Endometriosis is defined as the presence of endometrial glands in stroma outside the confines of the uterine cavity and musculature. Growth and maintenance of endometriotic implants is dependent upon the presence of ovarian steroids, and therefore endometriosis occurs almost exclusively among women of reproductive age or among those on estrogen replacement therapy . However, endometriosis has also been described in men receiving estrogen .
Endometriosis most commonly involves structures within the pelvis, particularly the ovaries, cul-de-sac, broad ligaments, and uterosacral ligaments. However, endometrial tissue can be found outside of the pelvis in the abdomen, thorax, brain, and skin .
The thoracic endometriosis syndrome (TES) will be reviewed here. Issues related to the general clinical features and treatment of endometriosis are discussed separately. (See "Pathogenesis, clinical features, and diagnosis of endometriosis" and "Overview of the treatment of endometriosis".)
Several hypotheses have been proposed to explain the pathogenesis of pelvic and thoracic endometriosis. So-called retrograde menstruation may cause movement of shed endometrial tissue through the fallopian tubes and into the peritoneal cavity . Decreased apoptosis may contribute to the failure of the normal mechanisms that clear ectopic endometrial cells . Alternatively, altered immune responses may contribute to the failure to clear peritoneal endometrial tissue, as suggested by genomewide transcriptional studies of endometriosis tissue that identified a gene signature consistent with an autoimmune profile . Serum B lymphocyte stimulating protein (BLyS), also associated with autoimmune disease, is elevated in patients with endometriosis, but its role remains unclear . Another theory is that ectopic endometrial tissue develops from a group of multipotential cells in the endometrial basal cell layer and that these cells are capable of transformation into endometrial cells with the appropriate hormonal stimulation [7,8].
The potential mechanism for migration of endometrial tissue from the pelvis to the thoracic cavity also remains uncertain. Possible routes include microembolization through the pelvic veins [1,9,10], and trans-diaphragmatic movement through diaphragmatic defects [11-13]. The hypothesis that microembolization might be a pathogenic mechanism has been questioned, as micro-embolization would be expected to affect both hemithoraces approximately equally, but TES has an overwhelming occurrence in the right hemithorax . In addition, the relatively high prevalence of pelvic endometriosis in women of reproductive age suggests that a comparably high number of TES cases would occur, if micro-embolization were the mechanism . Instead, the relatively low prevalence of TES makes micro-embolization a less likely explanation.