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In some cases the disease occurs within families, especially when it affects young people.Most late onset cases are sporadic(no known cause). The term "parkinsonism" refers to any condition that involves a combination of the types of changes in movement seen in Parkinson's disease, which happens to be the most common condition causing this group of symptoms. Parkinsonism may be caused by other disorders or by external factors (secondary parkinsonism).

Parkinson's disease is caused by progressive deterioration of the nerve cells of the part of the brain that controls muscle movement (the basal ganglia and the extrapyramidal area). Dopamine, which is one of the substances used by cells to transmit impulses (transmitters), is normally produced in this area. Deterioration of this area of the brain reduces the amount of dopamine available to the body. Insufficient dopamine disturbs the balance between dopamine and other transmitters, such as acetylcholine. Without dopamine, the nerve cells cannot properly transmit messages, and this results in the loss of muscle function. The exact reason that the cells of the brain deteriorate is unknown. The disorder may affect one or both sides of the body, with varying degrees of loss of function.

In addition to the loss of muscle control, some people with Parkinson's disease become severely depressed. Although early loss of mental capacities is uncommon, with severe Parkinson's the person may exhibit overall mental deterioration (including dementia, hallucinations, and so on). Dementia can also be a side effect of some of the medications used to treat the disorder.

Parkinson's disease is rare in children. When present, it appears to be due to decreased sensitivity of the nerves (post-synaptic) to dopamine rather than deterioration of the area of the brain that produces dopamine.

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