| Abstract No.: | B-F2174 |
| Country: | Canada |
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| Title: | MEASURING DETERMINANTS OF FIBRE TRACT INTEGRITY USING IMAGING GUIDED FOCAL TMS |
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| Authors/Affiliations: | 1 Sasan Ghinani*; 2 Basia Radlinska; 1 Alexander Thiel;
1 1) Neurological Sicences & Neurosurgery McGill University 2)Lady Davis Institute for Medical Research; 2 Lady Davis Institute for Medical Research, Montreal, QC, Canada
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| Content: | Objective: Transcranial Magnetic Stimulation (TMS) and Motor evoked potentials (MEP) have extensively been used in the past to assess gross corticospinal tract (CST) function. The introduction of neuroimaging guidance and focal coils has further increased the precision of TMS so that traditional parameters for description of MEP’s like suprathreshold amplitude and resting motor threshold may no longer be valid descriptors of CST function. It has recently been proposed that measures derived from dose-response relationships between stimulator output and MEP-amplitude may be better descriptors. We tested these new descriptors in normal subjects and patients with subcortical stroke.
Methods: 7 healthy hemispheres and 4 hemispheres affected by subcortical stroke were examined. fMRI was performed on all subjects to localize cortical activation corresponding to the hand representation in the motor cortex. A neuronavigation system was used to position the TMS coil at the area of maximum activation. Amplitude and latency of the MEP were collected over the full range of TMS stimulus intensities via surface EMG from the first dorsal interosseus muscle (FDI). Amplitudes of the MEPs were plotted against stimulator output to obtain a sigmoidal dose-response relationship for each hemisphere. The tangent to the steepest section of the sigmoidal curve and the intercept of the tangent with the abscissa were calculated using least-squares curve-fitting algorithm. This extrapolated motor threshold (EMT) can be used as a better standardized measure of MT because it is not based on stochastic definition. Other extrapolated parameters include the slope of the sigmoidal function and the maximum MEP.
Results: As expected, the maximum amplitude was lower over the affected side as compared to the non-affected side, corresponding to the loss of CST-fibres (mean of non-affected side 2.575 mV. Mean of affected side 1.441 mV). The EMT of the affected hemispheres was on average lower than those of non affected hemispheres (mean of affected side 77.42. Mean of non-affected side 85.20). This indicates a decreased cortical excitability of the primary motor-cortex caused by subcortical lesions. A positive linear relationship exists between the slope of the sigmoid function and the maximum amplitude of the MEP independent of damage to the CST. This may allow a more reliable measurement of MEP amplitude even in those patients where supra-maximal stimulation intensities cannot be reached. EMT was neither related to slope or amplitude indicating an independent parameter of cortical excitability.
Conclusion: These data may suggest an effect of subcortical CST-damage on motor-cortex excitability. Furthermore our method of measuring CST integrity provides new analytical parameters for describing CST function in patients with damage to this area. |
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