Visceral nociception: consequences, modulation and the future.

It is widely appreciated that visceral pain differs significantly from pain that arises from cutaneous structures. Visceral pain is difficult for both the patient and physician to localize because it is diffuse in character and is typically referred to cutaneous structures. Further, there are differences between acute, post-operative visceral pain and the altered sensations associated with the so-called functional bowel disorders (e.g. non-ulcer dyspepsia, non-cardiac chest pain and irritable bowel syndrome). A variety of considerations suggests that sensory inputs from the fiscera, like nociceptive inputs from the skin, can be sensitized. Accordingly, inputs from the viscera that are not typically perceived may give rise to discomfort and pain if either visceral afferent fibres are sensitized or central neurones undergo a change in excitability ('central sensitization') after persistent visceral input. The anatomy and potential mechanisms associated with visceral hyperalgesia will be considered as will new information about opioid modulation of visceral inputs to the spinal cord.