Principles of Invasive EEG

The growing requirement for invasive EEG in presurgical evaluation of intractable focal epilepsy has been driven largely by the increasing complexity of epilepsy surgery cases. Extratemporal surgeries now exceed anterior temporal lobe resections for mesial temporal sclerosis, and the proportion of patients undergoing invasive EEGs has significantly increased. Half of all patients undergoing stereotactic EEG (SEEG) evaluations are MRI-negative (usually with focal cortical dysplasia type 1 or 2) and a third are reoperations for failed resective or palliative surgery. Certain principles guide the decision to use invasive EEG and the choice of invasive EEG technique. SEEG has distinct advantages, as do subdural grid evaluations and intraoperative corticography. The consequences of loose hypotheses in the decision to invasively evaluate a patient, and of inappropriate choice of technique, include poor seizure outcomes after surgery, morbidity, and mortality. This chapter discusses the guiding principles for invasive studies of the human epileptic brain.