Diurnal time course of heat pain perception in healthy humans

Rapid skin heating by infrared lasers can be used to investigate the integrity of the nociceptive system by activating A-delta and C fibers. The aim of our study was to analyze if healthy humans exhibit any clinically relevant diurnal variations in their heat pain sensitivity. Circadian A-delta fiber function was analyzed by studying N2 and P2 components of laser-evoked potentials (LEP) and pain thresholds evoked by laser stimulation of the foot every 2h from 8a.m. to 10p.m. in 15 healthy subjects. Heat stimuli were generated by an infrared Tm-YAG laser and were delivered to an area of 4 cm × 4.5 cm on the dorsum of the right or left foot in 3 runs of incremental and decremental intensities. After each stimulus subjects were asked to classify the intensity of pain with a numeric rating scale (NRS). LEPs were recorded with fixed stimulus intensities that were 1.5× of the pain threshold. Data were collected with the SynAmps System (Neuroscan, El Paso, USA) and averaged across 35-40 trials. Laser-induced heat pain thresholds and circadian latencies of LEP did not significantly vary during the day. Our results correspond with previous studies that did not detect any consistent significant diurnal variations in perception of heat pain perception using contact thermodes. The intensity of pain perception did not demonstrate any correlation with mood or sleep parameters as measured with the Beck Depression Inventory (BDI), the subjective sleep scales Pittsburgh Sleep Quality Index (PSQI) and Epworth Sleepiness Scale (ESS).