Identify whether ictal thalamic EEG signals could be derived from ambulatory recordings in patients with pharmacoresistant epilepsy treated with deep brain stimulation (DBS) targeting bilateral anterior thalamic nuclei (ATN) or centromedian nuclei (CMN).

The DBS system is one modality that can target thalamic nuclei for neuromodulation. The most recently available model features the capability to record live streamed intracranial EEG (local field potentials, or LFP).

We selected four patients who had habitual electroclinical and electrographic seizures during live streaming (based on the simultaneous scalp EEG). All the devices were Percept^TM PC (by Medtronic®), utilizing the BrainSense® feature to record bipolar LFP signals between two contacts of each of the bilateral four-contact leads. LFP time series were extracted using MATLAB and exported as EEG for review and visualization. 

Three patients were adults (mean age 43, median age 37), and one was a child (10 years). Two had generalized forms of epilepsy (acquired Lennox-Gastaut Syndrome due to diffuse bilateral polymicrogyria; generalized non-lesional epilepsy characterized by absence and myoclonic seizures). One patient had unilateral hemispheric epilepsy due to dual pathology (adult-onset Rasmussen's encephalitis and focal cortical dysplasia) and prior frontal topectomy. The last patient presented with non-lesional multifocal epilepsy and a history of left parietal topectomy. The DBS for two types of generalized epilepsy targeted bilateral CMN, while the other two targeted bilateral ATN. 

Generalized ictal patterns involving the four contacts of both thalamic nuclei have been identified as ictal patterns. In contrast, in hemispheric or multifocal epilepsies, only unilateral thalamic nuclei exhibited ictal EEG changes. Ictal EEG alterations consisted of a combination of rhythmic activity and sharp waves or rhythmic spiking activity. Our figures depict the ictal thalamic recordings with the corresponding ictal thalamic waves. 

Our observations call for a more systematic investigation of ictal thalamic signature in various types of epilepsy.