as loss of tact and concern for others, impaired judgment, and loss of the ability to plan ahead. Loss of language skills occurs later, while memory and knowledge of such things as where one is and the time of day are preserved until near the end. In contrast, memory and time-space orientation are among the first things lost in Alzheimer disease, while personality changes and loss of language skills are late symptoms.
Both Parkinson disease and Huntington chorea initially affect deeper brain structures, those concerned with motor functions (that is, movement of the voluntary muscles). Indeed, most descriptions of Parkinson disease focus on the muscular rigidity that the disorder produces. In the later stages, however, nearly all patients with the disease will develop some degree of dementia as well.
Shortly after appearance of the choreiform movements that typify Huntington disease, most patients will begin to have trouble thinking clearly and remembering previous events. By the time they die, Huntington patients are intellectually devastated.
Although degenerative disorders account for the majority of dementia cases, a respectable minority result from interference with blood flow in or to the brain. Most such cases are due to a series of small strokes. Each stroke in the series may be unnoticeable, but the long-term result is a continuing and eventually severe decline in mental function.
(A stroke, known technically as an infarct, is a failure of blood flow beyond a certain point in an artery. Usually this is due to a blood clot at that point, but sometimes it results from a break in the artery allowing much or all of the blood to escape. Although the fundamental causes are almost diametrically opposite—a clot at the wrong place versus no clot where one is needed—the effects are virtually the same.) Unlike the degenerative dementias, which follow a relatively predictable course, vascular dementias can be quite variable. When and precisely where the next stroke occurs will determine both how quickly the dementia progresses and the extent to which different mental abilities are affected. Typically, however, vascular dementias are characterized by sudden onset, step-wise progression, and occurrence of motor symptoms early in the disorder. High blood pressure is usually present as a predisposing factor. Most, but not all, physicians believe that other heart attack risk factors, such as diabetes, cigarette smoking, and high cholesterol, also increase the risk of developing vascular dementia.
Traditionally, physicians have distinguished two major types of vascular dementia. In multiple-infarct dementia, examination of the brain after death shows a number of small but individually identifiable areas where strokes have destroyed the brain tissue. In Binswanger's disease, individual areas of destruction cannot be identified. Almost the entire "white matter" of the brain—the portion occupied primarily by axons rather than nerve cell bodies—is affected to some degree. There is no sharp line between the two disorders, however, just as there is none between multiple-infarct dementia and the effect of two or three large strokes.
Dementia may also result from a reduction in blood flow to the brain as a whole. The most common cause is a severe narrowing of the carotid arteries in the neck. This may be considered analogous to partial plugging of an automobile's fuel line, whereas the local damage resulting from a stroke is more like knocking out a piston. Most other dementias similarly represent damage to the engine itself. (Alzheimer disease might perhaps be likened to cylinder-wall deposits causing the pistons to stick, although we do not know enough about the origin of the disease to be sure this analogy is entirely accurate.)
Many infections either attack the brain as their primary target or can spread to it. If enough brain tissue is destroyed as a result, the outcome may be dementia. Brain infections can be due to viruses, bacteria, fungi, or parasites. For example, the measles virus will occasionally attack the brain, producing a condition known as subacute sclerosing panencephalitis that causes dementia and eventually death. The herpes (cold sore) virus can also cause dementia if it attacks the brain.
Infection by mosquito-borne encephalitis virus may leave survivors with significantly reduced mental function. The frequency with which this occurs depends, however, both on the particular virus involved and the age of the individual. Dementia is rare following infection with Western encephalitis virus, but is found in more than half those under five years of age who survive an Eastern encephalitis virus attack. Similarly, equine encephalitis virus produces severe illness, often leading to serious brain damage or death, in children under 15; in older people, however, the disease is typically quite mild and causes no lasting problems.
Nevertheless, serious viral infections of the brain are relatively uncommon. The one exception is infection with the human immunodeficiency virus (HIV)—the virus that causes AIDS. This is the most common infectious cause of dementia in the United States today, and the number of people affected continues to grow.
Although popular accounts of HIV infection focus on the damage it causes to the immune system, the virus also typically attacks the brain. Nearly all HIV-infected people will develop dementia at some time during their illness. However, how soon this dementia occurs and how severe it may become varies widely. About 20% of people with HIV infection develop dementia before they develop the opportunistic infections that define progression to full-blown AIDS.
Over the past half-century, antibiotics have greatly reduced the threat from bacterial infection of the brain or the meninges that surround it. In one respect, however, the situation may be said to have worsened. Formerly, 95% of those with acute bacterial meningitis died; today, most survive, but the disease often leaves them with reduced mental capacities or other central nervous system problems.
On the other hand, tuberculous meningitis, which once accounted for up to 5% of children's hospital admissions,has now been almost eliminated. There has also been a major reduction in syphilis of the central nervous system, a disease whose effects once resulted in 15–30% of mental hospital admissions. Unfortunately, both diseases are undergoing resurgences. The incidence of tuberculosis has increased 20% since 1985, while estimates suggest that 50,000 undetected and untreated new cases of syphilis occur each year. In the absence of treatment, 25–30% of syphilis cases will spread to the brain or meninges and result, over the course of years, in paralysis and dementia.
Fungal infections of the brain and meninges are generally rare except in people with weakened immune systems. Parasitic infections are also rare in this country. Elsewhere, however, the well-known African sleeping sickness, spread by a type of biting fly found only in equatorial Africa, is due to a parasite known as a trypanosome. Malaria, a mosquito-born parasitic disease, may also at times attack the brain and result in dementia.
Two infectious dementias that are quite rare but of tremendous scientific interest are Creutzfeldt-Jakob disease and kuru. The probable cause of these diseases are prions, infectious agents made up of gene-lacking proteins.
The dementia that can result from medication side effects or overdoses has been discussed in connection with diagnosis. Certain vitamin deficiencies may also cause dementia. The only one that is not extremely rare in developed countries, however, is Korsakoff's syndrome. This results from thiamine deficiency produced by intense, prolonged alcohol abuse. Yet another potential cause of dementia is deficiency of thyroid hormone; unlike many other dementias, this is usually reversible once adequate amounts of the hormone are available.
In yet other cases, diseases of the kidney or liver may lead to build-up of toxic materials in the blood; dementia then becomes one symptom that these materials have reached poisonous levels. Chronic hypoglycemia (low bloodsugar), often due to disorders of the pancreas, may also impair mental function.
Although both head injuries and brain tumors usually affect only a single sphere of mental activity (and thus, by definition, do not produce dementia) this is not always the case. Prize fighters in particular are likely to have experienced multiple blows to the head, and as a result often suffer from a generalized dementia. Conditions such as near-drowning, in which the brain is starved of oxygen for several minutes, may also result in dementia.
Almost 3% of dementia cases are due to hydrocephalus (literally "water on the brain;" more precisely, an accumulation within the brain of abnormal amounts of cerebrospinal fluid). This usually results from an injury that makes it difficult for the fluid to reach the areas where it is supposed to be reabsorbed into the bloodstream. In the most common form, and the one most easily overlooked, pressure within the brain remains normal despite the fluid build-up. The extra fluid nevertheless distorts the shape of the brain and impairs its function. Installing shunts that allow the fluid to reach its proper place usually cures the dementia.
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W. A. Thomasson