Experts said the research needs to be confirmed by further study, but the findings suggest another issue for premenopausal women and their doctors to discuss as they consider ovary removal.
If they decide to go ahead with surgery, they need to consider the risks and benefits of taking estrogen to age 50, said Dr. Walter Rocca, a Mayo Clinic neurologist and lead study author.
Hormone therapy has been linked to a greater risk of dementia and heart attacks when given to women after age 65. But recent research indicates that when given before menopause or just afterward, it doesn't raise heart attack risk and may protect against dementia.
The study did not include women who had ovaries removed as part of cancer treatment, and Rocca said the results do not apply to such women. The work was published Wednesday in the online edition of the journal Neurology.
Ovaries produce estrogen. Rocca said the likeliest explanation of the study results is that removing ovaries causes a sudden deficiency of that hormone, which in turn affects the brain.
Hundreds of thousands of women have their ovaries removed each year in the United States. In women around age 45, approaching menopause, ovaries are often removed during hysterectomies as a precaution against developing ovarian cancer. In addition, some women at unusually high risk of developing ovarian cancer have ovaries removed without hysterectomies, as do others who have ovarian problems like endometriosis.
Women younger than 45 often take estrogen after ovary removal because of symptoms like hot flashes and concerns about developing osteoporosis, noted Dr. Nancy Chescheir of Vanderbilt University. But older women who have the surgery are less likely to start estrogen therapy, said Chescheir, who didn't participate in the new research.
The new study found the risk of later mental impairment was higher when the surgery was done at younger ages.
The research examined the fates of about 1,500 women who had one or both ovaries removed from 1950-87, and compared them to about 1,500 other women. Interviewers spoke with either the women themselves or somebody who knew them, asking about signs of memory impairment and any diagnosis of dementia or Alzheimer's disease.
Overall, the study found that women who'd had one or both ovaries removed showed about a 50 percent increase in risk of the later mental problems.
A second study, which included about 2,300 women who'd had the surgery and about 2,400 who hadn't, found about a 70 percent increased risk for a diagnosis of Parkinson's disease or Parkinson's symptoms like tremors.
Still, that outcome was far less common than mental impairment, and experts said the evidence behind it was weaker than that provided in the mental-impairment paper. The Parkinson paper finding is "not quite ready for prime time" in terms of affecting patient care, said Dr. JoAnn Manson, chief of preventive medicine at Harvard's Brigham and Women's Hospital. She was not involved with either study.
The mental-impairment paper suggests that a premenopausal woman without a family history of ovarian cancer who has to decide on whether to have her ovaries removed should ask her doctor whether that step is really necessary, she said.
"It's very reasonable and important to have that conversation with her doctor," Manson said.
Chescheir noted that estrogen therapy carries its own risks, such as a higher rate of blood clots and breast cancer, but that ovary-removal patients younger than 50 may want to have a serious discussion of that option after surgery.
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