A comparative study of cortical responses evoked by transcutaneous electrical vs CO(2) laser stimulation.

OBJECTIVES: A novel non-invasive method for nociceptive electrical stimulation of the skin has been recently introduced by using a planar concentric stimulating electrode (CE). We compared the cortical potentials induced by a CE vs laser stimulator in healthy subjects using a multichannel recording. METHODS: Cortical potentials were recorded in 11 healthy subjects by 54 scalp electrodes, stimulating the skin of the right hand and the right supra-orbital zone by the CE and laser stimulator settled two levels above the individual pain threshold. RESULTS: The latency difference between N1, N2, and P2 evoked by the CE vs laser stimulator was larger than the receptor activation time of 40 ms and larger following stimulation of the upper limbs than of the head. The amplitudes and topographic distribution of the cortical waves did not differ between the two stimulation types. CONCLUSIONS: A-beta fibre co-activation may be induced by CE electrodes, as suggested by latency gaps. Nevertheless, CE-evoked potentials showed similarity in amplitude, morphology and topographic representation with laser-induced ones. SIGNIFICANCE: At present, CE-evoked potentials cannot be considered a reliable measure of nociceptive pathway function.

Laser-evoked potentials habituation in fibromyalgia.

UNLABELLED: Abnormalities of central pain processing play an important role in the pathophysiology of fibromyalgia (FM). The aims of the present study were to: 1) evaluate habituation of laser-evoked potentials (LEP) to repeated painful stimulation of 1 tender and 2 nontender points; and 2) determine correlations between LEP abnormalities and major clinical features of FM. Fourteen consecutive FM outpatients and 13 normal controls were included. LEP were recorded from scalp designations Fz, Cz, Pz, T3, and T4. The dorsum of the right hand, the right supra-orbital zone, and the right knee (a tender point in all patients) were subjected to repeated CO2 laser stimuli. For each stimulation site, recordings were obtained for 3 consecutive series of 20 stimuli. The 3 main findings in FM patients were: 1) an increased amplitude of vertex LEP and subjective laser pain; 2) decreased habituation of vertex LEP and subjective laser pain; and 3) a correlation between reduced N2 wave habituation and the severity of self-reported depressive symptoms. As with other chronic pain syndromes, the pathophysiology of FM may involve a generalized increase in the perception of painful stimuli and reduced habituation of the sensory cortex. PERSPECTIVE: Reduced habituation of cortical responses to laser stimuli in FM patients suggests alterations in the pattern of cortical excitability. This is facilitated by depressive symptoms and abnormalities in central neurotransmission. These findings provide further support for the use of medications with effects on the central nervous system in the management of FM.

Effects of high-frequency repetitive transcranial magnetic stimulation of primary motor cortex on laser-evoked potentials in migraine.

The aim of this study was to examine the effects of high-frequency (HF) repetitive transcranial magnetic stimulation (rTMS) of the left primary motor cortex (M1) on subjective pain and evoked responses induced by laser stimulation (LEPs) of the contralateral hand and supraorbital zone in a cohort of migraine patients without aura during the inter-critical phase, and to compare the effects with those of non-migraine healthy controls. Thirteen migraine patients and 12 sex- and age-matched controls were evaluated. Each rTMS session consisted of 1,800 stimuli at a frequency of 5 Hz and 90% motor threshold intensity. Sham (control) rTMS was performed at the same stimulation position. The vertex LEP amplitude was reduced at the trigeminal and hand levels in the sham-placebo condition and after rTMS to a greater extent in the migraine patients than in healthy controls, while the laser pain rating was unaffected. These results suggest that HF rTMS of motor cortex and the sham procedure can both modulate pain-related evoked responses in migraine patients.