The effects of A-fiber pressure block on perception and neurophysiological correlates of brief non-painful and painful CO2 laser stimuli in humans.
This study examined the relative capacity of Adelta- and C-fibers to encode non-painful and painful brief CO(2) laser stimuli by comparing the effects of Adelta/C-fiber activation versus C-fiber activation alone. In nine normal subjects, brief CO(2) laser pulses of four different intensities (range 5.8-10.6mJ/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+/-4.8mJ/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 1355ms. The late LEPs, attributed to the activation of Adelta-fibers, disappeared and ultra-late LEPs were recorded at Cz with a positivity peaking around 800ms. Collectively, these observations lead to the conclusion that Adelta-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.
Nahra
Hicham
H
Unité de Réadaptation Université Catholique de Louvain, Avenue Mounier 53 B-1200, Brussels, Belgium.
Plaghki
Léon
L
D000328:Adult; D002245:Carbon Dioxide; D005071:Evoked Potentials; D006801:Humans; D007834:Lasers; D008297:Male; D009407:Nerve Block; D009413:Nerve Fibers, Myelinated; D036421:Nerve Fibers, Unmyelinated; D009420:Nervous System / Q000503:physiopathology*; D010146:Pain / Q000209:etiology* / Q000503:physiopathology*; D017288:Pain Threshold; D011930:Reaction Time; D013997:Time Factors
D002245:Carbon Dioxide