Effects of paired-pulse transcranial magnetic stimulation of the motor cortex on perception of experimentally induced pain.

OBJECTIVE: We investigated the influence of paired-pulse transcranial magnetic stimulation (ppTMS) of the motor cortex (M1) on perception of noxious electrical stimuli. The nociceptive flexion reflex response was assessed to determine spinal effects. METHODS: In the first experiment, the effect of ppTMS of M1 on perception of noxious stimulation of the sural nerve was assessed by varying the stimulus onset asynchronies (SOAs) and the order of the stimulations (-400, -75, -25, 25, 125, 400 ms and control ppTMS, negative sign: ppTMS precedes the noxious stimulation). Effects of a preceding ppTMS on the RII and the RIII response of the nociceptive flexion reflex were investigated for SOAs of -400 and -75 ms. The effects of ppTMS of M1 and of the occipital cortex (Oz) on noxious stimulation of the radial nerve were investigated in a second experiment. Visual analogue scales were used to assess pain intensity and unpleasantness. RESULTS: The results revealed increased pain unpleasantness scores for SOAs of -75, -25, 25, and 400 ms and decreased pain intensity scores for a SOA of -400 ms, when the sural nerve and M1 were stimulated. An increase of the area of the RII response was found for a SOA of -75 ms. For stimulation of the radial nerve, ppTMS of Oz but not of M1 increased the perceived pain at a SOA of 25 ms. DISCUSSION: The facilitatory component of ppTMS led to increased pain perception when applied during the cortical process of Adelta fiber-mediated input, whereas the subsequent inhibitory component may lead to the opposite effect on the subsequent noxious stimulation.

High-frequency rTMS of the motor cortex does not influence the nociceptive flexion reflex but increases the unpleasantness of electrically induced pain.

The aim of this study was to investigate whether a 10-Hz repetitive transcranial magnetic stimulation (rTMS) applied over the motor cortex, using a stimulus paradigm employed for pain control in chronic pain, affects acute electrically induced pain. We investigated whether rTMS modulates the nociceptive flexion reflex (NFR) in addition to subjective pain perception. Pain threshold, NFR threshold, supra-threshold NFR response, and the concomitant pain intensity and pain unpleasantness visual analogue scale (VAS) scores were compared before and after 20 min of rTMS. Effects of 20 trains of 5 s' duration (55 s intertrain interval) of 10-Hz rTMS at 80% of the resting motor threshold (RMT) applied over the dominant motor cortex were compared to sham rTMS in 12 healthy volunteers. Supra-threshold NFR stimulation significantly increased pain unpleasantness VAS scores with real rTMS compared to sham rTMS (F(1,10)=6.91; P=0.025). There was no significant effect of 10-Hz rTMS on the subjective pain threshold or on the NFR, neither at threshold nor at supra-threshold noxious stimulation. The rTMS paradigm used to control chronic pain is not suitable for controlling Adelta fiber-mediated acute experimentally induced pain since the effects on pain perception were only marginal, with an increase in the VAS unpleasantness scores but with no effect on the NFR. The increased activity of cortico-thalamic projections might modulate the perception of Adelta fiber-mediated pain within the lateral pain pathway. The type of fiber that is stimulated and neuroplastic changes in chronic pain and are thought to be critical for rTMS to have an effect.