Induction and assessment of experimental visceral pain

Abstract

Experimental studies related to visceral pain have been concentrated on the gastrointestinal tract which is the structure involved in the largest number of clinical complaints. In recent years the fundamental knowledge of the visceral pain pathways has increased and in the nearest future the number and quality of experimental visceral pain studies are expected to increase. A number of direct and indirect procedures are available to investigate mechanisms of visceral pain in man. Various physiological reactions have also been used to assess the visceral pain pathways. The GI tract transducts multiple sensory modalities: mechano-, thermo- and chemosensitive stimuli. Thermal stimulation, together with mechanical and electrical stimuli, may provide the possibility to determine differentiated responses to the different stimulus modalities. As for cutaneous stimulation, this multi-modal approach can be used to assess patients with various disorders of the GI tract and the efficacy of pharmacological interventions. The spatial and temporal summations are also important mechanisms for visceral pain. In various visceral pain conditions the central nervous system may be hyperexcitable. Summation of nociceptive input from viscera is expected to play an important role in visceral pain conditions. The other aspect of visceral hyperexcitability and summation is related to the referred pain areas. Referred pain has been suggested as a manifestation of central hyperexcitability related to nociceptive visceral input. In experimental studies it is possible to investigate the effect of repeated nociceptive visceral stimuli. The major obstacles for human studies on pharmacological intervention of experimental visceral pain are; large inter- and intra-individual variability, and lack of experimental techniques to study differentiated responses.