The findings will be presented Wednesday at the American Academy of Neurology meeting in San Diego.
The researchers studied 1,209 women who had surgical removal of both ovaries and 1,302 women who had only one removed from 1950 to 1987 in Olmsted County, Minn., home of Mayo Clinic. They compared each of the women who had undergone ovary removal with women who had no ovaries removed and followed them over time to see whether they developed dementia or cognitive impairment. Dementia or cognitive impairment was determined by interviewing a family member who reported a diagnosis of dementia, or by a low score on a telephone cognitive test given to the affected individual.
Walter Rocca, M.D., M.P.H., Mayo Clinic neurologist and epidemiologist, and lead study author, proposes two possible theories to explain the increased risk: 1) low estrogen due to the ovary removal leads to decreased protection of a woman's brain from cognitive decline, or 2) the ovariectomy is innocent and variations in the genes a woman is born with dictate the need for ovary removal and also prompt development of dementia or cognitive impairment.
The researchers hypothesize that the risk of dementia or cognitive impairment is higher when the surgery takes place at a younger age -- removal of both ovaries before age 46 or one ovary before age 38 -- due to insufficient estrogen. Prior epidemiologic and laboratory studies have pointed to a role for estrogen in protecting the brain from aging.
Even though women with both ovaries removed are given hormone replacement therapy, Dr. Rocca says it's not for long enough for women who have the surgery at a young age.
"Estrogen supplementation normally is not that long -- women receive it for five or six years, till the menopausal symptoms diminish," says Dr. Rocca. "The current treatment is often not sufficient to get women who have ovariectomy through the age of natural menopause, especially if the surgery is performed at a young age. Thus, these women don't get quite enough estrogen."
Dr. Rocca emphasizes that his study's findings must be discussed by those contemplating ovary removal with their physicians in the context of the woman's overall situation -- such as medical history, genetic makeup and other factors.
"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."
In addition to Dr. Rocca, members of the Mayo Clinic research team included: Brandon Grossardt; Mariza de Andrade, Ph.D.; James Bower, M.D.; and Demetrius Maraganore, M.D. The study was financially supported by a grant from the National Institutes of Health.