The generation and sense of EAP is multifactorial, and confounded by the complexity of endometriotic lesion development, and diversity of lesion placement. Contributing factors include altered distribution of pelvic nerve fibers, inflammation, secreted factors, and neuronal growth (46–49).

Endometriosis-associated pain involves stimulation of nerve fibers of the peritoneal and visceral tissues. The parietal peritoneal tissue is highly innervated with sensory nerve fibers, sympathetic nerve fibers, myelinated and unmyelinated nerve fibers, presynaptic vesicles and neuropeptides that sense pain, pressure, touch, friction, cutting and temperature (50–52). The visceral tissue is innervated by the autonomic nervous system, with unmyelinated sensory and myelinated mechanosensory neurons that sense stretching, tearing, distention and contraction. The sensation of pain may arise from these networks. Thin unmyelinated nerve fibers lie just below the peritoneal surface (51). Myelinated nerve fibers arise from the sub-mesothelial tissue and penetrate the peritoneal cavity with demyelinated nerve endings. These nerve endings may then sense the peritoneal fluid for noxious or nociceptive stimuli (52).

Endometriotic lesions are sources of inflammation, cytokines, angiogenic factors and nerve growth factors. Inflammatory mediators at both lesion sites and within the peritoneal fluid have been investigated as potential activators of nociceptive pathways (39). Inflammatory mediators including interleukin-1β (IL-1β), interleukin-6 (IL-6), interleukin-8 (IL-8), regulated upon activation, normal T cell expressed and secreted [RANTES, or chemokine ligand 5 (CCL5)] and tumor necrosis factor alpha (TNF-α), and chemokines including monocyte chemotactic protein 1 [chemokine ligand 2 (CCL2)] are increased in the peritoneal fluid of women with endometriosis (Table 1) (53–58).

It has been commonly suggested that, consistent with endometrial tissue, endometriotic lesions cycle and bleed in response to changes in estrogen and progesterone. As such, apoptosis, necrosis and shedding would trigger inflammation and pain (35, 59–61). Cyclic changes have been thought to be responsible for hemorrhage at lesion sites visualized at the time of laparoscopy (62). However, more recent data suggest endometriotic lesions rarely cycle in synchrony with the eutopic endometrium (63). Endometriotic lesions are heterogeneous in histological morphology and hormonal responsiveness, with only some lesions demonstrating cyclic changes (64). Hemorrhage within lesions is observed throughout the menstrual cycle and may be caused by local inflammation and angiogenesis (63). Recognizing the heterogeneity of endometriotic lesions is an important step toward understanding the complexity of EAP and its treatment.

Peripheral sensitization refers to “increased responsiveness and reduced threshold of nociceptive neurons in the periphery to the stimulation of their receptive fields” (37, 59). It is proposed that persistence of EAP is associated with sensitization of the peripheral nervous system. The following mechanisms may contribute to peripheral sensitization: altered nerve density at lesion sites, changes in the peritoneal fluid, perineural invasion, and alterations in the sympathetic nervous system.

The structure and function of peripheral nerves are altered in endometriosis. Changes in nerve density have been observed in peritoneal endometriosis, ovarian endometriomas and deep infiltrating endometriosis (DIE). Significantly more nerve fibers are found in endometriotic lesions compared with normal peritoneum (65). Ovarian endometriomas have been found to have more nerve fibers compared with normal ovarian tissue (66). DIE has a higher nerve fiber density compared with endometriosis at other sites; endometriosis in the rectovaginal septum has been demonstrated to have higher nerve fiber density compared with superficial peritoneal endometriosis (55, 67). Moreover, increased density of calcitonin gene-related peptide (CGRP) positive nerve fibers in lesions has been shown to positively correlate with pain severity in individuals with endometriosis (68).

Differences in receptors on peripheral nerve fibers have also been observed. The density of transient receptor potential vanilloid 1 receptor (TRPV1), involved in nociceptive pain pathways, is increased in endometriotic lesions, and its increased density has been associated with increasingly severe dysmenorrhea (69).

Alterations in the peritoneal fluid may cause sensitization of peripheral nociceptors. Changes in peritoneal fluid contribute to neuroangiogenesis in endometriotic lesions and in the adjacent peritoneum. Neurotrophins [nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), neurotrophin-4] and other growth factors [vascular endothelial growth factor (VEGF) and transforming growth factor beta (TGF-β)] are increased in the peritoneal fluid of women with endometriosis (Table 1). Growth factors stimulate new nerve growth in the periphery, potentially contributing to the altered nerve density in endometriotic lesions (70).

