Bowel endometriosis

Endometriosis is defined as endometrium-like glands and stroma outside the uterus (1). It clinically presents with cyclical or chronic pelvic pain and/or infertility. Deep endometriosis (DE) is one of three phenotypes of endometriosis in addition to peritoneal endometriosis and ovarian endometriosis (2). A recent AAGL, ESHRE and WES consensus defines DE as “endometrium-like tissue lesions in the abdomen, extending on or under the peritoneal surface. They are usually nodular, able to invade adjacent structures, and associated with fibrosis and disruption of normal anatomy” (1). This is different to peritoneal endometriosis on the bowel that is superficial peritoneal lesions not DE (1). Bowel endometriosis is most commonly located on the rectum (3). 

N80: endometriosis
N80.4: endometriosis of rectovaginal septum and vagina
N80.5: endometriosis of intestine

The prevalence of bowel endometriosis is 5-12% among women and individuals assigned female at birth that have been treated surgically for endometriosis (4), with up to 71% located on the rectosigmoid (3). Other gastrointestinal locations are the ileum, cecum and appendix (3). A French nationwide study found that 10-20% bowel surgery was performed in women treated surgically for endometriosis (5). In a retrospective study based on transvaginal ultrasound (TVS) examination of 373 women symptomatic of endometriosis found an 8% prevalence of bowel endometriosis (6).

The origin of bowel endometriosis remains unclear although it is likely multifactorial. In the past the most common theory was retrograde menstruation and metaplasia. With Sampson’s theory implantation of serosal endometrial cells into the muscularis layer is aided by repeated tissue injury and repair causing smooth muscle hyperplasia and fibrosis (7-9). Histologically bowel endometriosis involves most commonly the bowel muscularis layer, rarely affecting the mucosa (10). Thus, rectal bleeding is rare among individuals with bowel endometriosis. Thickening of the bowel wall and adhesions may cause a narrowing of the bowel lumen may in the worst cases lead to obstructive symptoms (11).The retrograde theory is further supported based on observations of the distribution of bowel endometriosis being most commonly located in the left posterior compartment (3) as the rectosigmoid and left adenexa that act as a barrier of retrograde menstruation leading to accumulation of blood in the peritoneal fluid (12). However, retrograde mentrutration is a normal physiological phenomenon as 90% of women have blood in their peritoneal fluid (13). Metaplasia has been proposed as alternative theory that endometriosis derives from peritoneal cells (14) or Müllerian remnants (15). Deep endometriosis has been proposed to be adenomyosis externa being histologically different to the other phenotypes based on histological studies of endometriosis of the rectovaginal septum found the presence of smooth muscle, as well as endometrial glands, similar to adenomyotic lesions (15). Although others have argued that deep enometriosis develops intraperitoneally and not from the rectovaginal septum (16). However, the adenomyosis externa theory has been challenged as smooth muscle components is found in other locations of endometriosis (17). The complexity of the origin of endometriosis is emphasised by evidence of lymphatic spread of endometrial cells among individuals with deep posterior compartment disease that may explain extra-pelvic presence of endometriosis (18). 

The recent genetic-epigenetic (GE) theory suggests that GE predispostion and incidents need to occur to iniate progression of the three phenotypes of endometriosis (19). The aggressive nature of bowel endometriosis may be explained by cancer associated mutations found in epithelial cells (20). GE theory can explain why not all individuals with retrograde menstruation and implantation get endometriosis. Previously predisposing factors may instead be the result of GE changes causing insults, like differences in the endometrium among individuals with endometriosis, or biochemical or immunological factors influencing the development of peritoneal, ovarian and DE (19). 

Clinical history and examination are fundamental in diagnosising bowel endometriosis. Individuals may report deep dysparunia, cyclical or chronic pelvic pain, dyschezia and altered bowel habits. Altough some individuals may be asymptomatic. Non-cyclical chronic pelvic pain is more common agong individuals with bowel endometriosis (21). Gastrointetinal complaints may occur among individuals withouth DE (22). This makes diagnosis challenging as individuals with bowel endometriosis have similar symptoms like bloating and alereted bowel habits to that of inflammatory bowel disease (23). Severity of dyschezia is correlated with rectosigmoid lesion size as defined by the revised Enzian classificaiton (24). Recently there has been a shift in diagnosis moving away from diagnostic laparoscopy to imaging with TVS and/or magnetic resonance imaging as the gold-standard for diagnosing DE (25, 26). Diagnosis of bowel endometriosis and describing DE extent is important in counselling and optimizing management of bowel endometriosis (27).

