Electrophysiologic characteristics of single-unit recordings for intraoperative targeting in deep brain stimulation for refractory epilepsy

Introduction: Anterior nucleus of the thalamus (ANT) deep brain stimulation (DBS) is an increasingly promising treatment option for refractory epilepsy. Greatest seizure reduction has been associated with stimulation at the junction of ANT and the mammillothalamic tract (MTT), but electrophysiologic markers of this target are lacking. We examined single cell microelectrode recordings (MER) in order to identify unique electrophysiologic characteristics of ANT and the ANT-MTT junction to aid in intraoperative targeting.

Methods: Ten patients with medically refractory epilepsy underwent MER during ANT-DBS implantation under general anesthesia. MER recording locations were determined based on coregistration of preoperative MRI, postoperative CT, and the Morel atlas. Single units were extracted using the spike sorting program wav clus in Matlab. Single unit bursts were defined 3 or more spikes with interspike intervals less than 10 ms. Spike and bursting rates were characterized at recording depths and compared to anatomic boundaries. Recordings outside of the thalamus (e.g., striatum and ventricle) were excluded.

Results: Twenty hemispheres with 394 recording locations were included. The average firing rate in the ANT among all the patients was 7.52 Hz. There were 101 recording locations that exhibited a thalamic bursting pattern. The proportion of bursting cells was highest 0-2 mm above the ANT-MTT junction (two-sample z-test, p=0.0001). The average bursting frequency was 0.40 Hz. Burst frequency was negatively correlated with distance from the ANT-MTT junction (linear regression, p=0.035).

Conclusion : The ANT and ANT-MTT junction exhibit unique electrophysiologic characteristics that can be appreciated intraoperatively to aid in electrode targeting. Single cell data can be utilized in identifying patterns within these structures.