Definition of Alzheimer's disease:

Dementia is a loss of brain function that occurs with certain diseases. Alzheimer's disease (AD), is one form of dementia that gradually gets worse over time. It affects memory, thinking, and behavior.

Memory impairment, as well as problems with language, decision-making ability, judgment, and personality, are necessary features for the diagnosis.

Causes, incidence, and risk factors:

Age and family history are risk factors for AD.

• As you get older, your risk of developing AD goes up. However, developing Alzheimer's disease is not a part of normal aging.
• Having a close blood relative, such as a brother, sister, or parent who developed AD increases your risk.
• Having certain combination of genes for proteins that appear to be abnormal in Alzheimer's disease also increases your risk.

Other risk factors that are not as well proven include:

• Longstanding high blood pressure
• History of head trauma
• Female gender

There are two types of AD -- early onset and late onset.

• In early onset AD, symptoms first appear before age 60. Early onset AD is much less common than late onset. However, it tends to progress rapidly. Early onset disease can run in families. Several genes have been identified.
• Late onset AD, the most common form of the disease, develops in people age 60 and older. Late onset AD may run in some families, but the role of genes is less clear.

The cause of AD is not entirely known, but is thought to include both genetic and environmental factors. A diagnosis of AD is made when certain symptoms are present, and by making sure other causes of dementia are not present.

The only way to know for certain that someone has AD is to examine a sample of their brain tissue after death. The following changes are more common in the brain tissue of people with AD:

• "Neurofibrillary tangles" (twisted fragments of protein within nerve cells that clog up the cell)
• "Neuritic plaques" (abnormal clusters of dead and dying nerve cells, other brain cells, and protein)
• "Senile plaques" (areas where products of dying nerve cells have accumulated around protein).

When nerve cells (neurons) are destroyed, there is a decrease in the chemicals that help nerve cells send messages to one another (called neurotransmitters). As a result, areas of the brain that normally work together become disconnected.

The buildup of aluminum, lead, mercury, and other substances in the brain is no longer believed to be a cause of AD.