Background and aims: Experimental pain models based on single stimuli have to some degree limited visceral pain studies in humans. Hence, the aim of this study was to investigate the effect of multi-modal visceral pain stimuli of the oesophagus in healthy subjects before and after induction of visceral hyperalgesia. We used a multi-modal psychophysical assessment regime and a neurophysiological method (nociceptive reflex) for the characterisation of the experimentally induced hyperalgesia.
Methods: A probe for multi-modal (cold, warm, electrical, and mechanical) visceral stimulation was positioned in the lower part of the oesophagus in eleven healthy subjects. Mechanical stimuli were applied as distensions with a bag, which also had electrodes mounted for electrical stimulation. Thermal stimulation with temperatures from 0 to 60 degrees C was applied with re-circulating water in the bag. To assess the interaction between visceral and somatic pathways, the nociceptive withdrawal reflex to electrical stimuli at the ankle was measured with and without simultaneous mechanical oesophageal distension to painful levels. Finally, the oesophageal sensitisation was induced by perfusion with hydrochloric acid. Multimodal responses (pain threshold, stimulus response function, size of nociceptive reflex, and referred pain areas) were assessed before and after the induced hyperalgesia.
Results: The multi-modal psychophysical responses and reflex sizes were assessed twice before sensitisation, and the parameters were reproducible. Sensitisation of the oesophagus resulted in hyperalgesia to electrical and mechanical stimuli (29 and 35% decrease in pain threshold) and allodynia to cold and warmth stimuli (11% increase in sensory rating). After sensitisation, the referred pain area to mechanical stimuli increased more than 300% with a change in the localisation of the referred pain to all stimuli, and the amplitude of nociceptive reflex increased 100%, all indicating the presence of central hyperexcitability.
Conclusions: Visceral hyperalgesia/allodynia can be induced experimentally and assessed quantitatively by the newly introduced multi-modal psychophysical assessment approach. The significant changes of the experimentally evoked referred pain patterns and of the nociceptive reflex evoked from a distant somatic structure indicate that even short-lasting visceral hyperalgesia can generate generalised sensitisation.