Dementia Health Article

Definition

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.

Description

The definition of dementia has become more inclusive over the past several decades. Whereas earlier descriptions of dementia emphasized memory loss, the last three editions of the professional's diagnostic handbook, Diagnostic and Statistical Manual of Mental Disorders(also known as the DSM) define dementia as an overall decline in intellectual function, including difficulties with language, simple calculations, planning and judgment, and motor (muscular movement) skills as well as loss of memory. Although dementia is not caused by aging itself— most researchers regard it as resulting from injuries, infections, braindiseases, tumors, or other disorders— it is quite common in older people. The prevalence of dementia increases rapidly with age; it doubles every five years after age 60. Dementia affects only 1% of people aged 60–64 but 30%–50% of those older than 85. About four to five million persons in the United States are affected by dementia as of 2002. Surveys indicate that dementia is the condition most feared by older adults in the United States.

Causes

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.

Genetic factors in dementia

Genetic factors play a role in several types of dementia, but the importance of these factors in the development of the dementia varies considerably. Alzheimer's disease (AD) is known, for example, to have an autosomal (non-sex-related) dominant pattern in most early-onset cases as well as in some late-onset cases, and to show different degrees of penetrance (frequency of expression) in late-life cases. Moreover, researchers have not yet discovered how the genes associated with dementia interact with other risk factors to produce or trigger the dementia. One non-genetic risk factor presently being investigated is toxic substances in the environment.

EARLY-ONSET ALZHEIMER'S DISEASE.In early-onset AD, which accounts for 2%–7% of cases of AD, the symptoms develop before age 60. It is usually caused by an inherited genetic mutation. Early-onset AD is also associated with Down syndrome, in that persons with trisomy 21 (three forms of human chromosome 21 instead of a pair) often develop early-onset AD.

LATE-ONSET ALZHEIMER'S DISEASE.Recent research indicates that late-onset Alzheimer's disease is a polygenic disorder; that is, its development is influenced by more than one gene. It has been known since 1993 that a specific form of a gene (the APOE gene) on human chromosome 19 is a genetic risk factor for late-onset AD. In 1998 researchers at the University of Pittsburgh reported on another gene that controls the production of bleomycin hydrolase (BH) as a second genetic risk factor that acts independently of the APOE gene. In December 2000, three separate research studies reported that a gene on chromosome 10 that may affect the processing of a protein (called amyloid-beta protein) is also involved in the development of late-onset AD. When this protein is not properly broken down, a starchy substance builds up in the brains of people with AD to form the plaques that are characteristic of the disease.

MULTI-INFARCT DEMENTIA (MID).While the chief risk factors for MID are high blood pressure, advanced age, and male sex, there is an inherited form of MID called CADASIL, which stands for cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy. CADASIL can cause psychiatric disturbances and severe headaches as well as dementia.

FRONTAL LOBE DEMENTIAS.Researchers think that between 25% and 50% of cases of frontal lobe dementia involve genetic factors. Pick's dementia appears to have a much smaller genetic component than FLD. It is not yet known what other risk factors combine with inherited traits to influence the development of frontal lobe dementias.

FAMILIAL BRITISH DEMENTIA (FBD).FBD is a rare autosomal dominant disorder that was first reported in the 1940s in a large British family extending over nine generations. FBD resembles Alzheimer's in that the patient develops a progressive dementia related to amyloid deposits in the brain. In 1999, a mutated gene that produces the amyloid responsible for FBD was discovered on human chromosome 13. Studies of this mutation may yield further clues to the development of Alzheimer's disease as well as FBD itself.

CREUTZFELDT-JAKOB DISEASE.Although Creutzfeldt-Jakob disease is caused by a prion, researchers think that 5%–15% of cases may have a genetic component.