Helium-neon laser irradiation: Effect on the experimental pain threshold

The aim of this double-blind study was to examine the effects of helium-neon laser irradiation on the mechanical (pressure algometry) and electrical (1 ms monophasic square-wave pulses, 50 Hz) pain threshold. 32 pain-free subjects were randomly assigned to either the experimental group (helium-neon laser stimulation: 5 mW, 10 min) or the placebo group (sham stimulation). Laser or sham stimulation and pain threshold ascertainment were carried out on the dorsal aspect of the forearm area. The contralateral arm served as an untreated control. The groups were compared with each other and with the control arm. No significant differences were found between the laser stimulation and the sham stimulation in changes of either the mechanical or the electrical pain threshold. There were no changes in the mechanical pain threshold through laser stimulation and sham stimulation with respect to the untreated contralateral arm. After laser stimulation electrical pain threshold was significantly higher (p<0.01) in the treated arm than in the untreated contralateral arm, because this threshold decreased in the contralateral arm. This was not the case in sham treatment. The data suggest that helium-neon laser stimulation does not raise the experimental pain threshold in healthy subjects compared to placebo treatment. After helium-neon laser stimulation there was a decrease of the electrical pain threshold in the contralateral arm. To disclose the mechanism of this effect further experimental investigations under strict electrophysiological conditions are required.