Spinothalamic tract conduction velocity estimated using contact heat evoked potentials: What needs to be considered

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Laser-evoked potentials have been shown to be clinically useful for the electrophysiological assessment of nociceptive pathways. Contact heat evoked potentials (CHEP) are less established but might be advantageous for clinical purposes. This study aimed at determining the conduction velocity (CV) of central pain (spinothalamic tract, STT) pathways using contact heat stimulation in order to replicate previous findings using laser stimulation. Contact heat stimulation 3 degrees C higher than the pain threshold was applied at different body locations in 20 subjects. The CHEP latencies correlated significantly with the respective pain thresholds. Without normalization for this effect no significant linear regression between distance to the brain and the latencies was found. Conversely, if thresholds were considered, the regression was significant and the CV of the STT (ranging between 11.2 and 13.4m/s) was comparable to CVs estimated after laser stimulation. Pain thresholds seem crucial in interpreting CHEP latencies. It is suggested that the rather low heating rate is responsible for the dependence of latencies on the pain thresholds. This study shows the importance of pain thresholds and their control to attain valid CV of the STT after contact heat stimulation in healthy subjects.