NEW YORK (Reuters Health) ¿ Premenopausal women who undergo surgical removal of the ovaries (oophorectomy) have an increased risk of developing neurological disorders, including cognitive decline, dementia, and parkinsonism, a team of researchers reports in two studies published in the online edition of Neurology.

"The effect is age-dependent and suggests a critical age window for neuroprotection" by oestrogen, according to lead investigator Dr. Walter A. Rocca and his associates at the Mayo Clinic in Rochester, Minnesota.

The studies are based on women in Olmsted County, Minnesota, who underwent oophorectomy, between 1950 and 1987, for a benign ovarian condition before the onset of menopause.

The investigators interviewed 813 women who had one ovary removed, 676 with had both ovaries removed and 1,471 "controls" who were matched to the other women by age.

The study participants, or proxies when necessary, were interviewed over the telephone to determine the rates of memory impairment, limitations in activities of daily living, and diagnosis of dementia, senility or Alzheimer's disease.

The investigators also factored in the possible affects of age, educational level and history of depression.

Among subjects who had one or both ovaries removed, 150 had cognitive impairments or dementia, compared with 98 of the controls, a statistically significant difference.

Rocca and his colleagues observed a significant trend for increased risk of neurological deficits with younger age at surgery, unless the women received oestrogen replacement therapy until at least 50 years of age.

In a second study using the same group of women, Rocca and colleagues also identified a significantly increased risk of parkinsonism for women who had one or both ovaries removed if the surgery was conducted before 41 years of age.

These women were 75 percent more likely to have parkinsonism.

Parkinsonism refers to any condition that causes the symptoms of Parkinson's disease, one of the most common age-related neurological disorders.

These include tremor; stiffness of the arms, legs or trunk of the body; loss of facial expression; loss of control over movement; paralysis; and impaired mental processes (cognitive dysfunction).

As was the case for cognitive impairment and dementia, the effect of surgery was age-dependent, the investigators report, and the risk increased with younger age at surgery.

Based on findings from the two investigations, Rocca and associates conclude that "the neuroprotective effect of estrogen may be general and may involve multiple mechanisms" and multiple type of brain cells.

These studies are among the first "to support the hypothesis of a critical age window for the protective effect of oestrogen on the brain...in humans," the study added.