DIFFERENT PATTERNS OF HYPERALGESIA INDUCED BY EXPERIMENTAL INFLAMMATION IN HUMAN SKIN

Different types of hyperalgesia were studied after experimental induct ion of inflammation in small skin areas of healthy volunteers either b y topical application of capsaicin solution (1% in 70% ethanol) or by briefly freezing a skin area of similar size to -28 degrees C. Sensory tests were performed 30 min after capsaicin application and 22 h afte r freeze lesions. Heat pain thresholds were lowered after both treatme nts, probably due to nociceptor sensitization. Hyperalgesia to four ty pes of mechanical stimulation was studied (i) Hyperalgesia to punctate stimuli was encountered at the skin site directly affected by the nox ious chemical or freeze stimulus (1 degrees zone) and in the surroundi ng skin (2 degrees zone) in both models though the area of 2 degrees h yperalgesia to punctate stimuli after freezing was smaller than after capsaicin. (ii) Hyperalgesia to gently blushing the skin was prominent after capsaicin in 1 degrees and 2 degrees zone, but almost absent af ter freezing. It was concluded that both hyperalgesia to punctate stim uli and brush-evoked pain are due to central nervous plasticity change s rather than nociceptor sensitization. As revealed by differential ne rve blocks, brush-evoked pain is mediated by low threshold mechanosens itive A beta-fibres, whilst hyperalgesia to punctate stimuli can be el icited when only C-fibres conduct. in contrast to hyperalgesia to punc tate stimuli it requires continuous background discharges in nocicepto r units. (iii) Pressure hyperalgesia to tonic stimulation with a blunt probe was encountered in the 1 degrees zone of both types of inflamma tion and is probably due to recruitment of sensitized nociceptor units . (iv) Impact hyperalgesia was studied by shooting small bullets again st the skin at predetermined velocities. It was found in the 1 degrees zone after freezing and absent in the capsaicin model. Differential n erve blocks revealed that it is probably mediated by sensitized C-fibr es. in conclusion, different types of inflammatory changes may result in characteristic different patterns of hyperalgesia.