Parkinson's disease is a progressive neurological disorder that results from degeneration of neurons in a region of the brain that controls movement. This degeneration creates a shortage of the brain signaling chemical (neurotransmitter) known as dopamine, causing the movement impairments that characterize the disease. Parkinson's disease was first formally described in "An Essay on the Shaking Palsy," published in 1817 by a London physician named James Parkinson, but it has probably existed for many thousands of years. Its symptoms and potential therapies were mentioned in the Ayurveda, the system of medicine practiced in India as early as 5000 BC, and in the first Chinese medical text, Nei Jing, which appeared 2500 years ago.

Often, the first symptom of Parkinson's disease is tremor (trembling or shaking) of a limb, especially when the body is at rest. The tremor often begins on one side of the body, frequently in one hand. Other common symptoms include slow movement (bradykinesia), an inability to move (akinesia), rigid limbs, a shuffling gait, and a stooped posture. People with Parkinson's disease often show reduced facial expressions and speak in a soft voice. Occasionally, the disease also causes depression, personality changes, dementia, sleep disturbances, speech impairments, or sexual difficulties. The severity of Parkinson's symptoms tends to worsen over time.

In the United States, at least 500,000 people are believed to suffer from Parkinson's disease, and about 50,000 new cases are reported annually. These figures are expected to increase as the average age of the population increases. The disorder appears to be slightly more common in men than women. The average age of onset is about 60. Both prevalence and incidence increase with advancing age; the rates are very low in people under 40 and rise among people in their 70s and 80s. Parkinson's disease is found all over the world. The rates vary from country to country, but it is not clear whether this reflects true ethnic and/or geographic differences or discrepancies in data collection.

Although there are many theories about the cause of Parkinson's disease, none has ever been proved. Researchers have reported families with apparently inherited Parkinson's for more than a century. However, until recently, the prevailing theory held that one or more environmental factors caused the disease. Severe Parkinson's-like symptoms have been described in people who took an illegal drug contaminated with the chemical MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and in people who contracted a particularly severe form of influenza during an epidemic in the early 1900s. Recent studies of twins and families with Parkinson's have suggested that some people have an inherited susceptibility to the disease that may be influenced by environmental factors. The strong familial inheritance of the chromosome 4 gene is the first evidence that a gene alteration alone may lead to Parkinson's disease in some people.

Parkinson's disease is usually diagnosed by a neurologist who can evaluate symptoms and their severity. There is no test that can clearly identify the disease. Sometimes people with suspected Parkinson's disease are given anti-Parkinson's drugs to see if they respond. Other tests, such as brain scans, can help doctors decide if a patient has true Parkinson's disease or some other disorder that resembles it. Microscopic brain structures called Lewy bodies, which can be seen only during an autopsy, are regarded as a hallmark of classical Parkinson's. Autopsies have uncovered Lewy bodies in a surprising number of older persons without diagnosed Parkinson's -- 8% of people over 50, almost 13% of people over 70, and almost 16% of those over 80, according to one study. As a result, some experts believe Parkinson's disease is something of an "iceberg; phenomenon," lurking undetected in as many as 20 people for each known Parkinson's patient. A few researchers contend that almost everyone would develop Parkinson's eventually if they lived long enough.

There is no cure for Parkinson's disease. Many patients are only mildly affected and need no treatment for several years after the initial diagnosis. When symptoms grow severe, doctors usually prescribe levodopa (L-dopa), which helps replace the brain's dopamine. Sometimes doctors prescribe other drugs that affect dopamine levels in the brain. In patients who are very severely affected, a kind of brain surgery known as pallidotomy has reportedly been effective in reducing symptoms. Another kind of brain surgery, in which healthy dopamine-producing tissue is transplanted into the brain, is also being tested. Finally, researchers are trying to identify substances that will prevent dopamine-producing brain cells from dying.