Perineural invasion (PNI) may also be associated with pelvic sensitization. PNI, more commonly associated with malignancy, is defined in this context as the migration or integration of endometriotic lesions along nerve fibers (71–73). PNI in patients with deep infiltrative endometriosis has been shown to be associated with higher pain scores, dysmenorrhea, dyspareunia, persistent pelvic pain, sciatica and unilateral leg pain (73, 74). PNI contributes to increased neurogenesis and angiogenesis (75). Increased endometriotic lesion secretion of NGF (76) and TGF-β (77) also correlates with lesional invasion of pelvic nerves (70).

Dysregulation of the autonomic nervous system (ANS) may further amplify peripheral sensitization. Some endometriotic lesions are found to have a lower density of sympathetic nerve fibers while sensory nerve fiber density is unchanged (78). A loss of sympathetic nerve fibers is associated with chronic inflammation, and is seen in other chronic inflammatory diseases (79). However, the mechanisms by which ANS dysregulation contributes to EAP remain unclear.

Central sensitization refers to the heightened excitability of the central nervous system in response to noxious stimuli (37). Central sensitization has been classically described as increased excitability of nociceptive neurons in the dorsal horn of the spinal cord following continued or recurrent exposure to noxious stimuli, tissue injury or nerve damage (80). Changes in dorsal horn neurons are exhibited as a decreased pain threshold and an increased pain response. Subsequent alterations in synaptic efficacy in surrounding nerves gives rise to central facilitation, with pain experienced due to innoxious stimuli. Impaired descending pain modulation at the level of the dorsal horn neurons may also contribute to central sensitization (81).

It has been demonstrated that women with endometriosis and persistent pelvic pain demonstrate hyperalgesia in response to noxious stimuli compared to healthy pain-free controls (82). Evidence is mixed when women with pelvic pain with endometriosis and women with pelvic pain without endometriosis are compared, with studies finding increased hyperalgesia in those with biopsy proven endometriosis (82), or no difference (40).

Central sensitization may also be described as arising due to primary changes in brain activity or structure. Changes in brain structure, function and activity have been evaluated in individuals with persistent pain using functional MRI and PET imaging (83). Women with EAP have been found to have reduced volume in areas of the brain associated with pain processing, including the thalamus, insula and putamen (84). It is suggested that reduced tissue volume occurs due to neuronal atrophy, neurodegeneration or medication effects (44).

Using functional MRI, it has been demonstrated that women with EAP have increased resting-state connectivity between pain processing regions including the anterior insula compared to women with endometriosis and no pain, and healthy controls (85). Women with dysmenorrhea have been found to have increased activation of pain processing regions in response to noxious stimuli both within the pelvis and in the periphery (86).

Finally, central sensitization may occur as a consequence of changes in the hypothalamic-pituitary-adrenal (HPA) axis. Women with persistent pelvic pain display alterations in their HPA axis and HPA axis-mediated pain response. Persistent pain alters the HPA axis and its capacity to mount a stress response to noxious stimuli. Women with dysmenorrhea have been demonstrated to have reduced levels of cortisol compared to women without pain (86). Acute stress leads to activation of the HPA axis but with chronic insult, this response hypo-attenuates (44).

Cross sensitization refers to nociceptive inputs from a diseased tissue impacting the perception of pain arising from normal tissue in close proximity (87). Cross sensitization has been studied between the pelvic organs of the bladder, colon, uterus and vagina. It is suggested that sensitized afferent nerves of the uterus and vagina give rise to sensitization in visceral afferents of other organs. These peripheral afferent nerve pathways overlap, converging on similar areas of the spinal cord. This is referred to as viscero-visceral convergence. Cross sensitization has also been observed between pelvic floor muscles and pelvic viscera. This is referred to as somato-visceral convergence (88).

It is hypothesized that shared innervation pathways have evolved to facilitate the coordination of pelvic organ functions—urination, defecation and sexual function—and that the presence of endometriosis causes maladaptation of converging pain pathways. In endometriosis, neurogenesis within lesions may also play a role, with newly sprouted nerve fibers converging on existing innervation pathways (35).

