Fast ripples reflect increased excitability that primes epileptiform spikes

Area of Science:

• Neuroscience
• Epilepsy Research
• Computational Biology

Background:

• Neuronal circuit disturbances driving epilepsy remain unclear.
• Understanding inter-ictal and ictal epileptiform discharges is crucial for epilepsy treatment.

Purpose of the Study:

• Investigate the role of high-frequency oscillations in epileptiform discharge generation.
• Determine the predictive value of pre-spike fast ripples for seizure onset zones.
• Explore the clinical significance of fast ripples in epilepsy surgery.

Main Methods:

• Used extra-operative macro- and micro-electrode recordings during non-rapid eye movement sleep.
• Analyzed inter-ictal intracranial EEG spikes and associated fast ripples (200-600 Hz).
• Correlated fast ripple occurrence and characteristics with neuronal firing and seizure onset zones in two patient cohorts.

Main Results:

• Fast ripples, not ripples (80-200 Hz), frequently preceded inter-ictal spikes in the seizure onset zone.
• Pre-spike fast ripples were linked to higher spectral power and more vigorous neuronal firing.
• Sites with high pre-spike fast ripple proportion correlated with seizure onset zones, but resection did not ensure seizure freedom.

Conclusions:

• Fast ripples preceding EEG spikes suggest increased local excitability that primes discharges in the seizure onset zone.
• Epileptogenic brain regions are necessary but not sufficient for initiating inter-ictal epileptiform discharges.
• Fast ripples may serve as a biomarker for seizure onset zones, but their role in seizure generation is complex.