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The four primary symptoms of Parkinson's disease often appear gradually but increase in severity with time. They are:

• Tremor or trembling in hands, arms, legs, jaw, and face
• Rigidity or stiffness of the limbs and trunk
• Slowness of motor movements
• Postural instability or impaired balance and coordination

People with Parkinson's disease may have trouble walking, talking, or completing simple tasks that depend on coordinated muscle movements.

Scientists aren't sure why Parkinson's disease develops. Most scientists think it happens because of a combination of genetic and environmental factors (perhaps a toxin or virus that alters the activities of specific genes) but there is no scientifically sound evidence for this yet. A rare form of Parkinson's disease is the result of an inherited genetic mutation, but the common form of Parkinson's disease is sporadic, meaning that it happens to people randomly for reasons that aren't clear.

Is there any treatment?
There is no cure for Parkinson's disease. Since most of the symptoms are due to the lack of dopamine in the brain, effective medications aim at temporarily replenishing or mimicking dopamine's actions. These drugs - levodopa and the dopamine agonists ropinirole, pramipexole, and pergolide - reduce muscle rigidity, improve speed and coordination of movement, and relieve tremor. In some cases, surgery may be appropriate if the disease doesn't respond to drugs. Early results using deep brain stimulation or transcranial magnetic stimulation to control tremor have been promising. Many doctors recommend physical therapy or muscle-strengthening exercises to help people handle their daily activities.

What is the prognosis?
At present there is no way to predict or prevent Parkinson's disease. Symptoms usually appear after 80 percent or more of the dopamine-producing neurons in the substantia nigra have died, which significantly narrows the window of opportunity for preventive or protective treatments. Medications can relieve symptoms for a period of time but won't slow or stop the natural progression of the disease. The course of the disease varies widely. Some people have mild symptoms for many years, while others have severe symptoms and a quicker progression.

What research is being done?
Hundreds of research studies supported by the NINDS are exploring the basic biomechanisms of Parkinson's disease and looking for ways to stop the degenerative process that causes the disorder. Clinical trials across the country are investigating new medications and therapies to find ways to improve the lives of people with Parkinson's disease. For example:

• Studies are looking at how Lewy bodies - the characteristic dense clumps of abnormal protein that are found in the neurons of people with Parkinson's disease - are involved in the disease process. Some researchers believe Lewy bodies are the cause of neuron death, others see them more as a byproduct of the neurodegenerative process, and still others believe they are a protective mechanism employed by the neuron to lock away abnormal and harmful proteins. Discovering how Lewy bodies work, and why, may open doors to potential interventions that could prevent the cascade of biological events that cause the neurodegeneration of Parkinson's disease.
• Several groups of researchers are exploring new opportunities for drug development by studying neurotrophins, molecules in neurons that support growth and survival. Understanding how neurotrophins work could lead to the development of drugs that stop the degeneration of dopamine cells and heal those that are already damaged. Another avenue of exploration involves neuroprotective substances in cells - molecules that protect neurons from harm. NINDS is currently supporting a series of pilot clinical trials to test the effects of four potential neuroprotectants.
• Cell transplantation is one way of repairing the damage Parkinson's disease causes in the brain. In laboratory experiments, researchers are studying embryonic stem cells, adult stem cells, and other types of promising cells to see if they can be coaxed into turning into dopamine-producing neurons. In studies using animal models, researchers are looking at what these new neurons need to survive, make the proper connections, and become fully functional cells in the brain.