gms | German Medical Science

Objective: Intraoperative targeting during deep brain stimulation (DBS) is based on electrophysiological recordings, e.g. spike activity, test stimulation and the patient’s report. However, there are targets such as the thalamus in which classical targeting tools fail. Targets with complex interconnections ultimately require a tool that has the potential to elicit anatomical and functional connectivity to distant parts of the brain. Here we present our experiences with nucleo-cortical evoked potential (NCEP) during DBS surgery.

Methods: The implantation of 17 DBS electrodes in the thalamus was examined. Patients underwent DBS in Vim, VoA and the anterior thalamus (AT) for Essential Tremor, Holmes-Tremor and epilepsy. After the insertion of 1-4 microelectrodes and the determination of the individual threshold for side effects (SE), NCEPs were evoked by stimulating (30-60 pulses, 1-2 Hz, 500 µs, 90% SE, 5 mA in general anesthesia) in 1-2 mm steps until target depth was reached. Concurrently, multi-channel electroencephalography (EEG) was recorded. Data was processed instantaneous in the operating room. In order to prove functional connectivity, coherence between local field potentials (LFP) and EEG activity during rest and movement was calculated in a subgroup of patients.

Results: NCEPs were found 82% of the cases. The NCEP appears depending on the target as a positive or negative deflection between 30 and 120 ms after the stimulus. For Vim stimulation NCEPs are found over the ipsilateral motor cortex, whereas in VoA and AT stimulation, the NCEP appeared frontotemporal, consistent with the anatomical connections. In 85%, the microelectrode evoking the highest NCEP on target depth was selected for implantation of the permanent electrode considering electrophysiological findings and results of the test stimulation. The depth with the strongest NCEP correlated in 82% with the contacts used for chronic stimulation. In 89%, depth and microelectrode with the strongest NCEP correlated with peak of oscillatory activity in LFP recordings. Furthermore, this activity was coherent with EEG activity indicating functional connections.

Conclusions: Cortical-evoked potentials are a feasible tool for intraoperative targeting during DBS surgery with the potential to elicit not only anatomical but also functional connections between the thalamus and the cortex. Thus, this technique might help to improve accuracy of DBS surgery for targets with no clear electrophysiological marker or acute stimulation effects such as the anterior thalamus.