A Brain Disorder Parkinson's disease is a brain disorder that leads to shaking, stiffness, and difficulty with walking, balance, and coordination. It affects about half a million people in the United States although the numbers may be much higher. The average age of onset is 60 years, and the risk of developing Parkinson's goes up with age. Parkinson's disease was first described in 1817 by James Parkinson, a British doctor who published a paper on what he called "the shaking palsy." In this paper, he described the major symptoms of the disease that would later bear his name. Four Main Symptoms Parkinson's disease belongs to a group of neurological conditions called movement disorders. The four main symptoms of Parkinson's are: - tremor, or trembling in hands, arms, legs, jaw, or head  - rigidity, or stiffness of the limbs and trunk  - bradykinesia, or slowness of movement  - postural instability, or impaired balance. tremor, or trembling in hands, arms, legs, jaw, or head rigidity, or stiffness of the limbs and trunk bradykinesia, or slowness of movement postural instability, or impaired balance. Parkinson's symptoms usually begin gradually and get worse over time. As the symptoms become more severe, people with the disorder may have difficulty walking, talking, or completing other simple tasks. They also experience non-motor, or movement, symptoms including mental and behavioral changes, sleep problems, depression, memory difficulties, and fatigue. Parkinson's disease not only affects the brain, but the entire body. While the brain involvement is responsible for the core features, other affected locations contribute to the complicated picture of Parkinson's. Parkinson's disease is both chronic, meaning it lasts for a long time, and progressive, meaning its symptoms grow worse over time. It is not contagious. Diagnosis Can Be Difficult About 60,000 Americans are diagnosed with Parkinson's disease each year. However, it's difficult to know exactly how many have it because many people in the early stages of the disease think their symptoms are due to normal aging and do not seek help from a doctor. Also, diagnosis is sometimes difficult because there are no medical tests that can diagnose the disease with certainty and because other conditions may produce symptoms of Parkinson's. For example, people with Parkinson's may sometimes be told by their doctors that they have other disorders, and people with diseases similar to Parkinson's may be incorrectly diagnosed as having Parkinson's. A persons good response to the drug levodopa may support the diagnosis. Levodopa is the main therapy for Parkinsons disease. Who Is at Risk? Both men and women can have Parkinsons disease. However, the disease affects about 50 percent more men than women. While the disease is more common in developed countries, studies also have found an increased risk of Parkinson's disease in people who live in rural areas and in those who work in certain professions, suggesting that environmental factors may play a role in the disorder. Researchers are focusing on additional risk factors for Parkinsons disease. One clear risk factor for Parkinson's is age. The average age of onset is 60 years and the risk rises significantly with advancing age. However, about 5 to 10 percent of people with Parkinson's have "early-onset" disease which begins before the age of 50. Early-onset forms of Parkinson's are often inherited, though not always, and some have been linked to specific gene mutations. Juvenile Parkinsonism In very rare cases, parkinsonian symptoms may appear in people before the age of 20. This condition is called juvenile parkinsonism. It is most commonly seen in Japan but has been found in other countries as well. It usually begins with dystonia (sustained muscle contractions causing twisting movements) and bradykinesia (slowness of movement), and the symptoms often improve with levodopa medication. Juvenile parkinsonism often runs in families and is sometimes linked to a mutated gene. Some Cases Are Inherited Evidence suggests that, in some cases, Parkinsons disease may be inherited. An estimated 15 to 25 percent of people with Parkinson's have a known relative with the disease. People with one or more close relatives who have Parkinson's have an increased risk of developing the disease themselves, but the total risk is still just 2 to 5 percent unless the family has a known gene mutation for the disease. A gene mutation is a change or alteration in the DNA or genetic material that makes up a gene. Researchers have discovered several genes that are linked to Parkinson's disease. The first to be identified was alpha-synuclein or SNCA. Inherited cases of Parkinsons disease are caused by mutations in the LRRK2, PARK2 or parkin, PARK7 or DJ-1, PINK1, or SNCA genes, or by mutations in genes that have not yet been identified.