When visceral pains are experienced, they are often associated with poor and unreliable localization. They are generally deep and diffuse and often the only localization of pathology comes with a physical examination in which manipulations directly stimulate the painful organ. Whereas superficial sensations from a specific site are always reliably localized to the same site and do not migrate to other body areas in the absence of nerve injury, the same cannot be said for visceral pain. Visceral pain can be felt in several different areas at the same time or can migrate throughout a region even though pathology is localized within a single organ. Visceral pain is classically described as being referred to other sites and this referral has two separate components: (1) the sensation is transferred to another site (e.g. angina can be felt in the neck and arm) and (2) other sites become more sensitive to inputs applied directly to those other sites (e.g. flank muscle becomes sensitive to palpation when passing a kidney stone). This latter phenomenon is a form of secondary hyperalgesia which can involve both somatic and other visceral structures.
Based on clinical experience, stimuli which can lead to the production of pathological visceral pain can be categorized into four groups:
1. acute mechanical stretch/distension of visceral structures;
2. ischemia of visceral structures;
3. chemical stimuli from a local pathological process (e.g. an infiltrating tumor);
4. functional alterations leading to atypical patterns of afferent activity.
Visceral pains may also occur secondary to iatrogenic damage of the viscera and their associated nerves produced by interventional therapies, surgery, chemotherapy, and/or radiation. There is a poor correlation between the amount of visceral pathology and the intensity of associated pain. For example, very extensive processes with ongoing tissue damage (e.g. ulcerative colitis or gastric perforation) may produce little or no pain in some individuals, while minimally discernable pathology may produce out-of-control pain in others.
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