Background: In the early clinical trials, human experimental pain models are important tools for defining the analgesic efficacy of drugs. The Ultraviolet-B (UVB) pain model is used for the induction of a local cutaneous inflammation in a circumscribed skin area with primary hyperalgesia (increased sensitivity to painful stimuli) and possible area of secondary hyperalgesia (sAREA).
Aim: To investigate the potential effect of the UVB-irradiation in this pain model and the reliability of this model by detecting the inter- and intra- individual variations in this model and to produce sample size estimates for a parallel and a cross over study design.
Methods: Inflammatory hyperalgesia was induced on the upper arms of 15 healthy male volunteers by irradiating a circular spot (5 cm diameter) with three times minimal erythema dose (MED) of the UVB-irradiation. The inflammatory response assessed by measures of erythema, superficial blood flow and skin temperature. The sensory tests including the brush induced allodynia, von Frey and pinprick stimuli, pressure pain threshold (PPT) and heat pain threshold (HPT), all measurements were performed at baseline, 24h, 48h and 72h after the irradiation, the whole procedure was repeated on the opposite arm with a two week’s interval. sAREA was assessed to both von Frey filament and pinprick after the UVB-exposure.
Results: A significant increase in the erythema, mean blood flow (BF) and skin temperature was detected after the irradiation (P <0.001 in all three cases), but no changes in the BF in the sAREA was detected (P=0.664). Brush induced allodynia was detected 48h after the irradiation (P=0.02). No significant increase was detected in primary hyperalgesia to von Frey filament (P=0.124). A significant increase in the pinprick induced primary hyperalgesia was detected (P<0.001) and a significant difference between the pin sizes (P<0.001). Both heat pain threshold (HPT) and pressure pain threshold (PPT) significantly decreased after the irradiation (P<0.001 in both cases), but the PPT decreased differently in both arms (P<0.001).
Conclusion: The present study concluded that the UVB-pain model was successful in inducing a local cutaneous inflammation in a circumscribed skin area, as both alteration in the BF and visually erythema was noticed. Furthermore, significantly increase in both skin temperature and erythema were detected.
For detection of mechanical induced primary hyperalgesia the present study concluded that the pinprick was the most suitable test in comparison to von Frey filaments.
Detection of the heat pain threshold was found to be a highly reliable test in the UVB-pain model. PPT only seemed to be reliable on the dominant side. Area of secondary hyperalgesai was detected to both von Frey filament and pinprick stimuli and both test seemed to be reliable, the issues of which test should be preferred need to be addressed further.