Objective: Assessing the predictive value of a volume of tissue activation (VTA) model for directional deep brain stimulation (DBS) leads on the response threshold of the corticospinal tract.

Background: In DBS, unintended current spread into adjacent structures like the corticospinal tract (CST) leads to side-effects like muscle cramps and is best avoided. Clinical testing for individual lead contacts has been standard of care, but time-consuming and uncomfortable for the patient. Computer models for the prediction of the brain tissue activation surrounding the stimulation leads have been available for some time, but few studies have tried to validate their predictions on the individual patient level.

Method: Akinetic-rigid PD patients with directional DBS leads in the subthalamic nucleus were eligible for this study. Inclusion criteria were the presence of unvoluntary muscle contractions at sufficiently high stimulation amplitudes (capsular response) and the availability of individual MRI imaging with diffusion tensor imaging (DTI). Exclusion criteria were tremor or dyskinesia interfering with the recordings. 12 patients were included in the study, 13 leads measured in total. Surface EMG electrodes were mounted in a belly-tendon montage on the muscles for recordings, another pair was placed on the connection cable between impulse generator and DBS leads for triggering. Low-frequency (20Hz) stimulation of the leads triggered a 40 msec recording period of which 2x 20 were averaged. Stimulation amplitude was increased in discrete steps until a clear muscle action potential became discernible which defined the capsular respond threshold. This measurement was repeated for every single contact.
With the help of a commercial software (Brainlab Elements with GuideXT), lead localization in relation to the individual CST was determined. For every contact, the threshold at which the VTA reached the CST in the computer model was noted.

Results: A highly significant correlation between the modelled as well as measured thresholds could be determined in a mixed linear effect model. The mean bias between measured and modelled thresholds as determined by the Bland-Altman plot was only 0.3 mA.

Conclusion: This computer model based upon patient-specific anatomy and tractography can provide a valid and precise prediction of the capsular response threshold for directional DBS leads.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/clinical-and-electrophysiological-validation-of-a-volume-of-tissue-activation-model-for-directional-deep-brain-stimulation-leads-the-recaps-study/