Non-invasive methods for high-precision localization of epileptic foci

Together with neurosurgical clinics we are developing methods of non-invasive localization of epileptic foci. Epilepsy affects hundreds of millions of people across the world and for 20% of them drug therapy does not have a significant effect. In this case, neurosurgical intervention is an option. The operation success depends on the accuracy of the epileptic focus localization. Using MEG we are currently able to localize epileptic foci with high accuracy in the majority of cases. Effectiveness of our approach to the source localization is confirmed by results of invasive pre-surgical evaluations, as well as by outcomes of the neurosurgical intervention performed at of the Neurological Scientific Research Institute of Neurosurgery N. N. Burdenko.

One of our approaches is detection and localization of local high-frequency activity (60-90 Hz) that preceds the first clinical manifestations of the seizure and usually co-localize with the epileptogenic zone. Standard clinical evaluation is often limited to the frequency range of up to 40 Hz, whereas the higher frequency range is often contaminated with muscle artifacts and cannot be reliably detected with electroencephalography (EEG), a routine method for pre-surgical diagnosis. MEG allows reliable detection of this high-frequency activity (60-90 Hz) generated by a local area of the cerebral cortex - the target of surgical treatment.

The application of mathematical methods of signal analysis (e.g, 'Wavelet analysis') helps detecting such high-frequency activity. One example is given below. MEG has been registered in a patient with suspected temporal epilepsy during three typical seizures. All three seizures were preceded by an "electro-decrement," a short-term sudden suppression of the slow background activity. The electro-decrement was accompanied by an increase in the local high-frequency activity (60-90 Hz), as can be seen in the figure below.

We hope that further development of our approach will increase reliability of pre-surgical diagnostics and, as a result, would improve quality of surgical treatment of epilepsy.

Fig.1. Enhancement of the high-frequency activity immediately before the onset of three typical seizures in a patient.
A – Wavelet spectrograms. The spectrograms are based on the data preceding clinical manifestation
of the seizure  onset and not contaminated with muscle artifacts. The white vertical lines indicate the moment
of the ‘electro-decrement’ - the suppression of the spectral power in the range of 10-30 Hz.
Simultaneously, a sharp increase in power in the range of 60-90 Hz is observed.
Time periods: pre-ED - the moment before the electrodecrement; ED - electrodecrement;
ictal onset - further development of the ictal pattern.
B - Topography of high-frequency activity in the range (60-90 Hz) during electro-decrement.
In all three attacks the maximal enhancement of the high-frequency activity
preceding the seizures is located in the left hemisphere.

This study is conducted in collaboration with the Scientific Research Institute of Neurosurgery N. N. Burdenko, as well as with the RCCH FGBOU IN THE RENNIU them. N.I. Pirogova (Chadayev V.A.)

You can learn more about this work from the articles published in the Journal of Neurosurgery N.N. Burdenko and Epilepsy Research [doi: 10.1016/j.eplepsyres.2017.12.014]