Glioma Intraoperative MicroElectroCorticoGraphy

Electroencephalography (EEG) is a widely-used tool for studying brain activity, developed in humans in the late 1920s. Its indications have evolved as brain imaging techniques have developed, but it remains extremely useful in the fields of epileptology, comas, brain death, post-anoxic, metabolic, toxic encephalopathies and encephalitis. Alongside scalp EEG, intracranial recordings have been developed to optimize the localization of epileptic activity, with a view to epilepsy surgery.

Two main categories of intracranial electrodes are currently in use.

1. StereoElectroEncephalopGraphy electrodes consist of a stylus with a string of cylindrical contacts along its length, implanted directly into the brain parenchyma.
2. ElectroCorticoGraphy (ECoG) electrodes, non-penetrating, organized in strips or grids of 4 to 64 electrodes, disc-shaped, 0.2 to 1 cm in diameter, 0.5 to 1 mm thick, used to collect cerebral activity at the cortical surface.

Among its indication, surgery is a favorable circumstance for using ECoG. During brain surgery, cortical exposure enables cortical EEG recording without additional invasiveness. Intraoperative ECoG is of particular interest in epilepsy surgery, recently reactivated by the discovery of better electrophysiological biomarkers. In glioma surgery, often performed under awake conditions, cortical recording helps to establish a functional cerebral map and to adapt the resection. Indeed, ECoG recordings can be performed during awake surgery, without any disturbance of neuronal activity by anesthesia, to map epileptic activity (using visual identification of spikes) and to look for seizures induced by electrical stimulation. The evolution of these recording techniques is moving towards the development of tools for automatic analysis and mapping of new biomarkers, more relevant but more difficult to record because they are more focal and characterized by amplitudes close to background noise, such as high-frequency oscillations. New types of cortical electrodes are being developed to improve the spatial resolution (better localization), temporal resolution (faster activity detection) and detection sensitivity (signal-to-noise ratio) of cortical electrical signals.

New electrodes have recently been developed, based on the use of conductive organic polymer coatings such as poly(3,4-ethylenedioxythiophene (PEDOT: PSS), to optimize the signal-to-noise ratio, organized in electrode arrays that are 100 times thinner (5 microns vs. 0.5 mm for standard electrodes), 250 times smaller (between 30 and 500 microns vs. 8 mm for standard electrodes) and with more electrodes (128 vs. 6 or 8 for standard electrodes). This type of electrode, known as microECoG, has been validated in animals and has already been used in human research for intraoperative cortical recordings. They enable us to improve the quality of the recorded signal and to go down to a new scale, at the level of the neuron. Moreover, the very low thickness of the electrodes improves their conformability to the cortex, making them much more tolerable and enabling extended recordings to be envisaged as part of future man/machine interfaces.