Helium–Neon Laser Irradiation: Effect on the Experimental Pain Threshold

Abstract

The aim of this double-blind study was to examine the e#ects 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 di#erences 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 e#ect further experimental investigations under strict electrophysiological conditions are required.

Low-Level Laser Therapy in Acute Pain: A Systematic Review of Possible Mechanisms of Action and Clinical Effects in Randomized Placebo-Controlled Trials

“…Seven studies found none, or only minor, changes in neurophysiological processes or nerve conduction velocities in intact peripheral nerves after LLLT. 20,[25][26][27][28][29][30] One study in healthy subjects found LLLT reduced nerve conduction velocity and increased negative peak latency with energy dose of 1 Joule per stimulation point, but there were no effects from energy doses at 0.5 or 1.5 Joules when applied over the sural nerve. 31 There was no convincing evidence that LLLT could act by substantial rapid modulation of neurophysiological processes in intact peripheral nerves in the absence of inflammation.…”

Analgesic Effect of He-Ne (632.8 nm) Low-Level Laser Therapy on Acute Inflammatory Pain

“…Mester 11 showed in his pioneering work in 1966 that LLLT presented beneficial effects on tissue; since then, other researchers have found that LLLT affects tissue healing, 12,13 anti-inflammatory action, and analgesic effects. [16][17][18][19] Although different studies have shown the efficacy of that treatment, particularly of inflammatory pain, the mechanism by which radiation produces analgesia and anti-inflammatory effects remains elusive. Previous studies in the 1980s suggested that laser radiation could promote release of neurotransmitters, such as serotonin 19 or ␤-endorphin.…”

Efeitos local e sistêmico do laser de baixa potência no limiar de dor por pressão em indivíduos saudáveis

“…Schuhfried et al 32 relataram que o PPT pode ser influenciado pela sensibilização dos nociceptores pelas medidas aferidas rapidamente, uma após a outra. Como todos os grupos estudados apresentaram-se negativos, pode ter havido uma sensibilização na região da perna.…”

“…Porém, há uma carência de estudos que utilizaram a mesma metodização deste estudo com sujeitos saudáveis e com a algometria por pressão. O único trabalho encontrado foi o de Schuhfried et al 32 , os quais não observaram diferença no PPT nos sujeitos saudáveis comparado com o Grupo Placebo, o que corrobora com este estudo quando comparado com as energias de 6 e 12 J por ponto. Também utilizou-se uma energia de 3 J por ponto, porém com um laser de hélio neônio (λ=632,8 nm, Pm=5 mW).…”

“…Acredita-se também que o LBP pode gerar um efeito sistêmico pelas mudanças metabólicas tanto no local da irradiação como em áreas mais distantes devido ao fato de substâncias serem liberadas na circulação sanguínea 26,27 , como também ocorrer vasodilatação e aumento do fluxo sanguíneo [28][29][30][31] . No entanto, são encontradas poucas evidên-cias sobre as hipoalgesias local e sistêmica durante as aplicações do LBP associadas à algometria de pressão 32 . Portanto, o objetivo do presente estudo foi avaliar os efeitos local e sistêmico do LBP no limiar de dor por pressão (PPT) em indivíduos saudáveis.…”