Parkinson's disease is the most common of the major movement disorders, a category of conditions in which a malfunction in the nervous system affects communication between the brain and the muscles. Other common movement disorders include dystonia, characterized by involuntary muscle spasms, and essential tremor, a neurological condition in which part of the body shakes or trembles.
Parkinson's disease is a chronic, progressive neurological disorder that was called "shaking palsy" when it was first described by James Parkinson in 1817. The symptoms include tremor, stiffness, difficulty moving, and problems with walking and balance. Parkinson's disease may be difficult to diagnose in the early stages, because the symptoms begin gradually and progress over time.
There isn't a cure for Parkinson's, but treatments exist to help control symptoms. People with the condition can live a normal life span, although they may develop complications such as pneumonia in the later stages and are at heightened risk of falling or choking.
According to the National Institutes of Health, Parkinson's affects at least 500,000 people in the United States, and some estimates put the number much higher; the Parkinson's Disease Foundation estimates that as many as 1 million people are affected. Some 50,000 new cases are diagnosed each year, a number that is expected to rise as the population ages. Most cases begin between the ages of 50 and 65, with the average age of onset about 60—though younger people are affected, too.
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Parkinson's disease occurs when, for unknown reasons, nerve cells that produce dopamine in the brain die. Dopamine is a "neurotransmitter," which means that it carries messages between cells. The region of the brain affected—the basal ganglia—plays a crucial role in regulating smooth motor function such as body posture, muscle tone, and muscle movements. The basal ganglia and their connections are a group of gray matter nuclei deep within the brain. Examples include the putamen, globus pallidus, subthalamic nucleus, substantia nigra, and caudate nucleus.
In Parkinson's disease, the number of dopamine-producing cells in the substantia nigra declines. The falloff in dopamine makes it difficult for an individual to control muscle movements and leads to symptoms such as slow movements, tremor, and rigidity.
On autopsy, collections of proteins inside the nerve cells, known as Lewy bodies, are seen in the midbrain of people with Parkinson's disease. These are a sign of cell death. How Lewy bodies form and the role they play in the neurodegenerative process are poorly understood, but they are a hallmark of Parkinson's disease. Lewy bodies are also linked with Lewy body dementia, a disorder whose symptoms are similar to those of Parkinson's patients (slow movements, rigidity, tremor, shuffling gait) and also Alzheimer's patients (confusion, loss of memory). Researchers do not yet understand whether Lewy body dementia is a distinct disorder or possibly a variant of Alzheimer's or Parkinson's. Hallucinations early in the course of a patient's decline are another sign supporting the diagnosis of Lewy body dementia.
Parkinson's disease occurs when dopamine-producing nerve cells die in a region of the brain that plays a critical role in smooth motor movements. The underlying causes for the brain cell death are unknown, though some interplay of environmental factors and genetic susceptibility is thought to be responsible. Because people who live in rural areas and drink well water seem to have a higher incidence of the disease, for example, researchers have explored whether exposure to pesticides might play a role. Exposure to heavy metals such as copper has also been associated with a higher incidence of the disease.
In a small number of cases, genetic mutations are responsible for the disease. Flawed copies of the parkin gene, for instance, allow a buildup of defective proteins in brain cells that may contribute to neuron death in a small number of people. In addition, new research suggests that a defect in a different gene that influences the level of phosphates in brain cells may be responsible for about a third of all cases of Parkinson's in Jews of eastern European descent as well as in a significant percentage of Arabs and North Africans. Other genes for Parkinson's disease have been identified as well.
The reasons Parkinson's disease develops are not well understood, but they are believed to be an interplay of genetic and environmental factors. Having a first degree relative with Parkinson's disease may double the risk, for example. Mutations in the parkin gene allow a buildup of proteins in brain cells that may contribute to neuron death in a small number of people, and a defect in a different gene appears to play a role in many cases of Parkinson's in Jews of eastern European descent and in Arabs and North Africans. In addition, exposure to heavy metals (such as copper), pesticides, and well water has been associated with increased incidence of the disease. On the other hand, cigarette smoking and caffeine intake have been associated with a decreased risk.
Most people who develop Parkinson's are in their 60s, although younger people also are affected. The disease is slightly more prevalent in males and in Caucasians.
Last reviewed on 04/11/2006
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