Viscero-visceral hyperalgesia refers to increased pain experienced by women with endometriosis and an associated pelvic pain syndrome. Viscero-visceral hyperalgesia is thought to contribute to pain severity in women with endometriosis (89).

Pain processing is further influenced by genetics, psychological state and cognitive factors (44). Women with persistent pelvic pain and endometriosis report high levels of mood disorders, with population-based studies demonstrating a bidirectional association of diagnosis with endometriosis and psychiatric disorders (90). The mechanisms underlying these associations are complex (91), and impacted by individuals' perception of social functioning (92). Subsequent alterations in pain perception have been observed (93–95). Depressed mood, anxiety, pain catastrophizing, pain anticipation and increased attention to pain are associated with higher pain intensity (39). Evidence suggests that psychosocial factors impact pain processing at all levels, local, peripheral and central (44).

Bladder pain syndrome (BPS) is characterized by pain in the bladder and/or pelvis associated with lower urinary tract symptoms including urinary urgency and frequency (97). In women diagnosed with endometriosis at laparoscopy the rate of co-existing BPS is in the order of 43–60% (98–100). Women with endometriosis have a 4-fold risk of diagnosis with BPS within 3 years of diagnosis with endometriosis (101). Like EAP, pain in BPS is mediated by inflammation. High levels of urothelial inflammatory mediators including IL-6 and TNF-α are observed in individuals with BPS (102). Central pain amplification and altered processing of afferent signals may also contribute (103).

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder defined by the Rome IV criteria as the presence of abdominal pain related to defecation and/or change in bowel habit, in the absence of other gastrointestinal disease (104). In women diagnosed with endometriosis the rate of IBS is as high as 60% (105). In a cohort of adolescents with endometriosis, 24% self-reported a concurrent diagnosis of IBS, with an increased association between pain severity and diagnosis with IBS (106). The pathogenesis of IBS is thought to relate to alterations in the enteric nervous system and brain-gut interactions. Peripheral sensitization may contribute with similar mechanisms of sensitization observed in IBS as to those seen in EAP, including increased activation of the TRPV1 receptor (107). Altered visceral sensation and pain in IBS may also be a consequence of central sensitization, with changes observed in the activation of brain regions associated with emotional arousal and pain modulation in individuals with IBS (108).

Abdomino-pelvic myalgia refers to pain felt in the abdominal or pelvic muscles and surrounding connective tissue. Myalgia refers to tenderness (allodynia or hyperalgesia) of the muscles on palpation, in the absence of increased tone (109). If increased muscle tone is detected, the condition is termed abdominal or pelvic tension myalgia, and further defined as pelvic floor muscle (PFM) tension myalgia if the location of the tenderness and tone are specific to the PFM (109). Women with EAP are highly likely to be found to have PFM tension (110). PFM tension is more common in those with biopsy proven endometriosis than in women with persistent pelvic pain without endometriosis (82). Individuals with DIE have been found to have PFM tension in 29%, with inappropriate and weak PFM contraction, and inability to completely relax in up to 45% (111). Both peripheral and central nervous system changes have been implicated in the development of myalgia (112). Peripheral nociceptors in pelvic floor muscles may contribute to referred pain through central mechanisms, at the level of the dorsal horn (113).

Vulvodynia refers to vulvar pain of at least 3 months' duration, without clear identifiable cause (114). The most commonly reported subtype is provoked vestibulodynia (PVD) where pain is provoked and localized to the vulvar vestibule. Diagnostic criteria sometimes include positive cotton swab testing. PVD gives rise to superficial dyspareunia and has a negative impact on sexual functioning (114). Vulvodynia and endometriosis are comorbid in ~11% of affected individuals (115, 116). PVD has been diagnosed in individuals with persistent pelvic pain (with or without a diagnosis of endometriosis) in 33% (117). Moreover, PVD is reported in up to 74% of individuals with BPS (118). Central sensitization may contribute. Individuals with PVD exhibit decreased pain thresholds in regions distant to the vulva including the thumb and deltoid (119, 120). Studies related to changes in brain structure in individuals with PVD demonstrate mixed results, with a trend toward increased gray matter in pain modulating regions in younger women and decreased gray matter in older women (121). In pre-menopausal women, increased gray matter volume has been observed in the basal ganglia, hippocampus and sensorimotor cortices (122). PVD is associated with psychological distress including pain related anxiety and depression (123).