What Is Parkinson's Disease?

Actor Michael J. Fox has battled Parkinson's Disease for years
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(CNN) -- Robin Williams was in the early stages of Parkinson's disease when he died this week, his wife says. Williams was found dead in his Northern California home Monday after what investigators suspect was a suicide by hanging.

What is Parkinson's disease?

Parkinson's disease is a "progressive disorder of the nervous system," according to the Mayo Clinic, that primarily affects a patient's movement. It often starts with a small tremor in the hand or muscle stiffness and gets worse over time. There is no test for Parkinson's, so it is occasionally misdiagnosed.

Parkinson's patients often have trouble walking and talking. Symptoms include slowness of movement, a loss of balance and slurred speech. With Parkinson's disease, "you may have a decreased ability to perform unconscious movements, including blinking, smiling or swinging your arms when you walk," the Mayo Clinic says. The symptoms are often worse on one side of the body.

Whom does it affect?

Approximately 1 million people have Parkinson's disease in the United States, according to the Parkinson's Disease Foundation. Men are more likely to get it than women, and it usually affects people over 50.

What causes Parkinson's disease?

Your body uses a chemical called dopamine to control movement, according to the National Institutes of Health. Dopamine is produced by neurons in the brain, which begin to die with Parkinson's disease. With fewer live cells, a dopamine deficit occurs, causing abnormal brain activity, which in turn causes movement issues.

Scientists don't know what exactly causes these cells to start to deteriorate but believe it's a combination of genes and environmental causes.

Can you treat it?

There is no cure for Parkinson's disease, according to the National Institutes of Health, but medications can help patients cope with symptoms.

A medicine called levodopa is often given to patients to help their brains make more dopamine. It's often prescribed with carbidopa, which helps bring the levodopa into the brain.

A recent study done at Harvard University found that patients with Parkinson's improved after researchers transplanted tissue from fetal dopamine cells into their brains. Patients with severe symptoms experienced 50% fewer symptoms in the years after surgery. People who had been taking medication to control their Parkinson's but found that the medicine no longer worked also saw significant improvements after surgery.

Scientists are also experimenting with deep brain stimulation.

