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REVIEW: Evidence for asymmetric distribution of lower intestinal tract endometriosis

The aetiology of endometriosis is controversial. Investigating the anatomical distribution of endometriotic lesions may provide insights into the pathogenesis of the disease. If ectopic endometrium is due to retrograde menstruation, the pattern of lesions should be determined mainly by anatomical and physiological variables, whereas if coelomic metaplasia is the cause of endometriosis, lesions should not be distributed in relation to factors influencing the spreading and implantation of endometrial cells. The pattern of involvement of bilateral and symmetric pelvic organs has been studied specifically to verify whether endometriotic lesions are equally distributed on the two sides. In this regard, it has been demonstrated that the left ovary, ureter, and uterosacral ligament, as well as the right inguinal canal structures and sciatic nerve are affected significantly more frequently than the contralateral corresponding organ. – 12 This preference for the left side constitutes indirect evidence against the coelomic metaplasia theory, which is more likely to be associated with equal distribution, and the laterality has been attributed to the presence of the sigmoid colon on the left hemipelvis. – 12 The lower intestinal tract is the most common site of extragenital endometriosis. This is not unexpected, given its proximity to the fallopian tubes. However, if the menstrual reflux theory is true, the left terminal colon should be affected more frequently than the right one. In fact, the sigmoid is in close contiguity to the ipsilateral tube and creates a sort of shelter which should facilitate implantation of regurgitated endometrial cells, whereas the caecum is more cranial and hence less prone to be involved. It has been observed that endometriotic lesions of the descending colon are more common than those of the ascending colon, but the issue has never been approached formally. Moreover, the evidence in this regard is confusing, because endometriosis of the rectal tract is often considered among lesions of the left side. This may not be true, as the relatively frequent involvement of the rectum could originate from implants in the pouch of Douglas and be considered a primarily midline lesion. Finally, deep, infiltrating and obstructive lesions were not always distinguished from superficial, non-obstructing implants. To verify the hypothesis of an asymmetrical distribution of endometriotic lesions of the ileocaecum and colon, we reviewed all data published on the topic in the Englishlanguage scientific literature in the last two decades. A detailed description of the patients’ symptoms and signs and of the diagnostic and treatment alternatives was not among the aims of the study.