The effects of A-fiber pressure block on perception and neurophysiological correlates of brief non-painful and painful CO 2 laser stimuli in humans

Abstract

This study examined the relative capacity of Aδ- and C-fibers to encode non-painful and painful brief CO 2 laser stimuli by comparing the effects of Aδ/C-fiber activation versus C-fiber activation alone. In nine normal subjects, brief CO 2 laser pulses of four different intensities (range 5.8– 10.6 mJ/ mm 2 ) were delivered at random on the first intermetacarpal zone of the dorsum of the hand. A-fiber pressure block of the superficial radial nerve was performed to fully isolate the activity of C-fibers. Quality and intensity (VAS) of percepts, reaction time (RT) and laser-evoked potentials (LEPs) were examined in baseline and A-fiber block conditions. During A-fiber block, absolute detection threshold increased dramatically from 4.8±1.8 to 10.9±4.8 mJ/ mm 2 , proportion of detected stimuli decreased from 87% to 47% and proportion of pain reports from 39% to 10%. The quality of sensations became mainly ‘light touch’ and the ‘pricking’ sensation almost vanished. The stimulus-VAS curve shifted to the right and the slope was reduced. Signal Detection Theory analysis revealed that discrimination performance (P(A)) was significantly depressed and that response bias (B) evolved from a neutral towards a stoical attitude. Median RT increased from 492 to 1355 ms. The late LEPs, attributed to the activation of Aδ-fibers, disappeared and ultra-late LEPs were recorded at Cz with a positivity peaking around 800 ms. Collectively, these observations lead to the conclusion that Aδ-fibers are the main peripheral mediators for the perception of brief CO 2 laser stimuli and that they provide more sensory information than C-fibers.