Dementia is a mental disorder that affects your ability to think, speak, reason, remember and move. Many types of dementia exist. Some are progressive and permanent. That is, they get worse with time and cannot be cured. Other types can be treated and reversed.

A person with mild dementia may not realize he or she just asked the same question or may have trouble finding the right words to communicate. He or she may recognize faces but struggle to retrieve names. A person with mild dementia may also have trouble with more complex tasks, such as balancing a checkbook.

An individual with moderate dementia may struggle to name common objects or express needs and desires. He or she may be unable to learn new things and may become lost when away from home. As the dementia progresses, personality and behavior changes become more noticeable. He or she may become silent and withdrawn or verbally and physically aggressive. The person may not immediately recognize his or her grown children.

With more advanced dementia, the individual becomes confused and may have trouble operating routine equipment, such as the stove or a car. He or she may need help washing, dressing, eating and using the toilet. As the dementia progresses, the person may have difficulty swallowing, walking, standing or even sitting.

Because many conditions can cause dementia, it's important to consult a doctor so treatment, if available, can begin as early as possible.

Many disorders can cause dementia. The three most common are Alzheimer's disease, Lewy body dementia and vascular dementia. These causes can overlap.

Alzheimer's disease. Alzheimer's is the most common cause of dementia. Alzheimer's disease involves a loss of nerve cells in the areas of the brain vital to memory and other mental functions. This loss is associated with the development of abnormal clumps and tangles of protein in the brain. These are known as amyloid plaques and neurofibrillary tangles. Individuals with Alzheimer's disease also have lowered levels of brain chemicals that carry messages back and forth between nerve cells.

The first sign of Alzheimer's disease is usually forgetfulness. As the disease progresses, it affects language, reasoning and understanding. Eventually, a person with Alzheimer's may become withdrawn, anxious or aggressive.

The precise cause of Alzheimer's disease is unknown. Research indicates the following risk factors, but the disease usually results from a combination of factors.

• Age. Most people with Alzheimer's disease are older than 65. The average age at diagnosis is 80. Alzheimer's disease is uncommon in middle age and extremely rare in young adults.

• Heredity. A family history of Alzheimer's disease, especially in a first-degree relative, such as a parent or sibling, is associated with an increased risk of the disease. But even in families in which several people have had Alzheimer's, most family members don't develop it.

Alzheimer's disease is irreversible, but some medications may be able to slow its progression.

Lewy body dementia. Lewy bodies are microscopic protein deposits found in deteriorating nerve cells. They often appear in areas deep within the brain and are associated with the tremor and rigidity of Parkinson's disease. When widespread within the brain, Lewy bodies cause signs and symptoms similar to those of Alzheimer's disease. Unlike Alzheimer's, though, Lewy body dementia may affect speed of thinking, attention and concentration, and visual spatial abilities more severely than memory and language. It can also cause hallucinations and large fluctuations in degree of alertness.

Although there is no cure for Lewy body dementia, some people do receive some benefit from the same drugs used to treat Alzheimer's disease.

Vascular dementia. This type of dementia results either from extensive narrowing and blockage of the arteries that supply blood to the brain or from strokes caused by an interruption of blood flow to the brain. The initial onset of symptoms usually is abrupt, but sometimes the disease progresses slowly, making it difficult to distinguish it from Alzheimer's disease. It's common for vascular dementia to cause problems with thinking, language, walking, bladder control and vision. Preventing additional strokes by treating underlying diseases, such as high blood pressure, may halt the progression of vascular dementia.

Other diseases that directly affect brain cells and that may result in progressive dementia include:

Parkinson's disease. People with Parkinson's disease may experience stiffness of limbs, shaking at rest (tremor), speech impairment and a shuffling gait. Some people with Parkinson's develop dementia late in the disease. This type of dementia begins with a slowing of the thought processes and difficulty concentrating and eventually progresses to confusion.

Huntington's disease. This disease stems from an inherited disorder that causes certain nerve cells within the brain to waste away. As the disorder progresses, a person with Huntington's develops personality changes and a decline in intellect, memory, speech and judgment. Dementia commonly develops in the later stages of the disease.

Creutzfeldt-Jakob disease. This rare and fatal brain disorder can occur in young or middle-aged people. Creutzfeldt-Jakob disease is thought to be the result of prions, infectious agents that can transform normal protein molecules into transmissible, deadly ones. The earliest signs and symptoms of the disease are usually memory impairment and behavioral changes. The disease usually progresses rapidly with mental deterioration, muscle jerking (involuntary movements), weakness in the arms or legs, blindness and eventually coma.

Frontotemporal dementia. This uncommon brain disorder is characterized by disturbances in behavior and personality, and eventually memory loss. It is relentless in its progression and may ultimately lead to language impairment, erratic behavior and dementia. Pick's disease is one form of frontotemporal dementia.

Although the symptoms of dementia caused by these illnesses are sometimes improved with treatment, the underlying dementia cannot be cured.

For some medical conditions that cause dementia, some degree of recovery is possible and some treatments may even lead to a cure. These conditions include:

• Hydrocephalus
• Brain tumors
• Infections of the brain, including encephalitis, meningitis, HIV/AIDS and syphilis.

In addition, appropriate treatment may reverse dementia caused by:

• Disorders of the thyroid, kidney, liver and pancreas.
• A shortage of oxygen due to lung disease
• Overuse or abuse of medications, especially sleeping pills, tranquilizers and antidepressants
• A deficiency in vitamins, such as B-12
• Chronic alcoholism
• Drug abuse

Some conditions produce signs and symptoms similar to those of dementia, particularly in older adults. Two conditions sometimes mistaken for dementia are:

• Depression. A mental disorder, depression can cause difficulty in remembering, thinking clearly and concentrating. Sometimes depression occurs in conjunction with dementia. In those cases emotional and intellectual deterioration can be especially severe.

• Delirium. Unlike dementia, delirium is a severe but temporary state of mental confusion. It tends to be most common in older adults who have heart or lung disease, infections, poor nutrition, medication interactions or hormone disorders. A person who experiences the sudden onset of disorientation, loss of mental skills or loss of consciousness is more likely to have delirium rather than dementia.