Depth electroencephalography in selection of refractory epilepsy for surgery. Our experience with the suboccipital approach.

Despite new diagnostic tools, the precise localisation of an epileptic focal discharge remains an important step in the surgical treatment of epilepsy. Conventional EEG not always gives enough information to decide about surgery and more invasive methods have to be used. Epidural, subdural, and intra-parenchymatous electrodes have been used to come closer to the epileptic foci. Superficial hemispheric foci are well recorded by conventional epidural or subdural, strip or grid electrodes. Deeper foci, located in the medial temporal lobe or limbic areas are much more difficult to access from surface electrodes and other methods have to be used. Stereotactic placed multielectrodes and foramen ovale electrodes are most commonly implanted. Since 1986 we have used multi-contact cylindrical soft subdural electrodes. At the beginning we made the electrodes in our Department. Later on they were commercially available. In our Clinic the electrodes are usually introduced via a suboccipital approach and directed to the medial aspects of the temporal lobes until the temporal poles on both sides. Usually a median electrode located in the interhemispheric fissure, and covering gyrus cinguli is also placed. Out of 60 procedures done for different kinds of epilepsy, including cases with tumour or other surgical epileptogenic lesions, a total of 14 patients with genuine temporal lobe epilepsy have been studied and operated by this method. The advantages of the method as compared to stereotactic intraparenchymatous implanted electrodes are: less risk of bleeding and the fact that the brain tissue in those sensitive areas remains intact. Compared to foramen ovale electrodes our technique covers larger area of the temporal lobes and allows to insert a medial limbic electrode, but demands 2 burr holes. A further advantage of our technique is that the electrodes remain in place until surgery, allowing for their use as anatomical landmarks for tailoring the extension of the resection. This approach has been found to be simple, safe and reliable. A further improvement may be the simultaneous use of PEG epidural electrodes to obtain an overall view of the electrical activity of the brain including surface, deep temporal and midline cortical areas.