Transcranial magnetic stimulation in neurology: A review of established and prospective applications

Abstract

Transcranial magnetic stimulation (TMS) is a neurophysiologic technique to noninvasively induce a controlled current pulse in a prespecified cortical target. This can be used to transiently disrupt the function of the targeted cortical region and explore causal relations to behavior, assess cortical reactivity, and map out functionally relevant brain regions, for example during presurgical assessments. Particularly when applied repetitively, TMS can modify cortical excitability and the effects can propagate trans-synaptically to interconnected cortical, subcortical, and spinal cord regions. As such, TMS can be used to assess the functional integrity of neural circuits and to modulate brain activity with potential therapeutic intent.

Figure 1. Neuronavigation equipment and a potential clinical application of transcranial magnetic stimulation

(A) A schematic representation of how central motor conduction time (CMCT) is calculated. Motor evoked potentials (MEPs) are shown following cortical stimulation (i) and after cervical spinal root stimulation (ii). Note that the EMG baseline during cortical stimulation is active because CMCT is typically measured with the target muscle under slight voluntary contraction in order to reduce the MEP latency to the target muscle. CMCT is calculated by subtracting the onset latency of the MEP resulting from spinal stimulation (T2) from the onset latency of the MEP induced by the cortical stimulation (T1). An alternative method is to measure the F-wave following median nerve stimulation (iii) in order to assess peripheral conduction time. This can allow more precise measurement of the CMCT. F = onset latency of F-wave by median nerve stimulation; M = onset latency of M-wave by median nerve stimulation. (B) The Nexstim eXimia Navigated Brain System (NBS) TMS device makes use of stereotactic neuronavigation to target a cortical region that is predetermined on the basis of the subject's own individualized MRI. In this example, the NBS system is being employed to assess the subject's resting motor threshold based on stimulation of the subject's left primary motor cortex hand area (M1). Each red dot on the cortical surface (right panel) represents a single stimulation point in an attempt to locate the hotspot in M1 that will yield the most robust MEP in the contralateral first dorsal interosseous muscle (depicted in the left panel). Once located, this hotspot is stimulated at varying intensities to determine the subject's motor threshold. In this example, the determined motor threshold will be used to perform concurrent TMS and EEG.