The relationship between TMS measures of functional properties and DTI measures of microstructure of the corticospinal tract.
ABSTRACT Recently, a link between resting motor threshold (RMT) and local tissue microstructure, as indexed by fractional anisotropy (FA), was demonstrated in large parts of white matter. However, regions showing such correlations were generally found outside of the corticospinal tract (CST). Therefore, the question arises whether other electrophysiologic measurements could be more locally related to microstructural properties of the CST. In this study, we explored the relationship between such measurements and regional FA in a group of healthy volunteers.
We hypothesized that RMT might be more related to an overall susceptibility of white matter to TMS, whereas other electrophysiologic markers might be more specifically related to properties of the CST only.
Thirty-seven subjects were included. We studied RMT, active motor threshold (AMT), intensity to evoke a motor-evoked potential (MEP) of 1 mV (S1mV), MEP input-output curve (IO-curve), and central motor conduction time (CMCT) using transcranial magnetic stimulation, and FA of the corticospinal tract using diffusion tensor magnetic resonance imaging. We performed voxel-wise and TBSS correlation analysis between these electrophysiologic measurements and FA. In addition, we tested for significant correlation between these parameters and mean diffusivity (MD).
On voxel-wise analysis, we did not detect significant correlations between any electrophysiologic parameter (RMT, AMT, S1mV, IO curve slope, CMCT) and FA. With TBSS, we detected correlations between FA and bilateral AMT, as well as left-hemispheric S1mV, but these correlations were found in locations unlikely to contribute to motor pathways.
Although a relationship between structure and function has been shown in many other regions of the brain, it seems to be much more challenging to demonstrate such a relationship in the CST of healthy subjects.
Article: Prognostic value of cortically induced motor evoked activity by TMS in chronic stroke: caveats from a revealing single clinical case.[show abstract] [hide abstract]
ABSTRACT: We report the case of a chronic stroke patient (62 months after injury) showing total absence of motor activity evoked by transcranial magnetic stimulation (TMS) of spared regions of the left motor cortex, but near-to-complete recovery of motor abilities in the affected hand. Multimodal investigations included detailed TMS based motor mapping, motor evoked potentials (MEP), and Cortical Silent period (CSP) as well as functional magnetic resonance imaging (fMRI) of motor activity, MRI based lesion analysis and Diffusion Tensor Imaging (DTI) Tractography of corticospinal tract (CST). Anatomical analysis revealed a left hemisphere subinsular lesion interrupting the descending left CST at the level of the internal capsule. The absence of MEPs after intense TMS pulses to the ipsilesional M1, and the reversible suppression of ongoing electromyographic (EMG) activity (indexed by CSP) demonstrate a weak modulation of subcortical systems by the ipsilesional left frontal cortex, but an inability to induce efficient descending volleys from those cortical locations to right hand and forearm muscles. Functional MRI recordings under grasping and finger tapping patterns involving the affected hand showed slight signs of subcortical recruitment, as compared to the unaffected hand and hemisphere, as well as the expected cortical activations. The potential sources of motor voluntary activity for the affected hand in absence of MEPs are discussed. We conclude that multimodal analysis may contribute to a more accurate prognosis of stroke patients.BMC Neurology 06/2012; 12:35. · 2.17 Impact Factor