Endometriosis is typically associated with pelvic pain (in the region below the belly button). To further examine this, we need to remember the 3 types of the disease. In ovarian endometriomas, pain is encountered in 80% of cases¹, whilst most women with deep infiltrating endometriosis (DIE) present with severe pain¹. However, in superficial peritoneal lesions (SUP), things are less clear. They may cause mild pain in 50% of cases¹. It is also worth noting that there is no clear correlation between the stage of the disease and the type or severity of symptoms².

Why does this enigmatic disease cause pain (at least in 2 out of its 3 types)? Generally, the pain can be nociceptive (irritation of peripheral pain sensors), neuropathic(as a result of damage or disease in the nervous system) or a combination of the two, and it is likely that the disease could cause pain through a combination of those mechanisms³. It has been demonstrated that endometriosis is associated with changes in the peripheral and central nervous system, as well as the presence of sensitive nerve fibres within the lesions (foci) of endometriosis^4,5.

But let us have a look in the micro-environment of a deep endometriosis lesion. Within this lesion, the following events occur⁶: Neuroangiogenesis (coordinated development of nerves and vessels as a result of increased levels of growth factors), neuroinflammation and hypersensitization of nerve fibres (as a result of high levels of cytokines and histamine) as well as altered immune surveillance. The above create a ‘’vicious circle’’, in which are important the oestradiol levels, sending painful stimuli to the central nervous system. But as we mentioned, endometriosis can also cause changes to the central nervous system, which is responsible for the final ‘’interpretation’’ of these stimuli (and, therefore, the feeling of pain). As proof of this, patients with endometriosis generally exhibit a higher pain sensitivity⁷.

In conclusion, the feeling of pain in endometriosis appears to be associated with complex changes in the peripheral as well as central nervous systems.

References:

1. Koninckx PR,Meuleman C, Demeyere S, Lesaffre E, Cornillie FJ. Suggestive evidence that pelvic endometriosis is a progressive disease, whereas deeply infiltrating endometriosis is associated with pelvic pain. Fertil Steril 1991; 55:759–65.

2. Fauconnier A, Chapron C. Endometriosis and pelvic pain: epidemiological

evidence of the relationship and implications. Hum Reprod Update 2005; 11:595–606.

3. Vercellini P, Viganò P, Somigliana E, Fedele L. Endometriosis: pathogenesis and

treatment. 312 Nat Rev Endocrinol 2014;10:261–75.

4. Brawn J, Morotti M, Zondervan KT, Becker CM, Vincent K. Central changes

associated 317 with chronic pelvic pain and endometriosis. Hum Reprod Update

2014;20:737–47.

5. Morotti M, Vincent K, Brawn J, Zondervan KT, Becker CM. Peripheral changes in

endometriosis-associated 318 pain. Hum Reprod Update 2014; 20:717–36.

6.Morotti M, Vincent K, Becker CM. Mechanisms of pain in endometriosis. Eur J Obstet Gynecol Reprod Biol. 2017 Feb; 209:8-13.

7. As-Sanie S, Harris RE, Harte SE, Tu FF, Neshewat G, Clauw DJ. Increased pressure pain sensitivity in women with chronic pelvic pain. Obstet Gynecol. 2013 Nov;122(5):1047-1055.