Psychophysical analysis of visceral and cutaneous pain in human subjects

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Abstract
Clinical evidence suggests that cutaneous and visceral pain differ in sensory, affective, and motivational realms, yet there has been little comparative characterization of these types of pain. This study uses psychophysical measures to compare directly visceral and cutaneous pain and sensitivity. Healthy subjects (10 males, seven females, age 19-29) evaluated perceptions evoked by balloon distention of the distal esophagus and contact heat on the upper chest. Subjects gave continuous ratings of pain intensity using an on-line visual analog scale (VAS), reported maximum pain intensity and unpleasantness on printed VASs, chose phrases from the McGill Pain Questionnaire and Spielberger State-Trait Anxiety Inventory, and drew the area of perceived sensation. For esophageal distention, the threshold for pain intensity was higher than that observed for unpleasantness, whereas for contact heat, pain and unpleasantness thresholds did not differ for either phasic (10s) or tonic (36s) stimulus application. The relative unpleasantness, calculated as the difference between the unpleasantness and the intensity ratings, was higher during esophageal distention than during either phasic or tonic cutaneous heat; this difference in relative unpleasantness was seen at all intensities of esophageal stimulation. Subjects chose significantly more affective words and reported more anxiety during visceral pain than during phasic cutaneous heat pain. A similar tendency was observed when visceral pain was compared to tonic cutaneous heat pain. Subjects also chose a wider range of words to describe visceral than cutaneous pain. On-line VAS ratings revealed greater pain sensation after stimulus termination during visceral than during phasic cutaneous pain; likewise, a similar tendency was observed between visceral and tonic cutaneous pain. Finally, visceral pain led to a more spatially diffuse sensation and was referred to the entire chest and sometimes to the back. Our results show that visceral pain is more unpleasant, diffuse, and variable than cutaneous pain of similar intensity, independent of the duration of the presented stimuli. The data suggest the likelihood of both similarities and differences in the neural substrates underlying visceral and cutaneous pain.