Inter-individual variability and the effect of catastrophizing 140 on the perception of thermal grill illusion

The Thermal Grill Illusion (TGI) has been suggested as a promising experimental model for studying paradoxical thermal painful sensation as present in e.g. neuropathic pain. The TGI can be evoked by simultaneous application of innocuous cold and warm temperatures. However, a large inter-individual variability has been reported for the provocation of painful TGI. So far, it is not known which factors contribute to this variability. The purpose of this study was to investigate the role of catastrophizing in the perception of thermal grill illusion. 37 healthy subjects were recruited (17 females and 20 males) and assessed using the pain catastrophizing scale. Subjects were asked to place the palm of their non-dominant hand on six bars interlaced with cold and warm temperatures of 20° and 40°, respectively. Subjects were also tested in a control condition where both uniformly warm (40°C) and cold (20°C) bars were presented. After each test, subjects were asked to report both the intensity and the unpleasantness of the stimulus using a visual analog scale (VAS). The scale ranged from 0 (no sensation) to 10 (unbearable pain) and where 4 indicated the pain threshold. Furthermore, the quality of the TGI was assessed. The VAS intensity scale was divided into two categories: Responders and Non Responders according to the perceived intensity of the TGI. Subjects who reported intensities ≥ 4 where classified as Responders and subjects who reported intensities < 4 where classified as Non-Responders. Ten subjects were found to be Responders (felt a painful TGI), while 27 were Non-Responders (felt a non-painful TGI). T-test analyses showed that Catastrophizing levels (p<.05) were significantly higher in the Responders as compared with the Non-Responders. Furthermore, positive correlations were found in subjects with high-levels of catastrophizing scores and VAS intensity (p<.0001) and VAS Unpleasantness (p<.01). Furthermore, both warm intensity (p<.005) and unpleasantness (p<.001) ratings were significantly higher (but not painful) in Responders as compared with Non-Responders. A large inter-individual variability was also observed in the quality of the responses. Only, 10% of the participants experienced a burning pain perception while 7.5% heat pain while a greater 60% experienced an alternation or warm/cold. The remaining 22.5% reported a mix of (tingling, freezing pain, heat no-pain, and warm). In conclusion, an inter-individual variability exists in perception of a TGI. Catastrophizing is a factor which may modulate some of the perceptual processes and underlie part of the inter-individual variability.