Abstract
Over the past decade, theta-burst stimulation (TBS) has become a focus of interest in neurostimulatory research. Compared to conventional repetitive transcranial magnetic stimulation (rTMS), TBS produces more robust changes in cortical excitability (CE). There is also some evidence of an analgesic effect of the method. Previously published studies have suggested that different TBS parameters elicit opposite effects of TBS on CE. While intermittent TBS (iTBS) facilitates CE, continuous TBS (cTBS) attenuates it. However, prolonged TBS (pTBS) with twice the number of stimuli produces the opposite effect. In a double-blind, placebo-controlled, cross-over study with healthy subjects (n = 24), we investigated the effects of various pTBS (cTBS, iTBS, and placebo TBS) over the right motor cortex on CE and pain perception. Changes in resting motor thresholds (RMTs) and absolute motor-evoked potential (MEP) amplitudes were assessed before and at two time-points (0–5 min; 40–45 min) after pTBS. Tactile and thermal pain thresholds were measured before and 5 min after application. Compared to the placebo, prolonged cTBS (pcTBS) transiently increased MEP amplitudes, while no significant changes were found after prolonged iTBS. However, the facilitation of CE after pcTBS did not induce a parallel analgesic effect. We confirmed that pcTBS with twice the duration converts the conventional inhibitory effect into a facilitatory one. Despite the short-term boost of CE following pcTBS, a corresponding analgesic effect was not demonstrated. Therefore, the results indicate a more complex regulation of pain, which cannot be explained entirely by the modulation of excitability.
Highlights
Repetitive transcranial magnetic stimulation of the primary motor (M1) cortex modulates cortical excitability (CE) with subsequent neuroplastic changes observed in the stimulated area and its association areas, as well (Cárdenas-Morales et al, 2010)
The mean baseline resting motor thresholds (RMTs) for the whole sample was 48.7 ± 8.6% of the maximum device output. Both prolonged TBS (pTBS) sessions were less tolerated than plcTBS; prolonged iTBS (piTBS) was less tolerated than prolonged cTBS (pcTBS) [Visual Analog Scale (VAS): piTBS: median 3 (IQR 1–6); pcTBS 1 (0–3), plcTBS 0 (0–0); Friedman’s ANOVA χ2 = 22.1, p < 0.001; post hoc: piTBS vs. pcTBS; z = 3.52, p < 0.001]
Our findings confirmed the effect of pcTBS on the facilitation of CE, demonstrated by the increase of motor-evoked potential (MEP) amplitudes and the decrease of RMT
Summary
Repetitive transcranial magnetic stimulation (rTMS) of the primary motor (M1) cortex modulates cortical excitability (CE) with subsequent neuroplastic changes observed in the stimulated area and its association areas, as well (Cárdenas-Morales et al, 2010). There is evidence that TBS produces even more robust changes in CE than those observed in the conventional rTMS protocols (Gamboa et al, 2010; Suppa et al, 2016). TBS offers the possibility to induce changes in CE with a more pronounced post-modulation effect in the regulation of corticospinal excitability and synaptic plasticity, with the potential to optimize clinical stimulation protocols (Gamboa et al, 2010). A prolonged form of cTBS (pcTBS) with twice the number of stimuli (1,200 pulses) produces a facilitatory effect similar to that of iTBS (Gamboa et al, 2010). CE can be differently modulated for the TBS frequency used (30 Hz vs. 50 Hz), whereby its effect depends on time (minutes after TBS application) and interindividual differences between subjects (Chung et al, 2016)
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