Dementia is not a specific disease. It is a descriptive term for a collection of symptoms that can be caused by a number of disorders that affect the brain. People with dementia have significantly impaired intellectual functioning that interferes with normal activities and relationships.
They also lose their ability to solve problems and maintain emotional control, and they may experience personality changes and behavioral problems such as agitation, delusions, and hallucinations.
While memory loss is a common symptom of dementia, memory loss by itself does not mean that a person has dementia. Doctors diagnose dementia only if two or more brain functions - such as memory, language skills, perception, or cognitive skills including reasoning and judgment - are significantly impaired without loss of consciousness.
There are many disorders that can cause dementia. Some, such as AD, lead to a progressive loss of mental functions. But other types of dementia can be halted or reversed with appropriate treatment.
With AD and many other types of dementia, disease processes cause many nerve cells to stop functioning, lose connections with other neurons, and die. In contrast, normal aging does not result in the loss of large numbers of neurons in the brain.
Symptoms of dementia vary depending on the cause and the area of the brain that is affected. Memory loss is usually the earliest and most noticeable symptom.
Other key symptoms of dementia include:
- Having difficulty recalling recent events.
- Not recognizing familiar people and places.
- Having trouble finding the right words to express thoughts or name objects.
- Having difficulty performing calculations.
- Having problems planning and carrying out tasks, such as balancing a checkbook, following a recipe, or writing a letter.
- Having trouble exercising judgment, such as knowing what to do in an emergency.
- Having difficulty controlling moods or behaviors. Depression is common, and agitation or aggression may occur.
- Not keeping up personal care such as grooming or bathing.
Some types of dementia cause key symptoms:
- People who have dementia with Lewy bodies often have highly detailed visual hallucinations. They may fall frequently.
- The first symptoms of frontotemporal dementia may be personality changes or unusual behavior. People with this condition may not express any caring for others, or they may say rude things, expose themselves, or make sexually explicit comments.
- Symptoms of dementia that come on suddenly suggest vascular dementia or possibly delirium-short-term confusion caused by a new or worsening illness.
All forms of dementia result from the death of nerve cells and/or the loss of communication among these cells. The human brain is a very complex and intricate machine and many factors can interfere with its functioning. Researchers have uncovered many of these factors, but they have not yet been able to fit these puzzle pieces together in order to form a complete picture of how dementias develop.
Many types of dementia, including AD, Lewy body dementia, Parkinson's dementia, and Pick's disease, are characterized by abnormal structures called inclusions in the brain. Because these inclusions, which contain abnormal proteins, are so common in people with dementia, researchers suspect that they play a role in the development of symptoms. However, that role is unknown, and in some cases the inclusions may simply be a side effect of the disease process that leads to the dementia.
Genes clearly play a role in the development of some kinds of dementia. However, in AD and many other disorders, the dementia usually cannot be tied to a single abnormal gene. Instead, these forms of dementia appear to result from a complex interaction of genes, lifestyle factors, and other environmental influences.
Researchers have identified several genes that influence susceptibility to AD. Mutations in three of the known genes for AD - genes that control the production of proteins such as amyloid precursor protein (APP), presenilin 1, and presenilin 2 - are linked to early-onset forms of the disease.
Variations in another gene, called apolipoprotein E (apoE), have been linked to an increased risk of late-onset AD. The apoE gene does not cause the disease by itself, but one version of the gene, called apoE epsilon4 (apoE E4), appears to increase the risk of AD.
People with two copies of the apoE E4 gene have about ten times the risk of developing AD compared to people without apoE E4. This gene variant seems to encourage amyloid deposition in the brain. One study also found that this gene is associated with shorter survival in men with AD. In contrast, another version of the apoE gene, called apoE E2, appears to protect against AD.
Studies have suggested that mutations in another gene, called CYP46, may contribute to an increased risk of developing late-onset sporadic AD. This gene normally produces a protein that helps the brain metabolize cholesterol.
Scientists are trying to determine how beta amyloid influences the development of AD. A number of studies indicate that the buildup of this protein initiates a complex chain of events that culminates in dementia. One study found that beta amyloid buildup in the brain triggers cells called microglia, which act like janitors that mop up potentially harmful substances in the brain, to release a potent neurotoxin called peroxynitrite. This may contribute to nerve cell death in AD.
Another study found that beta amyloid causes a protein called p35 to be split into two proteins. One of the resulting proteins triggers changes in the tau protein that lead to formation of neurofibrillary tangles. A third study found that beta amyloid activates cell-death enzymes called caspases that alter the tau protein in a way that causes it to form tangles. Researchers believe these tangles may contribute to the neuron death in AD.
Vascular dementia can be caused by cerebrovascular disease or any other condition that prevents normal blood flow to the brain. Without a normal supply of blood, brain cells cannot obtain the oxygen they need to work correctly, and they often become so deprived that they die.
The causes of other types of dementias vary. Some, such as CJD and GSS, have been tied to abnormal forms of specific proteins. Others, including Huntington's disease and FTDP-17, have been linked to defects in a single gene. Post-traumatic dementia is directly related to brain cell death after injury. HIV-associated dementia is clearly tied to infection by the HIV virus, although the exact way the virus causes damage is not yet certain.
For other dementias, such as corticobasal degeneration and most types of frontotemporal dementia, the underlying causes have not yet been identified.
While treatments to reverse or halt disease progression are not available for most of the dementias, patients can benefit to some extent from treatment with available medications and other measures, such as cognitive training.
Drugs to specifically treat AD and some other progressive dementias are now available and are prescribed for many patients. Although these drugs do not halt the disease or reverse existing brain damage, they can improve symptoms and slow the progression of the disease.
This may improve the patient's quality of life, ease the burden on caregivers, and/or delay admission to a nursing home. Many researchers are also examining whether these drugs may be useful for treating other types of dementia.
Many people with dementia, particularly those in the early stages, may benefit from practicing tasks designed to improve performance in specific aspects of cognitive functioning. For example, people can sometimes be taught to use memory aids, such as mnemonics, computerized recall devices, or note taking.
Behavior modification - rewarding appropriate or positive behavior and ignoring inappropriate behavior - also may help control unacceptable or dangerous behaviors.