BACKGROUND Adenomyosis and endometriosis were initially described as 'adenomyoma'. When the retrograde menstruation theory became widely accepted to explain the pathogenesis of endometriosis, since it does not explain adenomyosis, the two conditions came to be seen as distinct entities. However, emerging evidence suggests that both diseases may be linked to changes in the inner portion of the myometrium. In addition, similar anomalies were found in the eutopic endometrium of the two conditions and the debate has re-opened. A common origin for both adenomyosis and endometriosis would have relevance not only for understanding uterine function and pathophysiology, but also for clinical management and treatment. METHODS The Scopus and Medline databases were searched for all original articles published in English up to the end of 2012. Search terms included 'adenomyosis'; 'endometriosis'; 'endometrium'; 'eutopic endometrium'; 'inner myometrium'; 'junctional zone'. Special attention was paid to articles comparing features of eutopic endometrium in the two conditions. RESULTS A number of similarities exist between adenomyosis and endometriosis and, by using magnetic resonance and laparoscopy, it was found that, at least in some subgroups, the two conditions often coexist. In both situations the inner myometrium (or junctional zone) is altered, although alterations are much more marked in adenomyosis where a thickness >12 mm is today considered sufficient for diagnosis. Research has shown differences between the eutopic endometrium of women with both diseases when compared with controls. There is an immune dysfunction and there are alterations of adhesion molecules, cell proliferation and apoptosis. An increase in cytokines and inflammatory mediators has also been observed. Finally, the presence of oxidative stress and anomalies in free-radical metabolism may alter uterine receptivity. When the two conditions were compared, dissimilarities were also observed in the extent of apoptosis inhibition and in the expression of some inflammatory mediators. It is not clear if observed differences are primarily related to presenting symptoms. Finally, both conditions are steroid dependent and research suggests a role for epigenetic mechanisms. The analysis indicates that much of the published research may have been influenced by the method of diagnosis and/or has not been controlled for the presenting symptoms, the concomitant presence of both diseases or full consideration of fluctuations within cycle phase. CONCLUSIONS It is difficult to draw firm conclusions from existing evidence since major diagnostic limitations still exist and there is a systematic bias in clinical presentation. In addition, scanty information is available on the natural history of endometriosis and no studies exist on the natural history of adenomyosis. Notwithstanding these limitations, a number of similarities, but also some differences have been found between the eutopic endometrium in the two diseases. These findings need to be taken with considerable caution as the few instances where the research was repeated yielded conflicting results.
Keywords: adenomyosis; endometriosis; eutopic endometrium; myometrium junctional zone.