Visceral pain, referred hyperalgesia and outcome: new concepts.

Visceral pain is a poorly defined, midline sensation but after minutes or hours becomes 'referred' to a somatic region when it becomes sharper and better localized. Referred pain may manifest either with or without hyperalgesia. Hyperalgesia is more common and can be demonstrated by changes in pain threshold. This referred hyperalgesia is probably sustained by central mechanisms, involving a spinal focus facilitating neuronal signals coming from somatic structures. Once established, the central changes may become independent of the peripheral input necessary for their initiation. With respect to post-surgical pain associated with operations on the viscus, the main determinant is whether or not the visceral pathology prior to surgery was algogenic.