Comparisons of 256-channel dEEG recordings have been made with subsampling that shows what would be seen by conventional (19-channel) EEG. These studies confirm that not only accurate localization but in some cases the detection of neuropathology may require the channel densities of 128 or 256 provided by dEEG (Holmes, 2008; Holmes et al., 2004; Lantz, et al., 2003).
Neuropathology that is limited to a small area of brain near the skull (such as a gyrus of the cortex) may project an EEG field that is invisible to conventional EEG. With the 10-20 system placement of conventional EEG, the electrodes are as far as 7 cm apart.
Holmes, M. D. (2008). Dense array EEG: Methodology and new hypothesis on epilepsy syndromes. Epilepsia, 49, 3-14.
Holmes, M. D., Brown, M., & Tucker, D. M. (2004). Are "generalized" seizures truly generalized? Evidence of localized mesial frontal and frontopolar discharges in absence. Epilepsia, 45(12), 1568-1579.
Lantz, G., Grave de Peralta, R., Spinelli, L., Seeck, M., & Michel, C. M. (2003). Epileptic source localization with high density EEG: how many electrodes are needed? Clin Neurophysiol, 114(1), 63-69.
Techniques for source localization
EEG is the most central diagnostic tool for presurgical evaluation of medically intractable focal epilepsy. Typically, multichannel EEG synchronized with video monitoring is performed continuously for several days. The resulting EEG traces (or waveforms) are then analyzed for patterns typical for the patient's seizures. These patterns are assessed by the examination of the EEG traces during interictal discharges, as well as before, during, and after seizures. In the hands of an experienced epileptologist, this kind of trace analysis has a considerable value in localizing the epileptogenic area of the brain for surgery.
To obtain sufficient precision for surgery, it is important to use several complementary techniques to more accurately localize the epileptogenic foci. Modern presurgical evaluation centers generally include neurological and neuropsychological exams, high-resolution MRI, PET, and/or interictal and ictal SPECT, and sometimes MRI-based volumetry and spectroscopy in the battery of exams that aim to precisely determine the epileptogenic focus. In cases of unclear or nonconcordant results through these exams, invasive recordings from surgically implanted electrodes are usually needed as well.
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