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What Is Parkinson's Disease?

Parkinson's disease, which mostly afflicts older people, results from gradual degeneration of nerve cells in the portion of the midbrain that controls body movements. The first signs are likely to be barely noticeable - a feeling of weakness or stiffness in one limb, perhaps, or a fine trembling of one hand when it is at rest (activity causes the tremor to disappear). Eventually, the shaking will worsen and spread, muscles will tend to stiffen, and balance and coordination will deteriorate. Depression and other mental or emotional problems are common.

Usually the disorder begins between the ages of 50 and 65, striking about 1% of the population in that age group; it is slightly more common in men than in women. Medication can treat its symptoms, and the disorder is not directly life-threatening.

What Causes It?

Bodily movements are regulated by a portion of the brain called the basal ganglia, whose cells require a proper balance of two substances called dopamine and acetylcholine, both involved in the transmission of nerve impulses. In Parkinson's, cells that produce dopamine begin to degenerate, throwing off the balance of these two neurotransmitters. Researchers believe that genetics sometimes plays a role in the cellular breakdown, and in rare instances, Parkinson's disease may be caused by a viral infection or by exposure to environmental toxins such as pesticides, carbon monoxide, or the metal manganese. But in the great majority of Parkinson's cases, the cause is unknown.

Parkinson's disease is a form of parkinsonism - a more general term used to refer to the set of symptoms that is commonly associated with Parkinson's disease but sometimes stems from other causes. The distinction is important because these other causes of parkinsonism can often be treated to improve or eliminate symptoms. Other causes of parkinsonism include:

• An adverse reaction to prescription drugs.
• Use of illegal drugs.
• Exposure to environmental toxins.
• Stroke.
• Thyroid and parathyroid disorders.
• Repeated head trauma (for example, the trauma associated with boxing).
• Brain tumor.
• An excess of fluid around the brain (called hydrocephalus).
• Brain inflammation (encephalitis) resulting from infection.

Parkinsonism may also be present in persons with other neurological conditions, including Alzheimer's disease, amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease), Creutzfeldt-Jakob disease, Wilson's disease, and Huntington's disease.