Diurnal time course of pain perception in healthy humans

Objective: Rapid skin heating by infrared lasers can be used to investigate the integrity of the nociceptive system by activating Ad- and C-fibers. Laser-induced pain stimuli are sensed as dual pain sensation attributable to the two different fibers' conduction velocities. To date, only little is known about diurnal variation of laser-evoked pain perception and time course of laser-evoked potentials (LEP). Methods: We assessed circadian ad-fiber function by studying N2 and P2 components of LEPs and pain thresholds evoked by laser stimulation of the foot every two hours from 8 a.m. to 10 p.m. in 15 healthy subjects, aged 27–59 years. Subjects showed normal peroneal nerve somatosensory evoked potentials, normal peroneal nerve conduction velocity and no relevant history or concomitant neurological, psychiatric or other disease (including diabetes mellitus, drug abuse). Heat stimuli were generated by an infrared Tm-YAG laser (wavelength 2,01µm, pulse duration 1 ms, spot diameter 7mm) and were delivered to an area of 4×4,5cm on the dorsum of the right and left foot in 3–4 runs of incremental and decremental intensities in steps of 50 mJ, starting at a minimum of 100 and reaching a maximum of 800 mJ. LEPs were recorded with fixed stimulus intensities that were 1,5 x of the pain threshold. After each stimulus subjects were asked to classify their sensation with a numeric rating scale. LEP were assessed on electrodes Fz, Cz, Pz, T3, T4, A1, A2. Data were collected with the SynAmps System (Neuroscan, El Paso, USA) and averaged across 35–40 trials. Results: Laser-induced heat pain thresholds and circadian latencies of laser evoked potentials did not significantly vary during the day. Discussion: Our results confirm previous studies that did not detect any consistent significant diurnal variations in perception of heat pain perception using contact thermodes.