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.