Diane Ruby's whole body shook when she was involved in a fender bender seven years ago. It was nerves, her doctor told her. But her right hand never really stopped trembling. When her right leg started to drag two years later, she knew something was terribly wrong. Her friends convinced her to see another doctor. His diagnosis: Parkinson's disease.

Parkinson's is a central-nervous-system disease that affects muscle movement. One out of 100 people over age 60 is afflicted. But 10 to 20 percent of its victims are diagnosed before age 50. Some, like Diane Ruby, 40, may find that their symptoms first appear after a major stressful event, such as an accident, surgery or even childbirth.

Initial symptoms may be subtle: perhaps a little shakiness or general slowing down. Gradually, however, as the disease progresses, walking and standing up from a sitting position become increasingly difficult. Hand tremors may complicate simple tasks like writing or buttoning clothes. In fact, it's these hand tremors that often send Parkinson's sufferers to their doctor for diagnosis.

The disease is caused by damage to brain cells in an area deep in the center of the brain known as the substantia nigra. These cells produce a chemical, dopamine, that's essential for the brain to send messages to the muscles. As the cells die and dopamine levels drop, muscle control suffers. Exactly why these brain cells die is not known. Many researchers believe it's a combination of aging, genes and environmental triggers.

As we age, some of these cells die naturally. However, in Parkinson's disease, the cells are lost faster and earlier than normal. Most research on twins seems to indicate that genetics does not play a role. But studies at the Clinical Research Institute of Montreal suggest that people who lack certain liver enzymes that help neutralize toxins may be at risk.

The Canadian researchers have observed that more cases of Parkinson's are found in commercial agricultural areas with high pesticide use. They believe that certain chemicals in insect and weed killers may cause "clinically silent damage" to the brain. When that damage is followed by the normal cell loss of aging, Parkinson's symptoms appear.

Intrigued by these studies, U.S. researchers plan to conduct further work with liver enzymes.