"The risk is higher for women with both ovaries removed; however, it may also be somewhat increased when only one ovary is removed," says Walter Rocca, M.D., Mayo Clinic epidemiologist and lead study investigator. "In addition, there is a suggestion that for women with one ovary removed, removal of the uterus may also increase the risk."
Dr. Rocca emphasizes that his study's findings must be viewed by women contemplating an elective or preventive removal of the ovaries and uterus in consultation with their physicians in the context of a woman's medical history, her genetic makeup and other diseases or risk factors. He also stresses that physicians must carefully consider the age of the woman at the time of the surgery and the option of treating the woman after surgery with estrogen therapy.
"Like any medical or surgical decision, there is a trade between risk and benefit," says Dr. Rocca. "Our findings are important for situations where the removal of the ovaries is 'elective' -- that is, conducted to reduce the risk of ovarian cancer."
At issue with removal of the ovaries and parkinsonism risk is a woman's estrogen level. According to the Mayo Clinic investigators, removal of both ovaries is the major cause of reduced estrogen during a woman's fertile life, and this reduction is linked to parkinsonism risk.
"This is the first direct confirmation that reduced estrogen produced during adult life by the ovaries increases the risk of parkinsonism; this is also the first general population study addressing this question," says Dr. Rocca. "Removal of both ovaries directly eliminates the major source of estrogen." Even women who received estrogen treatment after the surgery, received doses that were generally lower than what is produced naturally by the ovaries and for a shorter number of years.
Prior Mayo Clinic epidemiologic and laboratory studies have pointed to a role for estrogen in defending the brain from attack as a woman ages. "We conducted a previous study suggesting that hysterectomy, that is, removing the uterus, and younger age at menopause are related to an increased risk of Parkinson's disease," says Dr. Rocca. "These findings suggested that the estrogen produced normally by the ovaries is involved in protecting the brain during aging.
"Also, there is biological evidence from animal studies that estrogen is protective for the specific part of the brain that is involved in controlling movements," he says. "Damage to this part results in Parkinson's disease. If an animal receives a toxic chemical able to destroy the cells of this part of the brain along with estrogen, the damage will be reduced compared to injecting the toxic substance alone."
Dr. Rocca indicates that the design of the current study does not provide the data needed to address the question of whether a woman's risk of parkinsonism after having both ovaries removed is altered by taking estrogen therapy.
The Mayo Clinic investigators studied 1,202 women who had both ovaries surgically removed and 1,283 women who had one ovary surgically removed. All women were recruited from the general population of Olmsted County, Minn., from 1950 through 1987. For each woman who underwent ovarian surgery, the researchers selected a woman of the same age who did not undergo ovarian surgery. All the women studied were followed through the onset of parkinsonism or Parkinson's disease, death, loss to follow up or the time the study was conducted.
Among women who had no ovaries removed, 29 cases of parkinsonism developed, 18 of which were Parkinson's disease. In contrast, among women who had one or both ovaries removed, 43 cases of parkinsonism developed, 25 of which were Parkinson's disease. This includes 24 cases of parkinsonism, 15 of which were Parkinson's disease, in women with both ovaries removed, and 19 cases of parkinsonism, 10 of which were Parkinson's disease, in women with one ovary removed.
Parkinsonism is a group of diseases characterized by tremor, rigidity or stiffness, slow movements and difficulty maintaining balance. Parkinson's disease is the most common type of parkinsonism.
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