INFLUENCE OF SKIN TEMPERATURE ON HEAT PAIN THRESHOLD IN HUMANS

We compared the effect of skin temperature on the critical threshold t emperature 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 st imulus ramp on pain threshold. Furthermore, we determined the effect o f skin temperature on mechanically induced pain. We found that the lat ency to the first pain sensation induced by a radiant heat stimulus wa s 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 fou nd only with subject-controlled ascending stimuli (method of limits) b ut not with experimenter-controled, predetermined stimulus ramps (meth od 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 st imulus duration (2.5-10 s), the threshold temperature eliciting the he at pain sensation was significantly decreased. The mechanically induce d pain threshold was not influenced by the skin temperature. The resul ts indicate that the critical temperature for eliciting heat pain is i ndependent of the skin temperature in humans. However, a change in ski n temperature is an important source of an artefactual change in heat pain sensitivity when the radiant heat method (latency or energy) is u sed as an index of pain sensitivity. With a method dependent on reacti on time (the method of limits), the heat pain threshold was artefactua lly 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 temperatur e for eliciting pain sensation was significantly decreased, which may be explained by central neuronal mechanisms (temporal summation).