Dementia is caused by damage to or changes in the brain. A variety of conditions can cause dementia, including:

• Injury to the brain caused by tumors, head injury, or strokes.
• Diseases, such as Parkinson's disease, dementia with Lewy bodies, and frontotemporal dementia.
• Long-term alcohol dependence.

After Alzheimer's disease, dementia caused by strokes (vascular dementia) is the most common type of dementia. Many people have mixed types of dementia. Mental function lost to vascular dementia cannot be restored, but future damage may be prevented by reducing the risk for stroke.

Some causes of dementia can be reversed with treatment, but most cannot. Common causes of dementia that cannot be reversed are:

• Parkinson's disease, which is a movement disorder. Dementia is common in people with this condition.
• Dementia with Lewy bodies, which causes protein deposits (Lewy bodies) in brain cells. It can cause short-term memory loss like some other brain diseases, but it can also cause the person to fall often and to see things that aren't there (hallucinations).
• Frontotemporal dementia, a group of diseases that includes Pick's disease. These diseases can cause changes in personality, behavior, or language.
• Severe head injury that caused a loss of consciousness.

Less common causes of dementia that cannot be reversed include:

• Huntington's disease, which is a rare, inherited illness.
• Leukoencephalopathies, which are diseases that affect the deeper, white-matter brain tissue.
• Vascular dementia that may occur in people with long-term high blood pressure or severe hardening of the arteries (atherosclerosis).
• Creutzfeldt-Jakob disease, a rare and fatal condition that destroys brain tissue.
• Brain injuries from accidents or boxing.
• Some cases of multiple sclerosis (MS) or amyotrophic lateral sclerosis (ALS).
• Multiple-system atrophy (a group of degenerative brain diseases affecting speech, movement, and autonomic functions).
• Infections such as mad cow disease and late-stage syphilis. Antibiotics can effectively treat syphilis at any stage, but they cannot reverse the brain damage already done.

Doctors can treat some causes of dementia and restore mental function. These include:

• Underactive thyroid gland (hypothyroidism).
• Vitamin B12 deficiency.
• Heavy-metal poisoning, such as from lead.
• Side effects of medicines or drug interactions.
• Some brain tumors.
• Normal-pressure hydrocephalus, which occurs when fluid builds up in the brain, creating pressure.
• Some cases of chronic alcoholism.
• Some cases of encephalitis, an infection of the brain.
• HIV/AIDS.

Some disorders that cause dementia can run in families. Doctors often suspect an inherited cause if someone younger than 50 has symptoms of dementia. For more information, see the topic Alzheimer's Disease.

It is important to know that memory loss can be caused by conditions other than dementia, such as depression, and that those conditions can be treated. Also, occasional trouble with memory (such as briefly forgetting someone's name) can be a normal part of aging. But if you are worried about memory loss or if a loved one has memory loss that is getting worse, see your doctor.