Parkinson's Disease

The National Institute of Environmental Health Sciences (NIEHS) is one of the lead research agencies studying the environmental causes of Parkinson’s disease, a neurodegenerative disease thought to be caused by the interaction of genes and environment. NIEHS works closely with other institutes and centers of the National Institutes of Health (NIH), as well as partners and researchers across the country, to look at every aspect of Parkinson’s.

What is Parkinson's Disease?

Parkinson's disease is a progressive neurodegenerative disease, the second most common disorder of this type after Alzheimer's disease. It progresses slowly as small clusters of dopaminergic neurons in the midbrain die. The gradual loss of these neurons results in reduction of a critical neurotransmitter called dopamine, a chemical responsible for transmitting messages to parts of the brain that coordinate muscle movement.

Parkinson's patients have less dopamine.

Studies have shown that the symptoms of Parkinson's usually appear when 50 percent or more of the dopamine neurons in the midbrain have been lost. Symptoms begin gradually and typically worsen over time.

How many people are affected by Parkinson's Disease?

It is difficult to know exactly how many people have Parkinson's disease, since there is no national registry, but it is estimated that at least 500,000 people in the U.S. currently have the disease.¹ The average age of onset is about 60, and prevalence is increasing as the population ages.

The majority of people diagnosed have late-onset sporadic Parkinson's, which does not have a clear genetic cause. About 10 percent have early-onset Parkinson's that often begins before the age of 50. There may be a genetic cause associated with many of these patients. Parkinson's strikes people of all races, ethnic groups, nationalities, and income levels. Actor Michael J. Fox, singer Linda Ronstadt, former U.S. Attorney General Janet Reno, and boxer Muhammad Ali, are among the celebrities living with Parkinson's.

What causes Parkinson's Disease?

The exact cause of Parkinson's disease is unknown. Most researchers agree that the disease is caused by both genetic and environmental factors, and by interactions among these factors.

A full understanding of Parkinson's risk requires integrated efforts to study both genetic and environmental factors. If environmental exposures can be identified, it may lead to new targets for prevention and intervention.

What are the symptoms of Parkinson's Disease?

Common motor symptoms include:

• Tremors or shaking in hands, arms, legs, jaw, and face
• Rigidity or stiffness of limbs and trunk
• Slowness of movement
• Difficulties with balance, speech, and coordination

There are also nonmotor symptoms which may develop years before the onset of motor problems. These may include:

• Poor sense of smell
• Constipation
• Depression
• Cognitive impairment
• Fatigue