When a seizure starts in focal epilepsy, bursts of high-frequency activity (HFA) are often described in stereo-EEG (SEEG)recordings. This HFA is one of the most reliable signs that the seizure has begun. However, it is unclear how exactly this fast activity emerges from the inter ictal activity with inter ictal spiking. The study by Thandar Aung, Aude Jegou, and Patrick Chauvel tries to determine if this seizure-related fast activity appear suddenly at seizure onset, or if it build up gradually from earlier inter ictal spikes that already contain high-frequency components.

To explore this, the authors focused on a special type of inter ictal spike named the Spike-associated high-frequency oscillations(Sp-HFOs) in which quick bursts of high-frequency energy that occur within a spike. They wanted to see whether these Sp-HFOs progressively become more and more similar to the sustained HFA that marks seizure onset.

They analyzed SEEG recordings from 6 patients with drug-resistant focal epilepsy.
For each seizure, they looked at long inter ictal and the “preictal” periods and then the seizure onset itself (when sustained HFA appears).

They used time–frequency analysis to study the spectral content of spikes and fast activities. However, since there was no existing quantitative method to compare the frequency content of spikes and ictal HFA, the authors created the I-Fusion, which measures how much the frequency ranges of Sp-HFO and ictal HFA overlap.

The key results are outlined below:

1.     There is always a preictal transition phase

2.     Sp-HFOs evolve into ictal HFA : they are spectrally related

3.     The I-Fusion values can quantify how closely spikes resemble seizure activity

4.     The epileptogenic zone exhibits a progressive increase in spectral synchronization before the seizure.

This article supports a progressive, not abrupt,transition. The brain gradually prepares the seizure through a progressive increase in fast oscillatory synchronization. It highlights also Sp-HFOs as potential early biomarkers, they could help identify the epileptogenic zone more accurately and be valuable targets for closed-loop stimulation therapies.

This brings us closer to understanding how seizures start and provides promising avenues for prediction and surgical planning.

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Reference: https://onlinelibrary.wiley.com/doi/10.1111/epi.18703?af=R

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