Stimulation Motor Threshold Research Articles

Transcranial electrical stimulation motor threshold can estimate individualized tDCS dosage from reverse-calculation electric-field modeling

BackgroundUnique amongst brain stimulation tools, transcranial direct current stimulation (tDCS) currently lacks an easy or widely implemented method for individualizing dosage. ObjectiveWe developed a method of reverse-calculating electric-field (E-field) models based on Magnetic Resonance Imaging (MRI) scans that can estimate individualized tDCS dose. We also evaluated an MRI-free method of individualizing tDCS dose by measuring transcranial magnetic stimulation (TMS) motor threshold (MT) and single pulse, suprathreshold transcranial electrical stimulation (TES) MT and regressing it against E-field modeling. Key assumptions of reverse-calculation E-field modeling, including the size of region of interest (ROI) analysis and the linearity of multiple E-field models were also tested. MethodsIn 29 healthy adults, we acquired TMS MT, TES MT, and anatomical T1-weighted MPRAGE MRI scans with a fiducial marking the motor hotspot. We then computed a “reverse-calculated tDCS dose” of tDCS applied at the scalp needed to cause a 1.00 V/m E-field at the cortex. Finally, we examined whether the predicted E-field values correlated with each participant’s measured TMS MT or TES MT. ResultsWe were able to determine a reverse-calculated tDCS dose for each participant using a 5 × 5 x 5 voxel grid region of interest (ROI) approach (average = 6.03 mA, SD = 1.44 mA, range = 3.75–9.74 mA). The Transcranial Electrical Stimulation MT, but not the Transcranial Magnetic Stimulation MT, significantly correlated with the ROI-based reverse-calculated tDCS dose determined by E-field modeling (R2 = 0.45, p < 0.001). ConclusionsReverse-calculation E-field modeling, alone or regressed against TES MT, shows promise as a method to individualize tDCS dose. The large range of the reverse-calculated tDCS doses between subjects underscores the likely need to individualize tDCS dose. Future research should further examine the use of TES MT to individually dose tDCS as an MRI-free method of dosing tDCS.

S40: Intraoperative neurophysiological monitoring in pediatric neurosurgery: a focus on the sacral system

s of Speakers / Clinical Neurophysiology 125, Supplement 1 (2014) S1–S339 S9 anodal direct cortical stimulation, activates the corticospinal tract close to the axon hillock at the border between the grey and the white matter. DCS remains the most focal stimulation possible and thus is favorable used for tumor removal within the vicinity of the motor cortex. MEPs became focus of neurooncological strategies for optimized glioma resection. Combining DCS-MEPs and monopolar cortical/subcortical stimulation mapping techniques in supratentorial tumor surgery as well as combination with intraoperative imaging could achieve supramarginal resection. The proposed subcortical stimulation motor threshold of 2 mA for safe resection has to be confirmed with regard to tumor type and precise location of stimulation. As been shown in cerebral aneurysm surgery, the ability to detect pure motor hemiparesis in carotid endarterectomy was more recently shown in a multicenter study. In spine surgery, presence or disappearance (“all-or-nothing” principle) of MEP amplitude criteria is commonly accepted: the presence of any MEPs is related to unaffected motor function or transient paresis, with the exception of radiculopathy. In cervical and thoracic intramedullary spinal cord surgery, the additional implementation of the D-wave helps to distinguish between permanent and transient motor deficit: D-wave preservation is always followed by a recovery of motor function. The latter seems to be applicable in supratentorial surgeries alike. In summary, MEPs are specific and sensitive for the assessment of the motor cortex and the corticospinal tract. In supratentorial tumor surgery MEPs are increasingly becoming a tool for “functional” guided resection strategies. S40 Intraoperative neurophysiological monitoring in pediatric neurosurgery: a focus on the sacral system F. Sala1, G. Squintani2, V. Tramontano2, C. Arcaro1 1Institute of Neurosurgery, Verona, Italy; 2Division of Neurology, Neurosciences, Verona, Italy Introduction: Intraoperative neurophysiologic monitoring (IOM) is nowadays extensively used in Pediatric Neurosurgery and tethered cord surgery is one of the most useful application. Our goal is to describe and discuss the standard IOM techniques used during tethered cord surgery, in the light of our clinical experience over the past ten years. Material and methods: Neurophysiological mapping of the conus-cauda is performed through direct stimulation of these structures and bilateral recording from segmental target muscles. While mapping identify ambiguous neural structures, their functional integrity during surgery can be assessed by monitoring techniques only, such as somatosensory evoked potentials (SEPs), transcranial motor evoked potentials (MEPs) from limb muscles and anal sphincters, and the bulbocavernosus reflex (BCR). Results: Between 2002 and 2012, we performed 48 surgical procedures in 47 patients with a tethered cord secondary to a variety of spinal dysraphisms. The monitorability rate was 84% for SEPs, 97% for limb muscle MEPs, 74% for the anal sphincter MEPs, and 59% for the BCR. In all patients but one SEP, MEP and BCR remained stable during surgery. Post-operatively two out of 47 patients presented a significant – though transient – neurological worsening. In six patients an unexpected muscle response was evoked by stimulating tissue macroscopically considered as not functional. Conclusions: Mapping techniques allow to identify and spare functional neural tissue and, vice-versa, to cut non-functional structures that may contribute to cord tethering. Monitoring techniques, MEP and BCR in particular, improve the reliability of intraoperative neurophysiology. IOM minimizes neurological morbidity in tethered cord surgery. S41 Neurophysiology of congenital mirror movements

