Dementia, because it is a group of symptoms rather than one disease, has many different causes. One way doctors classify dementias is by the location in the brain of the specific disease that is causing the dementia. If the disease is in the cerebral cortex, it is called cortical. If it is somewhere in the brain below the cortex, it is called subcortical. With treatment, some causes are reversible to some extent. Others are irreversible and progressive, meaning that the symptoms will get worse over time even with treatment. The most common form of dementia, Alzheimer's disease (AD), is responsible for about half of all cases of dementia. It is irreversible, progressive, and causes damage in both cortical and subcortical areas of the brain.

For someone with Alzheimer's disease, what begins as occasional memory loss soon progresses into more serious cognitive difficulties and behavioral changes that can leave the person confused and unable to care for herself or himself.

Whole conversations may be entirely forgotten. Other cognitive skills then begin to weaken. For example, the person may become disoriented and not know what day it is, where he is, or how to get home, even if he already is home. There may be problems with language as the person struggles to find the words they want to say.

Later, the person may not recognize familiar people, even his own children. Inability to plan or organize may mean that bills do not get paid and appointments get missed. He may believe others are stealing from him, or that family members have been replaced by strangers. He may become verbally or physically aggressive or believe that things that never happened actually did happen. He may stop taking care of himself, forgetting to bathe or eat. He may lose the ability to do basic everyday skills such as cooking, cleaning, or taking care of pets. Eventually, he is likely to need complete, round-the-clock care from others.

These symptoms of AD are the result of significant changes in the brain. When Dr. Alois Alzheimer, for whom AD was named, first began to study the disease in the early 1900s, he learned a great deal about it from microscopic examinations of the brains of people who had died from early onset dementia. He noted dark "peculiar material"(12) both inside and around the neurons in the brain. Later researchers discovered that the dark material consisted of abnormal clumps, or aggregates, of proteins that are normally found in the brain. The protein aggregates were found to be toxic to nerve cells and caused them to malfunction and then to die. Other kinds of protein aggregates are known to cause other diseases besides AD, such as the muscle- wasting disease amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig's disease) and the blood disorder sickle cell disease.

AD is the most common form of dementia, but it is by no means the only kind. After Alzheimer's disease, Lewy body dementia is the second most-common kind of dementia in elderly people. It is also one of the most frightening types of dementia and can be very difficult to deal with. Like AD, Lewy body dementia is also caused by abnormal deposits of protein in the nucleus of the neurons. These deposits are called Lewy bodies, after Dr. Friedrich Lewy, who first discovered them in 1912. Lewy bodies are smooth, round clumps of protein that appear mainly in the cerebral cortex but are also found in deeper parts of the affected brain. No one knows yet what causes the bodies to form in the brain, but, like other dementias, it is thought to be a genetic malfunction.

Lewy body dementia can occur by itself, or it can occur along with AD or another brain disease called Parkinson's disease. Lewy bodies have been found in the brains of people with these two diseases, and some of the symptoms of Lewy body dementia are very similar to their symptoms, such as the memory loss of AD or the muscle stiffness and shakiness of Parkinson's disease.

One of the common early signs of Lewy body dementia is detailed visual hallucinations.
The person with this type of dementia may also have hallucinations involving other senses, such as hearing sounds or smelling odors that are not there. They may also have delusions-hinking things that cannot possibly be true. The symptoms of Lewy body dementia tend to come and go, and the person may have "good" days, when he appears normal, followed by "bad" days, when he is confused, disoriented, and having hallucinations or delusions. At this time, there is no cure for Lewy body dementia.

Vascular Dementia
After AD, the second most-common form of dementia in people of all ages is vascular dementia (VaD). It accounts for about 20 to 40 percent of all dementias, and more than a million Americans have it. It is most common in people who also have high blood pressure, high cholesterol, heart disease, diabetes, and alcoholism, and in people who smoke. It is more common in men than in women.

Symptoms of vascular dementia can be very similar to AD, but the cause is very different. "Alzheimer's is more closely related to a build-up of plaques and tangles in the brain," says Dr. Murali Doraiswamy of Duke University Medical Center. "In vascular dementia it's more a blockage in the brain."15 Vascular dementia results when the arteries that carry blood to the brain become blocked with deposits of a fatty substance called cholesterol, dead blood cells, and other substances, which interfere with the brain's blood supply. The deposits also cause the walls of the arteries to become thick and rigid, a condition called atherosclerosis, once called "hardening of the arteries." These changes can slow or even completely cut off blood flow to the part of the brain supplied by the clogged artery. Blockage of an artery is called an infarction, so VaD is also sometimes called multi-infarct dementia.

Other Causes of Dementia
There are many other disorders that can cause dementia or include symptoms of dementia. Many of them are genetic and run in families or occur mostly in certain parts of the world. Some illnesses of other body systems can also cause dementia symptoms, such as diseases of the thyroid, liver, lungs, pancreas, or kidneys. People with type 2 diabetes, a disease of sugar metabolism, are at a much higher risk for dementia. Brain infections, tumors, and head injuries can all cause dementia symptoms that may or may not be reversible, depending on the cause and severity. The long-term abuse of alcohol and illegal drugs, as well as exposure to toxic chemicals (especially lead) can damage the brain and cause irreversible dementia. Even nutritional deficiencies, especially of vitamin B, can cause dementia symptoms if they are not treated.