Objective: To study the pyramidal tract side effects (PTSEs) induced by the spread of current from the subthalamic nucleus (STN) to the pyramidal tract (PT), in parkinsonian patients undergoing STN stimulation.
Methods: We assessed 14 patients bilaterally implanted with tetrapolar electrodes. For each side separately, we detected the threshold of adverse effects induced by monopolar stimulation delivered by the chronically used contact. The voltage was progressively increased until the patient experienced discomfort. We videotaped all the PTSEs induced at 130 Hz - high-frequency stimulation (HFS) - and at 2 or 3 Hz - low-frequency stimulation (LFS). Superimposing the pre- and postoperative MRIs, we measured the distance (R) from the centre of the used contact (cC) to the medial border of the PT.
Results: The progressive increase in voltage at HFS induced tonic motor contractions, mainly located in the face, in 27/28 electrodes. LFS induced synchronous rhythmic myoclonus in the same territory. PTSEs induced at threshold voltage by HFS were observed in the upper face at 13/28 electrodes (bilaterally in 6 cases) and in the contralateral lower face at 5 electrodes. A positive correlation was found between the stimulus intensity capable of eliciting motor contractions at HFS and R.
Conclusions: HFS of the STN preferentially activates the corticobulbar tract in comparison with the corticospinal tract. Therefore, cranial motor contractions need to be looked for during electrical parameter setting. The positive correlation between the electrical intensity threshold for PTSEs and R reflects the need for millimetre accuracy in electrode positioning.
- Deep brain stimulation
- Parkinson's disease
- Pyramidal tract
- Subthalamic nucleus