Relationship between adenomyosis and endometriosis; Different phenotypes of a single disease?

Adenomyosis and endometriosis are common gynecological disorders, but their pathophysiology is still under debate. The aim of this review is to discuss whether adenomyosis and endometriosis represent two different entities or different phenotypes of a single disease. We searched PubMed electronic databases published between January 2000 and April 2020. Endometriosis is classified into three phenotypes; superficial peritoneal disease (SUP), ovarian endometrioma (OMA) and deep infiltrating endometriosis (DIE) lesions. Adenomyosis presents several different subtypes, including intrinsic adenomyosis, extrinsic adenomyosis, adenomyosis externa and focal adenomyosis located in the outer myometrium (FAOM). Human uterus is embryologically composed of archimetra, originating from the Müllerian duct, and neometra, arising from the non-Müllerian duct, and adenomyosis and endometriosis are diseases of archimetra. The outer myometrial layer of the uterus is composed of highly differentiated smooth muscle cells (SMCs), while the inner myometrial cells are immature. Inappropriate uterine contractions can cause retrograde menstruation and chronic inflammation in the pelvic cavity, then influencing the development of pelvic endometriosis. Furthermore, hyperperistalsis results in physiological and pathological changes to the endometrial-myometrial junctional barrier, allowing invagination of the normal endometrial tissue into the inner myometrial layer. This can trigger the development of intrinsic adenomyosis. There are insufficient data available to draw conclusions, but extrinsic adenomyosis may result from pelvic endometriosis and FAOM from rectal and bladder DIE/adenomyosis externa. In conclusions, this paper contributes to the debate in the possibility that adenomyosis and endometriosis represent different phenotypes of a single disease.