Parkinson's disease causes muscle tremors, slow movements, difficulty with moving and stiff muscles. Clumps of a normally harmless protein called synuclein form in the brains of people with Parkinson's, damaging brain cells in areas that control muscles. The symptoms result from a lack of the chemical messenger dopamine in the brain and progress as the areas of the brain that produce dopamine accrue further damage. Doctors do not know exactly what prompts the brain changes in Parkinson's disease, but a few risk factors have been identified.
Age
Because Parkinson's disease results from accumulating brain changes, it takes time for symptoms to develop. According to the National Parkinson's Foundation, symptoms first appear when 60 percent to 80 percent of dopamine-producing cells stop functioning, so the risk of developing Parkinson's disease increases with age. The Merck Manual states that Parkinson's disease affects only 1 percent of people younger than 65, and up to 10 percent of people age 85 or older. The average age of onset is 57.
Sex
For unknown reasons, men develop Parkinson's disease more often than women of the same age. As reviewed by Drs. Elbaz and Moisan in the August 2008 issue of "Current Opinion in Neurology," some evidence suggests that the "female" hormone estrogen can protect against Parkinson's disease.
Heredity
Parkinson's disease has a strong hereditary component, with about 15 percent to 20 percent of patients having a family history of the disease, according to the Merck Manual. Scientists have identified several genes that increase the risk for Parkinson's disease. For some of the causative genes, you must inherit two bad copies, one from each of your parents, to develop the disease (recessive genes) while for others, you need to inherit only one bad copy (dominant genes). Generally, recessive genes cause early-onset Parkinson's disease (before the age of 50) while dominant genes cause Parkinson's later in life.
As outlined in Online Mendelian Inheritance in Man, recessive genes causing Parkinson's disease include parkin, whose product breaks down proteins in nerve cells, and PINK1 whose product protects against cellular stress. Dominant genes causing Parkinson's disease include the alpha-synuclein gene, UCHL1 and HTRA2, which regulate nerve cells, and LRRK2, which produces a protein that interacts with parkin.
Toxins
Although genetic factors can increase your risk for Parkinson's disease, they do not absolutely cause it by themselves. Studies of twins, as summarized in Online Mendelian Inheritance in Man, show that environment plays an important role as well. Often, one twin will develop Parkinson's, while the genetically identical sibling does not.
However, the nature of the factors is not known. Few studies clearly link environmental variables to increased risk for Parkinson's disease. As reviewed by Drs. Elbaz and Moisan in the August 2008 issue of "Current Opinion in Neurology," some evidence supports the role of pesticides in increasing the risk of developing Parkinson's disease.
References
- National Parkinson's Foundation: What is Parkinson's?
- Current Opinion in Neurology; "Update in the Epidemiology of Parkinson's Disease"; Alexis Elbaz and Frederic Moisan; August 2008
- Merck Manual Online: Parkinson's Disease
- Online Mendelian Inheritance in Man: Parkinson's Disease