The International Deep Endometriosis Analysis (IDEA) group consensus states how to describe and measure ultrasonographic features of DE (28). It divides the pelvis into the anterior and posterior compartment. Bowel endometriosis is located in the posterior compartment. Bowel endometriosis is characterized by hypoechoic thickening of muscularis propria, or as hypoechoic solid nodules with or without hyperechoic foci (28). To assist in identifing bowel endometriosis, the “sliding sign” can be applied by assessing if the rectum slides or is adherent to the retrocervical region and/or uterus (28, 29). A negative sliding sign means that the bowel is adherent and indicates Pouch of Douglas obliteration increasing the risk of bowel endometriosis (29-31). The presence of endometriomas can guide the clinician to look for bowel endometriosis as endometriomas indicates increased disease severity (32).

Bowel endometriosis can be isolated, multfocal, i.e. multipe lesions affecting the same segment of the bowel, and/or multicentric, i.e. multipe lesions affecting several bowel segments (28). A morphological description of type of lesion is recommended i.e. like the comet, moose antler sign and the distance from the inferior part of the lesion to the anal verge (28). Additionally, lesion length, thickness and tranverse measurements is advised (28). There are several classifications systems for endometriosis (33). The #Enzian classification is currently the only classification describing peritoneal, ovarian and DE(27) that correlates with pre-operative findings (34).

The aim of treatment is to alleviate symptoms and improve feritlity and should be tailored to inividual needs. Treatment of bowel endometriosis can be divided into medical and surgical treament. Endometriosis associated pain is complex to treat but first line management is usually medical treatment with analgesics and hormonal treatment, unless the individual has fertility wish (25, 35). If medical treatment fails and/or if the individual have symptoms of sub-occlusion surgical treatment may be necessary (36, 37). When considering surgical managment of bowel endometriosis the distance between the inferior border of the lesion and the anal verge can be measured using transvaginal ultrasonography (21). This is an important marker for estimating anastomosis height and assessing the risk of complications like anastomotic leakage and rectovaginal fistula (38, 39). Type of bowel procedure performed can be guided according to lesion size measured with ultrasongraphy (40, 41). Bowel procedures can generally be divided into three types. Firstly, shaving, which is cutting away the bowel lesion and fibrosis withtout entering the bowel lumen. Whilst discoid excision and segmental resections opens the bowel lumen and removing the lesion and is associated with a higher risk of complications (39, 42). 

Bowel endometriosis rarely occurs alone and is associated with increased disease severity and pain (24). It is a benign disease that may have a significant impact on the quality of life and fertlity which poses a socioeconomic burden on the individual but also on their families and society (43). Individuals with all types of endometriosis have an increased risk of ovarian cancer, in particular endometroid and clear cell carcinomas (44). The risk of ovarian cancer is highest among women with ovarian endometriomas (44). However, no incrased risk of gynecological cancers was found for peritoneal or DE (44). DE is not associated with an increased risk of other types of cancers (45). 

Disease development for bowel endometriosis is not yet established and can progress despite hormonal treatment (46, 47). However, some argue that lesion growth is self-limited and does not grow rapidly (48).The risk of recurrence of bowel endometriosis is higher among women treated with shaving compared to the other two techinques most likely due to residual disease (49). Pouch of Douglas obliteration diagnosed on TVS after bowel surgery may raise suspicion of residual disease or recurrence of bowel endometriosis (50, 51).

Bowel endometriosis may share similar symptoms to rectal cancer, inflammatory bowel disease, diverticulitis and irritable bowel syndrome, like abdominal pain, bloating, altered bowel habits and rarely rectal bleedning (23, 52). However, bowel endometriosis usually presents in women of reproductive age with worsening of pain and gastrointestinal symptoms during menstrual cycle or ovulation or that may have become chronic. Unlike rectal cancer bowel endometriosis usually is located on the anterior rectal wall and rarely penetrates the mucosa (53), whilst rectal cancer can be located on any part of the circumference of the bowel wall (54).

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