Diffuse noxious inhibitory controls triggered by heterotopic CO2 laser conditioning stimulation decreased the SEP amplitudes induced by electrical tooth stimulation with different intensity at an equally inhibitory rate

The purpose of this study was to investigate (1) whether selective Aδ-fiber stimulation with CO2 laser produces a diffuse noxious inhibitory controls (DNIC) effect in the trigeminal nerve territory; and (2) whether the DNIC effect differs depending on test stimulus intensities under constant conditioning stimuli. To examine whether the CO2 laser radiation on the dorsum of the hand selectively stimulates Aδ-fibers, laser evoked potentials (LEP) were recorded. The mean peak latency of LEP was 381.4 ms. The findings revealed that the CO2 laser selectively stimulated Aδ-fibers. Electrical tooth stimuli with 3 levels of intensities (1.2, 1.4, 1.6 times the pain threshold) were applied to subjects as test stimulation in randomized order, with a CO2 laser stimulus of 18 mJ/mm2 applied to the dorsum of the hand for 4 min as the noxious conditioning stimulus. Somatosensory evoked potentials (SEP) induced by electrical tooth stimulation were recorded and tooth pain intensity was evaluated using a visual analogue scale (VAS). The amplitudes of the SEP late component and VAS values were significantly decreased only during the conditioning stimuli without aftereffect. The inhibitory rates of the amplitudes ranged from 31.3% to 34.6% and the VAS values from 29.0% to 31.2%. There were no significant differences in their inhibitory rates between the 3 test stimulus intensities. The result indicated that selective Aδ-fiber stimulation with the CO2 laser produces a DNIC effect in the trigeminal nerve territory and suggested that the DNIC effect does not depend on the intensity of the test stimuli.