Discussion
Major Findings
Time trends in the incidence of UO are influenced by trends in the incidence of bilateral oophorectomy, which in turn are determined by evolving evidence and beliefs about the risk-benefit balance between conserving the ovaries to reduce menopausal symptoms and sequelae and removing the ovaries to decrease the risk of ovarian or breast cancer and the risk of reoperation. Many studies have influenced this balance across time, leading to major trends in gynecological practices. Our study showed a link between the trends in bilateral oophorectomy and the trends in UO rates in a well-defined US population in the most recent 58-year period.
In our study, the incidence of left oophorectomy was higher than the incidence of right oophorectomy until approximately 1985. UO without a medical indication has been influenced by surgical preferences and traditions. Between 1950 and 1985, preservation of ovarian function during concurrent hysterectomy was the tradition, so there was an attempt to save at least one ovary. Reviewing medical records, we learned that it was customary to remove one ovary to reduce the risk of future reoperation. If both ovaries appeared normal, the left ovary was preferentially removed because of its proximity to the sigmoid colon. The decline in this practice across time coincides with the increased use of ultrasound for diagnostic purposes.
In women who underwent UO for a medical indication, the pathology differed between the left side and the right side. We confirmed findings from previous studies indicating that endometriosis is more common in the left ovary than in the right ovary. Two theories behind the development of endometriosis may explain this laterality. First, the retrograde menstruation theory postulates that the endometrium is expelled from the Fallopian tubes into the peritoneum. Elimination of this endometrial tissue may be less efficient on the left side because of its anatomical proximity to the colon or the decreased flow of peritoneal fluid on the left side. Therefore, tissue deposited on the left ovary would have greater opportunity to adhere and implant. The second theory is based on the functional difference between ovaries. The right ovary ovulates more frequently than the left ovary and, therefore, has higher localized progesterone production, which suppresses endometriosis.
Strengths
Although the initial time segment of our study was based on previously collected data (38 y), our focus on laterality, indications, and pathological findings in UO was novel and prompted a complete reanalysis of the data. We combined old data with new data from the most recent 20 years to provide a long-term perspective on surgical practices (58 y). The strengths of our study include the documentation of the laterality of oophorectomy and of whether a contralateral oophorectomy had previously been performed (as indicated in medical records), avoiding recall bias and inaccuracy of reporting. In addition, the availability of incidence rates for bilateral oophorectomy in the same population from a previous study allowed the comparison of rates for both surgical operations across time.
Limitations
The first limitation of this study is the local nature of UO practices. Surgical practices typical of the Mayo Clinic and other care providers in Olmsted County may not reflect national trends. A literature review failed to reveal national or regional guidelines for UO in the first four decades of our study. Thus, local surgical preferences and traditions may have influenced the decision to remove the right ovary or the left ovary without a medical indication. Second, we may have undercounted women who resided in Olmsted County but underwent UO at a medical facility not included in the Rochester Epidemiology Project, or women who were incorrectly coded in their medical records or in the electronic indexes of the Rochester Epidemiology Project system. Based on our experiences in manually abstracting medical records, these two events are rare and have not changed across time. Finally, our population includes primarily white women of northern and central European descent; thus, our findings may not be generalizable to other populations or ethnic groups. However, the population is similar to a large segment of the US population.