High-Frequency-Oscillations (HFOs) - epileptic sources localization

About 30% of all epilepsy cases are pharmacoresistant cases that cannot be pharmacologically controlled and a significant portion of such patients profit from surgical management. Intracerebral or subdural electrodes are implanted within the scope of the epilepsy surgery program in particularly severe cases. These electrodes are usually inserted with the objective of precise identification of epileptogenic focus prior to its surgical removal.
Measurements from depth electrodes represent a rare opportunity to contribute to the latest knowledge. From the medical point of view, research into pharmacoresistant epilepsy is extremely valuable because it allows monitoring of the basic functions and behavior of the brain in its deep structures. Research into the neurosciences does not take place in isolation, but is supported by co-operation with parallel research in technical and technological progress.
HFOs are observed in some EEG contacts in the frequency bandwidths from 80 Hz to 2000 Hz. There is a hypothesis that some HFOs most probably have a physiological importance and the others rather reflect intrinsic epileptogenicity of the dysfunctional brain tissue. Determination of the contribution of interictal epileptic oscillations can significantly help localize the epileptogenic zone in patients suffering from intractable epilepsy, and could be helpful as a new diagnostic approach.
HFOs include Ripples (80-200 Hz) and Fast Ripples (FR) (> 200 Hz). Ripples are mostly understood as a reflection of normal brain activity. HFOs above 200 Hz are directly connected with epileptic activity and are labeled High-Frequency Epileptic Oscillations (HFEOs). Three types of brain areas have been named – the seizure onset zone, which includes epileptic sources, the irritative area where epileptic activity occurs but there is no source, and the remote area without epileptic activity.
The methods of analysis are based in particular on the calculation of the spatial time-frequency spectral power distribution for the gamma band, ripple and FR frequencies up to 2000 Hz. Visual record inspection is commonly applied for HFO identification, through successful semiautomatic and automatic HFO detection. The numerical outputs are statistically evaluated and presented in graphic form.