Neurophysiological Characterization of Thalamic Nuclei in Epileptic Anaesthetized Patients

Abstract

Deep brain stimulation (DBS) requires precise localization, which is especially difficult at the thalamus, and even more difficult in anesthetized patients. We aimed to characterize the neurophysiological properties of the ventral intermediate (V.im), ventral caudal (V.c), and centromedian parvo (Ce.pc) and the magnocellular (Ce.mc) thalamic nuclei. We obtained microelectrode recordings from five patients with refractory epilepsy under general anesthesia. Somatosensory evoked potentials recorded by microelectrodes were used to identify the V.c nucleus. Trajectories were reconstructed off-line to identify the nucleus recorded, and the amplitude of the action potential (AP) and the tonic (i.e., mean frequency, density, probability of interspike interval) and phasic (i.e., burst index, pause index, and pause ratio) properties of the pattern discharges were analyzed. The Mahalanobis metric was used to evaluate the similarity of the patterns. The mean AP amplitude was higher for the V.im nucleus (172.7 ± 7.6 µV) than for the other nuclei, and the mean frequency was lower for the Ce.pc nucleus (7.2 ± 0.8 Hz) and higher for the V.c nucleus (11.9 ± 0.8 Hz) than for the other nuclei. The phasic properties showed a bursting pattern for the V.c nucleus and a tonic pattern for the centromedian and V.im nuclei. The Mahalanobis distance was the shortest for the V.im/V.c and Ce.mp/Ce.pc pairs. Therefore, the different properties of the thalamic nuclei, even for patients under general anesthesia, can be used to positively define the recorded structure, improving the exactness of electrode placement in DBS.

Features of Action Potentials from Identified Thalamic Nuclei in Anesthetized Patients

“…For this work, we followed the Schaltenbrand–Wahren (SW) atlas [ 19 ] nomenclature. What is called the parafascicular/centromedian Pf/CM in the literature is the centromedian (Ce) of ventroposterolateral and ventroposteromedial nuclei (VPL/VML) and are equivalent to ventrocaudal (V.c) [ 12 ].…”

“…However, most of these responses (except for the response to tactile stimuli) are nonspecific, and all of them need the conscious collaboration of the patient. Nevertheless, we have recently shown that raw traces are different for different thalamic nuclei in anesthetized patients [ 12 ].…”

“…Nonetheless, most of the information obtained during MER in clinical practice is restricted to the mean frequency of discharge and firing pattern (e.g., tonic, phasic, or more or less paused) [ 12 , 13 , 14 , 15 , 16 ], considering the AP as a bimodal variable (present or absent) that is represented as a vertical line without an inner structure in binary plots, under the assumption that the morphology of an AP does not carry any information [ 17 ]. However, we hypothesize that identified thalamic nuclei have different electrophysiological properties of extracellular APs recorded by MER during DBS.…”

Network Substrates of Centromedian Nucleus Deep Brain Stimulation in Generalized Pharmacoresistant Epilepsy

“…DBS implantation into the CM was performed under general anesthesia with propofol and isoflurane using an MRIguided stereotactic protocol, previously described elsewhere [15]. In brief, a stereotactic frame (Leksell Frame G, Elekta, Stockholm, Sweden) was placed and used to determine the target coordinates (X = 9, Y = − 9, Z = 0).…”

Can We Put Aside Microelectrode Recordings in Deep Brain Stimulation Surgery?

“…Although it has been proposed that diffusion tensor imaging can be useful in identifying white pathways [14], not all the relevant targets have these long pathways (e.g., the posterior hypothalamus). However, some thalamic [15,16] or hypothalamic [17] subnuclei show bioelectrical-specific features that allow a confident identification, even under general anesthesia. An unequivocal way to identify the thalamic ventrocaudal subnucleus is by the MER of somatosensory evoked potentials [18].…”