Minimal Tissue Reaction after Chronic Subdural Electrode Implantation for Fully Implantable Brain–Machine Interfaces

There is a growing interest in the use of electrocorticographic (ECoG) signals in brain–machine interfaces (BMIs). However, there is still a lack of studies involving the long-term evaluation of the tissue response related to electrode implantation. Here, we investigated biocompatibility, including chronic tissue response to subdural electrodes and a fully implantable wireless BMI device. We implanted a half-sized fully implantable device with subdural electrodes in six beagles for 6 months. Histological analysis of the surrounding tissues, including the dural membrane and cortices, was performed to evaluate the effects of chronic implantation. Our results showed no adverse events, including infectious signs, throughout the 6-month implantation period. Thick connective tissue proliferation was found in the surrounding tissues in the epidural space and subcutaneous space. Quantitative measures of subdural reactive tissues showed minimal encapsulation between the electrodes and the underlying cortex. Immunohistochemical evaluation showed no significant difference in the cell densities of neurons, astrocytes, and microglia between the implanted sites and contralateral sites. In conclusion, we established a beagle model to evaluate cortical implantable devices. We confirmed that a fully implantable wireless device and subdural electrodes could be stably maintained with sufficient biocompatibility in vivo.

Electrical Stimulation Mapping of Brain Function: A Comparison of Subdural Electrodes and Stereo-EEG

Despite technological and interpretative advances, the non-invasive modalities used for pre-surgical evaluation of patients with drug-resistant epilepsy (DRE), fail to generate a concordant anatomo-electroclinical hypothesis for the location of the seizure onset zone in many patients. This requires chronic monitoring with intracranial electroencephalography (EEG), which facilitates better localization of the seizure onset zone, and allows evaluation of the functional significance of cortical regions-of-interest by electrical stimulation mapping (ESM). There are two principal modalities for intracranial EEG, namely subdural electrodes and stereotactic depth electrodes (stereo-EEG). Although ESM is considered the gold standard for functional mapping with subdural electrodes, there have been concerns about its utility with stereo-EEG. This is mainly because subdural electrodes allow contiguous sampling of the dorsolateral convexity of cerebral hemispheres, and permit delineation of the extent of eloquent functional areas on the cortical surface. Stereo-EEG, while having relatively sparse sampling on the cortical surface, offers the ability to access the depth of sulci, mesial and basal surfaces of cerebral hemispheres, and deep structures such as the insula, which are largely inaccessible to subdural electrodes. As stereo-EEG is increasingly the preferred modality for intracranial monitoring, we find it opportune to summarize the literature for ESM with stereo-EEG in this narrative review. Emerging evidence shows that ESM for defining functional neuroanatomy is feasible with stereo-EEG, but probably requires a different approach for interpretation and clinical decision making compared to ESM with subdural electrodes. We have also compared ESM with stereo-EEG and subdural electrodes, for current thresholds required to evoke desired functional responses vs. unwanted after-discharges. In this regard, there is preliminary evidence that ESM with stereo-EEG may be safer than ESM with subdural grids. Finally, we have highlighted important unanswered clinical and scientific questions for ESM with stereo-EEG in the hope to encourage future research and collaborative efforts.

Minimally Invasive, Endoscopic-Assisted Device for Subdural Electrode Implantation in Epilepsy

BACKGROUND Subdural grids and strip electrodes provide wide coverage of the cerebral cortex, precise delineation of the extent of the seizure onset zone, and improved spatial sampling to perform functional mapping for eloquent cortex. OBJECTIVE To describe a novel device that allows for a minimally invasive approach to implantation of subdural grid and strip electrodes. METHODS A skull mounted device was created to allow for implantation of subdural electrodes through a keyhole craniotomy with direct visualization using the aid of a flexible neurovideoscope. The initial studies in preparation for grid development performed on cadaveric skulls were analyzed to determine the size of craniotomy required for deployment, maximal distance of strip electrode deployment from center of craniotomy, and visual inspection of the cortex was performed for any underlying damage. RESULTS The device allowed for the placement of subdural electrodes through a 40-mm craniotomy. Subdural electrodes were deployed in multiple directions to a distance of a 70-mm radius from the center of the craniotomy. There was no visual damage to the underlying cortex after the procedures were completed. CONCLUSION Large craniotomies are typically desired to provide direct visualization of the implantation of subdural electrodes, but can increase the risk of subdural hemorrhages and infections. This study describes a novel minimally invasive endoscopically assisted device for the implantation of subdural strip electrodes under direct visualization. With this device, we are able to limit the size of the craniotomy, avoid incision through the temporalis muscle, and implant subdural electrodes with visualization of the cortex.

History of Invasive EEG

‘History of Invasive EEG’ traces the development of electroencephalography (EEG) from the pioneering animal studies of Richard Caton (1875) up to the more recent development of stereoencephalography (SEEG) and large subdural electrodes. The following historical highlights are discussed: the first recording of EEG seizures in animals (PY Kaufman); the first recording of EEG in humans (Hans Berger); the first recording of interictal and ictal epileptiform discharges in humans (Hans Berger); the important role played by Reginald Bickford, Carl Sem-Jacobsen, and Jose Delgado in the development of chronically implanted invasive recordings; the development of stereotactic techniques for precise insertion of depth electrodes (Jean Talairach and Jean Bancaud); the use of large subdural plates for recording of EEG and cortical stimulation of extensive cortical regions.

Subdural EEG in Temporal Lobe Epilepsy

Invasive extraoperative EEG using subdural grids and strip electrodes is indicated for well-selected patients with suspected temporal lobe epilepsy (TLE) to confirm and delineate the epileptogenic zone prior to resection. This chapter discusses indications, hypothesis, and strategy for implantation of subdural electrodes. The emphasis is on a practical approach, with patients with MRI-negative and MRI-positive TLE being discussed separately and with a number of illustrative cases being presented.

Intracranial Electroencephalographic Monitoring: From Subdural to Depth Electrodes

At the London Health Sciences Centre Epilepsy Program, stereotactically implanted depth electrodes have largely replaced subdural electrodes in the presurgical investigation of patients with drug-resistant epilepsy over the past 4 years. The rationale for this paradigm shift was more experience with, and improved surgical techniques for, stereoelectroencephalography, a possible lower-risk profile for depth electrodes, better patient tolerability, shorter operative time, as well as increased recognition of potential surgical targets that are not accessible to subdural electrodes.