Including prior knowledge for accurate and fast motor threshold estimation

Accurately estimating motor threshold (MT) is important in transcranial magnetic stimulation (TMS), because the MT is used as a reference to adjust individual subject’s machine output intensity. 1 Hallett M. Transcranial magnetic stimulation: a primer. Neuron. 2007; 55: 187-199 Abstract Full Text Full Text PDF PubMed Scopus (1072) Google Scholar MT is defined as the minimum TMS output intensity that can induce reliable motor evoked potentionals (MEPs) (usually > 100 μV), with a probability of 50%. The Parameter Estimation in Sequential Test (PEST) method, which is based on maximum likelihood regression, has previously been proposed to speed up MT estimation. 2 Awiszus F. TMS and threshold hunting. Suppl Clin Neurophysiol. 2003; 56: 13-23 Crossref PubMed Scopus (263) Google Scholar We recently proposed a new Bayesian PEST method that modifies the previous PEST method in two ways. 3 Qi F, Wu AD, Schweighofer N. Fast estimation of transcranial magnetic stimulation motor threshold. Brain Stimulation. [Epub ahead of print]. Google Scholar First, we used prior knowledge, in the form of a probabilistic model, to facilitate MT estimation. Second, we used the probability interval of the estimated MT distribution to control the estimation error. Specifically, the width of 95% of the area under the curve of MTs probability density function was taken as a measure of the estimation precision. We experimentally tested the accuracy, and hence safety, of our Bayesian method in 10 healthy subjects. 3 Qi F, Wu AD, Schweighofer N. Fast estimation of transcranial magnetic stimulation motor threshold. Brain Stimulation. [Epub ahead of print]. Google Scholar

A Review of Studies Comparing Methods for Determining Transcranial Magnetic Stimulation Motor Threshold: Observation of Movement or Electromyography Assisted

Early in the development of transcranial magnetic stimulation (TMS) as a research tool, scientists developed the concept of motor threshold (MT) as a method of quantifying dose and establishing safety. Neurophysiologists determined the MT with the aid of peripheral electromyography (EMG). More recently, clinicians started substituting the EMG with observed movements (OM) of the hand or digits. It is unclear whether the two approaches (EMG and OM) reach similar conclusions about an individual's cortical excitability and when best to use each approach. To review TMS studies directly comparing the EMG and OM MT methods. A systematic search of the literature on Medline from January 1995 through April 2008. Four articles directly compared the two approaches, reaching different and opposing findings. CONCLUSION S: The evidence supporting the substitution of EMG with OM is sparse and conflicting. Clinicians stimulating at or near safety guidelines should use the EMG method.

The transcranial magnetic stimulation motor threshold depends on the distance from coil to underlying cortex: a replication in healthy adults comparing two methods of assessing the distance to cortex

%Using transcranial magnetic stimulation (TMS), a handheld electrified copper coil against the scalp produces a powerful and rapidly oscillating magnetic field, which in turn induces electrical currents in the brain. The amount of electrical energy needed for TMS to induce motor movement (called the motor threshold [MT]), varies widely across individuals. The intensity of TMS is dosed relative to the MT. Kozel et al observed in a depressed cohort that MT increases as a function of distance from coil to cortex. This article examines this relationship in a healthy cohort and compares the two methods of assessing distance to cortex. mSeventeen healthy adults had their TMS MT determined and marked with a fiducial. Magnetic resonance images showed the fiducials marking motor cortex, allowing researchers to measure distance from scalp to motor and prefontal cortex using two methods: 1) measuring a line from scalp to the nearest cortex and 2) sampling the distance from scalp to cortex of two 18-mm-square areas. mConfirming Kozel’s previous finding, we observe that motor threshold increases as distance to motor cortex increased for both methods of measuring distance and that no significant correlation exists between MT and prefontal cortex distance. mDistance from TMS coil to motor cortex is an important determinant of MT in healthy and depressed adults. Distance to prefontal cortex is not correlated with MT, raising questions about the common practice of dosing prefontal stimulation using MT determined over motor cortex.