Introduction: The immunologic reaction to chronic intracranial implants has been thoroughly studied: an initial acute phase response to implantation is followed by a low-grade chronic inflammatory response attributable to frustrated phagocytosis. While investigations have addressed how such changes may affect overall signal quality, the potential for changes in signal properties to affect clinical decision making has not been evaluated. To address these concerns, we critically evaluated recordings obtained from chronically-implanted subdural macroelectrodes.

Methods: Four dogs with spontaneous seizures were implanted with bilateral 8-contact subdural strip electrodes for chronic intracranial electroencephalographic (iEEG) recording (45-458 days) from a wireless system. Of the commonly used methods of seizure onset detection with low computational burden, we focused on line length and mean spectral power within clinically relevant frequency bands. The features were extracted for each channel over one minute windows for the duration of recording [Fig1]. Stability was quantified as the average variation in line length over 5 day periods.

Results: Line length generally decreased in amplitude and variability over time for each channel [Fig2] and across all four dogs [Fig3]. Spectral analysis demonstrated changes most significant in beta and gamma bands [Fig4]. Variation consistently dropped below 25% initial values 30 days after implantation [Fig5].

Conclusions: Significant changes in iEEG properties occur during the first month after implantation. Given the congruent time course of these changes with previously described occurrences at the electrode-tissue interface, a foreign-body reaction with formation of a glial capsule, changes to the electrode itself, or neuroplasticity may be responsible. These changes generate signal variability that has the potential to affect accurate diagnosis and proper therapeutic interventions during this subacute phase. Although further investigation of other signal characteristics and analysis of human data is required, our early findings suggest intracranial recordings during the first month are less reliable than once believed.

Patient Care: A better understanding of how changes occurring at the brain-electrode interface may affect electroencephalographic (EEG) recordings will ultimately improve our ability to properly interpret the information recorded from these electrodes. Our findings may help to explain the relatively poor seizure control achieved after relatively short periods of intracranial recording – recordings obtained after EEG stabilization may more accurately identify the true seizure onset zone for surgical resection. In addition, these findings suggest that chronically-implanted closed-loop devices require recalibration after EEG stabilization in order to provide reliable therapy.

Learning Objectives: By the conclusion of this session, participants should be able to: 1) Develop a general understanding of the physiologic changes occurring at the brain-electrode interface. 2) Understand the electrophysiologic changes occurring over time with chronically-implanted subdural electrodes 3) Discuss the potential biological mechanisms for these observed electroencephalographic changes. 4) Acknowledge the clinical implications of electroencephalographic changes over time.

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