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Medical Content Created by the Faculty of the
Harvard Medical School

Parkinson's Disease

What Is It?

Parkinson's disease is a disease of the central nervous system that causes problems with body motions, including tremor (shakiness), rigidity (muscle stiffness), slowed body movements, unstable posture and difficulty walking. It happens when nerve cells (neurons) in a part of the brain called the substantia nigra gradually die. These cells normally produce a chemical called dopamine that helps to relay messages between areas of the brain that control body movement. The death of cells in this area of the brain leads to abnormally low levels of dopamine, which makes it difficult for a person with Parkinson's disease to control muscle tension and muscle movement, both at rest and during periods of activity.

Parkinson's disease affects about one million people in the United States and more than 4 million people worldwide. It usually occurs in middle age, typically beginning around age 60. However, about 5% of patients have early onset Parkinson's disease and are younger than 40 years old when symptoms begin. It affects Europeans and North Americans more often than Asians or Africans. It is more common in men than in women.

So far, scientists have not determined why some people develop Parkinson's disease and others do not. The genetics are complex and the influence of family history depends on the age when the disease started. A person who has a parent or sibling that was diagnosed with the disease at a younger age is at higher risk of getting Parkinson's, but not if the affected family member was older when diagnosed.