Influence of skin temperature on heat pain threshold in humans

We compared the effect of skin temperature on the critical threshold temperature eliciting heat pain with the effect of skin temperature on the response latency to the first heat pain sensation in healthy human subjects. Also, we determined the effect of the duration of a heat stimulus ramp on pain threshold. Furthermore, we determined the effect of skin temperature on mechanically induced pain. We found that the latency to the first pain sensation induced by a radiant heat stimulus was significantly decreased with an increase in the skin temperature (25-35 degrees C). However, independent of the rate of the stimulus rise (3-10 degrees C/s) and independent of the stimulus location (hairy vs glabrous skin), the threshold temperature for eliciting the heat pain sensation, determined with a contact thermostimulator, was not changed by a change in the skin temperature in the same subjects. With a fast rate of stimulus rise, a higher pain threshold was obtained than with a slow rise of stimulus temperature. However, this difference was found only with subject-controlled ascending stimuli (method of limits) but not with experimenter-controled, predetermined stimulus ramps (method of levels). The heat pain threshold was higher in the glabrous skin of the hand than in the hairy skin of the forearm. With increasing stimulus duration (2.5-10 s), the threshold temperature eliciting the heat pain sensation was significantly decreased. The mechanically induced pain threshold was not influenced by the skin temperature. The results indicate that the critical temperature for eliciting heat pain is independent of the skin temperature in humans. However, a change in skin temperature is an important source of an artefactual change in heat pain sensitivity when the radiant heat method (latency or energy) is used as an index of pain sensitivity. With a method dependent on reaction time (the method of limits), the heat pain threshold was artefactually increased, with fast rates of stimulus rise due to the long delay of slowly conducting heat pain signals in reaching the brain. With an increase in the duration of the heat stimulus, the critical temperature for eliciting pain sensation was significantly decreased, which may be explained by central neuronal mechanisms (temporal summation).