Chronic pelvic pain is associated with a wide range of clinical conditions that include endometriosis. The precise cause, mechanisms of pain, and natural history are imprecise. Patterns of uterine innervation have been studied after hysterectomy for chronic pelvic pain with and without endometriosis.

Tissue blocks were taken from the lower one half of the uterus after hysterectomy for advanced endometriosis (n = 16 specimens; group 1) and for chronic pelvic pain without endometriosis (n = 15 specimens; group 2). The control group consisted of uteri that were removed for painless gynecologic conditions (n = 25 specimens; group 3). Tissue sections from the lower one half of the uterus were stained with anti-S100 to demonstrate patterns of innervation, and nerve fiber profiles were counted by standardized techniques; qualitative differences were also recorded.

In uteri from women with advanced endometriosis, there were increased numbers of nerve fiber profiles compared with control specimens (group 1 vs group 3; P = .0013, Mann Whitney U test). There were also increased numbers of nerve fiber profiles in uteri that were associated with chronic pelvic pain without endometriosis (group 2 vs group 3; P = .04, Mann Whitney U test). There were no differences in nerve fiber count in uteri from groups 1 and 2 (P = .35, Mann Whitney U test). Comparing both groups of uteri with controls (groups 1 and 2 vs 3) demonstrated marked differences in nerve fiber counts (P = .002, Mann Whitney U test). Two distinctive patterns of reinnervation that were observed: disruption of nerve bundles (collateral sprouting with microneuroma formation) and ingrowth around blood vessels (perivascular nerve fiber proliferation). There were increased numbers of microneuromas (groups 1 and 2 vs 3; P = .001, chi-squared test with Yates correction) and perivascular nerve fiber proliferation (groups 1 and 2 vs 3; P = .008, chi-squared test with Yates correction) in the myometrium in chronic pelvic pain with, and without, endometriosis compared with the control group.

Nerve fiber proliferation and other features of reinnervation have been observed in the isthmic regions of uteri that were removed at hysterectomy for chronic pelvic pain with and without endometriosis. There were no quantitative differences between the groups with chronic pelvic pain and endometriosis. These observations provide an alternative explanation for the source of pain and other clinical symptoms in these clinical settings.