Dementia is a serious brain disorder that interferes with a person's ability to carry out everyday tasks. Because dementia is usually progressive, early signs may be vague and subtle.

• The key feature of dementia is a decline in cognitive functions. These are mental processes such as thinking, reasoning, learning, problem solving, memory, language, and speech.

• Other features that occur frequently in dementia include changes in personality and behavior.

• Generally, these symptoms are not considered dementia unless they have continued unabated for at least 6 months.

• Dementia has many different causes. Some may be reversible, such as certain infections, drug intoxication, and liver diseases. Of the irreversible causes, the most common in older adults is Alzheimer disease.

• Although dementia is frequently linked to old age ("getting senile"), it is not a normal part of aging. Even children with certain degenerative brain disorders can develop dementia.

Memory Loss: Is It Aging, Alzheimer's, or MCI?

Alzheimer disease usually begins with mild, slowly worsening memory loss. Many older people fear that they have Alzheimer disease because they can't find their eyeglasses or remember someone's name.

• These very common problems are most often due to slowing of mental processes with age. It is not clear whether this is a normal part of aging.

• While this is a nuisance, it does not significantly impair a person's ability to learn new information, solve problems, or carry out everyday activities, as Alzheimer disease does.

• Mild cognitive impairment (MCI) is the term used by medical professionals when memory loss is greater than what "normally" occurs with aging, but a person is still able to perform normal daily activities.

Memory loss follows a specific pattern in Alzheimer disease. The losses are mainly in short-term memory. This means that the person has problems remembering recent events.

• The person cannot remember what he did last week or instructions the doctor gave this morning for taking a new medicine.

• This often contrasts sharply with the person's strong ability to remember minor details and events from many years ago.

• The memory loss is followed by many other cognitive and behavioral symptoms. Eventually, over many years, the person loses many mental and physical abilities and requires around-the-clock care.

MCI is a transitional zone between age-related memory loss and early Alzheimer disease. A person is often said to have MCI when he or she has Alzheimer-like memory loss while overall thinking and reasoning skills are maintained.

• The person with MCI is able to think clearly, solve problems, learn new information, and communicate despite relatively minor memory loss.

• Memory loss in MCI is more severe than purely age-related memory loss.

There are other types of MCI, but the type involving short-term memory loss is the most common. Medical professionals call this type "amnestic" MCI. Amnestic has the same root as the word amnesia, meaning memory loss.

We are still learning about MCI. We don't know how common it is, for example.

• About 5 million people in the United States have Alzheimer disease, but we don't know how many have MCI.

• Part of the problem is that the boundaries between age-related memory loss, MCI, and Alzheimer disease have not been clearly defined.

From studies in the brains of people with Alzheimer disease or amnestic MCI, we know that the changes are similar. Therefore, amnestic MCI is generally considered to be due to an early form of Alzheimer disease.

Not everyone with MCI develops full dementia. However, people with MCI are more likely than other elderly people to develop Alzheimer disease.

• A large, three-year study reported last year demonstrated that treatment with the medication Aricept could slightly delay, but not prevent, the transition from MCI to Alzheimer's.

• At the moment, we do not know what factors contribute to progression from MCI to Alzheimer disease.

SOURCES: WebMD Medical Reference from EMedicineHealth: "Possible Early Dementia Overview." Report of the Quality Standards Subcommittee of the American Academy of Neurology, 2001. Petersen et al. The New England Journal of Medicine, 2005; vol 352: pp 2379-2388.