The anatomy of pain lies within the peripheral nerve, which contains four different neuron axons: A-β, A-δ, C-fiber, and sympathetic axons. Of these four, the A-δ and C-fiber axons fire with painful stimuli, and are labeled as primary afferent nociceptors (pain receptors). A-β fibers exist primarily in the skin and respond to light touch and movement, while sympathetic axons are silent nociceptors (i.e. in the presence of inflammatory mediators they may trigger a pain response). The signals sent by nociceptors reach the somatosensory cortex via the spinal nerves where the brain maps the location, intensity, and quality of pain.

Sensitization and convergence contribute to the primary physiologic cause of chronic pain. In the setting of chronic pain, sensitization of the primary afferent nociceptor pathway occurs with constant or prolonged stimulation of receptors in the presence of damaged or inflamed tissue. Because of the constant stimulation, the threshold of neuron activation decreases while the firing frequency of each neuron increases, thus allowing even innocuous stimuli to cause pain. Convergence contributes to the varied localization of chronic pelvic pain, otherwise known as referred pain. Since each peripheral axon contacts many spinal neurons and each spinal neuron receives input from several peripheral axons, localization of pain often relates to the area covered by a spinal neuron rather than the specific stimulate fiber or receptor.

In reviewing the innervation of the pelvis, the complexity of pelvic pain becomes more evident. Stimulation of the reproductive viscera travels through sympathetic nerves that run through the pelvic plexuses to the hypogastric nerves to end on spinal nerves T11 and T12. At the same time, T11 and T12 receive stimulation from the kidneys, ureters, bowel, and skin dermatomes. Pain fibers (a combination of somatic sensory and autonomic nerves) from the cervix and vagina synapse on spinal nerves S2–4. These same spinal nerves also receive information from the rectum and bladder [4]. Thus, the task at hand is to identify and target therapy at the etiology of the stimulation whether it lies in the reproductive, urinary, gastrointestinal or musculoskeletal organs.

Evaluation should begin with a thorough history and physical examination. The site, duration, pattern, quality, intensity, and association with bodily functions or activities add to the differentiation of diagnoses. Physical examination includes a thorough examination of the abdomen and pelvis. Pelvic exam should include inspection of the external genitalia, evaluation of vaginismus, assessment of cervical motion tenderness, notation of uterine mobility/contour/tenderness, enlargement of adnexa with respective tenderness, and finally rectal exam for nodularity/blood/hemorrhoids.

Laboratory testing helpful to the differentiation of possible pathologic processes includes urinalysis, complete blood count, and sedimentation rates. Stool guiaic tests may be helpful in uncovering possible gastrointestinal disorders such as Crohn’s disease or ulcerative colitis. Analysis for sexually transmitted infections with a wet mount and/or urinary chlamydia/gonorrhea tests may also delineate the cause of vague pelvic pain.