Ictal Onset Patterns in Focal Epilepsy Systems

Ictal onset patterns in focal epilepsy systems describe the initial electrophysiological signatures marking the transition from a resting state to seizure activity. Two principal patterns dominate clinical and research classifications: hypersynchronous (HYP) onsets, characterised by large-amplitude, low-frequency spikes, and low-voltage fast (LVF) onsets, defined by rapid, low-amplitude oscillations at seizure initiation. High-resolution intracranial recordings reveal that these patterns reflect distinct network dynamics and tissue excitability: HYP onsets often localise to mesial temporal regions with marked neuronal loss, whereas LVF onsets engage broader temporal and neocortical substrates with preserved neuronal density. Computational models suggest that the excitability of peri-ictal tissue determines whether a focal perturbation induces a global network transition or coalesces through localised patches. Incorporating structural MRI, metabolic PET and advanced signal-processing techniques has further elucidated the heterogeneity of both onset zones and propagation pathways. A deeper understanding of these patterns underpins improved surgical targeting, informs neuromodulation strategies and supports the development of biomarkers to predict surgical outcomes and reduce treatment failures.

Research from Nature Portfolio

Recent studies have introduced a spatial perturbation framework to optimise implantation of depth electrodes by modulating the coupling between epileptic biomarkers and their spatial distribution. By virtually removing candidate seizure-onset zones and measuring system responses, this approach quantifies sampling adequacy and correlates stronger responses with accurate localisation and favourable surgical outcomes. The derived spatial perturbation map provides a qualitative tool to assess electrode coverage and predict the likelihood of seizure freedom based on probabilistic modelling of resection completeness.

Complementary work has demonstrated that phase–amplitude coupling dynamics in intracranial recordings can precisely delineate seizure-onset zones. Electrodes adjacent to the onset region exhibit elevated coupling between low-frequency phases and high-frequency amplitudes during pre-ictal periods, followed by a characteristic drop in coupling immediately before clinical seizure onset. This temporal marker refines pre-surgical mapping by improving the identification of critical contacts and enhancing the precision of resection margins.

Ictal Onset Patterns in Focal Epilepsy Systems publication trend

The graph below shows the total number of articles in ictal onset patterns in focal epilepsy systems across all publications each year (not limited to Nature Index journals).

Technical terms

Stereoelectroencephalography (SEEG): Intracranial recording technique using depth electrodes to map electrical activity during seizures.

Seizure-onset zone (SOZ): Brain region where ictal discharges originate.

Phase–Amplitude Coupling (PAC): Interaction between the phase of low-frequency and amplitude of high-frequency brain oscillations.

Hypersynchronous onset (HYP): Ictal pattern characterised by high-amplitude, low-frequency synchronous spikes at seizure initiation.

Low-voltage fast onset (LVF): Seizure-onset pattern marked by rapid, low-amplitude oscillations.

References

1. A spatial perturbation framework to validate implantation of the epileptogenic zone. Nature Communications (2024).
2. Influences of electrode density on intracranial seizure localisation: a single-blinded randomised crossover study. EBioMedicine (2025).
3. Delineating structural and metabolic abnormalities in amygdala and hippocampal subfields for different seizure‐onset patterns via stereotactic electroencephalography. CNS Neuroscience & Therapeutics (2024).
4. High-rate leading spikes in propagating spike sequences predict seizure outcome in surgical patients with temporal lobe epilepsy. Brain Communications (2023).
5. Functional and Anatomic Correlates of Two Frequently Observed Temporal Lobe Seizure‐Onset Patterns. Neural Plasticity (2000).
6. Mechanisms underlying different onset patterns of focal seizures. PLOS Computational Biology (2017).
7. Ictal Depth EEG and MRI Structural Evidence for Two Different Epileptogenic Networks in Mesial Temporal Lobe Epilepsy. PLOS ONE (2015).
8. Seizure localization using pre ictal phase-amplitude coupling in intracranial electroencephalography. Scientific Reports (2019).