Endometriosis is a chronic condition characterised by growth of endometrial tissue in sites other than the uterus, most commonly in the pelvic cavity, but also in other parts of the body (RCOG 2006).

This ectopic tissue responds to the hormonal changes of the menstrual cycle, with subsequent bleeding, inflammation, and pain. If the ovaries are affected, endometriotic ovarian cysts may develop (Bulun 009). Although the condition may be asymptomatic, common symptoms include dysmenorrhoea, dyspareunia, non-cyclical pelvic and abdominal pain, and subfertility (RCOG 2006). When endometriosis remains untreated, the disease progresses in around a third of women, but seems either to resolve or does not progress in the rest (DTB 1999). The prevalence is estimated to vary from 2-22% of women and, in women with dysmenorrhoea, the incidence of endometriosis is 40-60% (Johnson 2007).

The cause of endometriosis is not known, but several factors are thought to be involved in its development. These include retrograde menstruation; embryonic cells giving rise to depositsin distant sites around the body; an abnormal quantity or quality of endometrial cells; failure of immunological mechanisms; angiogenesis; and the production of antibodies against endometrial cells (Gazvani 2002, Rock 1992, Seli 2003, Kyama 2003, Oral 1996).

Pain due to endometriosis can be functional, neuropathic, due to inflammation, or result from a combination of these. It may be evoked by a low intensity, normally innocuous stimulus (allodynia), it may be an exaggerated and prolonged response to a noxious stimulus (hyperalgesia), or it may be spontaneous in the absence of any apparent peripheral stimulus (Lundeberg 2008). In addition, oestrogens and prostaglandins probably play key modulatory roles in endometriosis and the pain it causes (Lundeberg 2008). Consequently, current medical treatments for the condition include drugs such as NSAIDs, combined oral contraceptives, progestogens, androgenic agents and gonadotrophin releasing hormone analogues, as well as surgical excision of endometriotic lesions. However, management of pain in women with endometriosis is often inadequate.
 

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