Intracranial volume conduction of cortical spikes and sleep potentials recorded with deep brain stimulating electrodes.

OBJECTIVE: To examine interictal epileptiform and sleep potentials recorded intracranially from deep brain stimulation (DBS) electrodes in patients treated with DBS for epilepsy. Specifically, this study sought to determine whether the DBS-recorded potentials represent: (a) volume conduction from surface neocortical discharges or (b) transsynaptic propagation along cortical-subcortical pathways with local generation of the subcortical potentials near the DBS targets.
METHODS: Six patients with intractable epilepsy treated with thalamic DBS of the central median nucleus (CM; one patient) or anterior thalamus (5 patients) who had focal interictal spikes were studied. Sleep potentials were also studied in a 7th patient with Parkinson disease treated with DBS of the subthalamic nucleus (STN).
RESULTS: Focal interictal cortical spikes recorded by scalp electroencephalography (EEG) were recorded synchronously, but with opposite polarity, from the DBS electrodes in CM as well as the more superficial anterior thalamic contacts situated in the anterior nucleus (AN) and dorsal medial nucleus (DM). In referential montages, the subcortical potentials were of highest amplitude ipsilateral to the focal cortical spikes, with a small but reproducible amplitude decrement present at each electrode contact more distant from the cortical source, irrespective of the specific DBS target. Subcortical sleep potentials (K-complexes and sleep spindles) were also recorded synchronously and with inverse polarity compared to the corresponding scalp potentials, and appeared in a similar fashion at all subcortical sites sampled by the DBS electrodes. Amplitude attenuation in the thalamus of intracranial volume conducted potentials with increasing distance from their cortical spike sources was measured at approximately 5-10 microV/mm. DISCUSSION: Recent reports on scalp-CM or scalp-STN EEG recordings in patients treated with DBS for epilepsy have interpreted the intracranial waveforms as evidence of transsynaptic cortical-subcortical transmission across neuroanatomical pathways presumed to be involved in the generation of sleep potentials (Clin. Neurophysiol. 113 (2002) 25) and epileptiform activity (Clin. Neurophysiol. 113 (2002) 1391). However, our results show that the intracranial spikes recorded from DBS electrodes in various regions of the thalamus (CM, AN and DM) represent subcortical volume conduction of the synchronous cortical spikes recorded with scalp EEG. The same is true for the intracranial reflections of scalp EEG sleep potentials recorded from DBS electrodes in CM, AN, DM and STN. These interictal DBS waveforms thus cannot be used to support hypotheses of specific cortical-subcortical pathways of neural propagation or subcortical generation of the DBS-recorded potentials associated with scalp EEG interictal spikes and sleep potentials. SIGNIFICANCE: Detailed analysis of the intracranial potentials recorded from DBS electrodes in association with scalp EEG spikes and sleep discharges shows that the intracranial waveforms represent volume conduction from discharges generated in the neocortex and not, as has been suggested, locally generated activity resulting from cortical-subcortical neural propagation.