[Changes in the RIII response of the flexion reflex with painful maneuvers]

In the search for reliable indirect indices of pain sensation, interest has recently focused on the nociceptive flexion reflex and late components of the brain evoked potentials. In ten volunteers with sciatica, the nociceptive flexion reflex (RIII) and the late component (N150-P220) of the evoked potentials were recorded, with the subjects at rest and during pain produced by the Lasegue manoeuvre. In recordings with the subjects at rest, both responses were stable. During the Lasegue manoeuvre, the RIII response was markedly suppressed, with a mean reduction in area of 40% (P less than 0.001). In contrast, the amplitude of the N150-P220 components did not change significantly. Our results indicate that, of the two parameters tested, RIII alone is a reliable index for measuring pain sensation.