Abstract
Motor cortex excitability can be measured by single- and paired-pulse transcranial magnetic stimulation (TMS). Repetitive transcranial magnetic stimulation (rTMS) can induce neuroplastic effects in stimulated and in functionally connected cortical regions. Due to its ability to non-invasively modulate cortical activity, rTMS has been investigated for the treatment of various neurological and psychiatric disorders. However, such studies revealed a high variability of both clinical and neuronal effects induced by rTMS. In order to better elucidate this meta-plasticity, rTMS-induced changes in motor cortex excitability have been monitored in various studies in a pre-post stimulation design. Here, we give a literature review of studies investigating motor cortex excitability changes as a neuronal marker for rTMS effects over non-motor cortical areas. A systematic literature review in April 2014 resulted in 29 articles in which motor cortex excitability was assessed before and after rTMS over non-motor areas. The majority of the studies focused on the stimulation of one of three separate cortical areas: the prefrontal area (17 studies), the cerebellum (8 studies), or the temporal cortex (3 studies). One study assessed the effects of multi-site rTMS. Most studies investigated healthy controls but some also stimulated patients with neuropsychiatric conditions (e.g., affective disorders, tinnitus). Methods and findings of the identified studies were highly variable showing no clear systematic pattern of interaction of non-motor rTMS with measures of motor cortex excitability. Based on the available literature, the measurement of motor cortex excitability changes before and after non-motor rTMS has only limited value in the investigation of rTMS related meta-plasticity as a neuronal state or as a trait marker for neuropsychiatric diseases. Our results do not suggest that there are systematic alterations of cortical excitability changes during rTMS treatment, which calls into question the practice of re-adjusting the stimulation intensity according to the motor threshold over the course of the treatment.
Highlights
Repetitive transcranial magnetic stimulation is capable of modulating cortical excitability in a frequency dependent manner
In this systematic review we report data from 29 studies, which were identified by systematic literature research, in which changes of motor cortex excitability induced by non-motor Repetitive transcranial magnetic stimulation (rTMS) or TBS have been investigated
In 19 of the 29 studies, the effect of different stimulation protocols on resting motor threshold (RMT) was investigated. The results of these studies did not show any clear evidence for a systematic influence of nonmotor rTMS on RMT. This is of high practical relevance, since stimulation intensity is typically adjusted to individual RMT
Summary
Repetitive transcranial magnetic stimulation (rTMS) is capable of modulating cortical excitability in a frequency dependent manner. High frequency rTMS (≥5 Hz) has been shown to induce long-term potentiation-like effects, whereas low frequency rTMS (≤1 Hz) typically leads to longterm depression like effects (Fitzgerald et al, 2006; Thut and Pascual-Leone, 2010). Review in non-motor rTMS of pulses with a frequency of 5 Hz [intermittent (iTBS) or continuous theta burst stimulation (cTBS)] (Huang et al, 2005). Basic mechanisms of different rTMS protocols over the motor cortex have been intensely evaluated. In these studies, motor cortex excitability has been measured before and after rTMS using electromyographic activity which has been recorded after single and paired pulses of TMS over the corresponding area of the motor homunculus. Typical measures are resting motor threshold (RMT), motor-evoked potentials (MEPs), shortinterval cortical inhibition (SICI), intracortical facilitation (ICF), and cortical silent period (CSP)
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