Science & Discovery
An EEG can be essential in epilepsy — and still fail to show exactly where a seizure began.
The problem: scalp EEG records electrical activity after it has travelled through brain tissue, skull and scalp. Deep or very local seizure activity may be difficult to detect at the surface.
For decades, clinicians have used scalp EEG to help classify and localise seizures. It remains one of the most important tools in epilepsy. But modern intracranial recordings have shown why the surface picture can sometimes be incomplete.
A deep seizure can become visible only after it spreads
The insula is a good example. It lies deep within the lateral sulcus and has extensive connections with temporal, frontal, parietal and limbic networks. Seizures beginning there can spread quickly into more superficial cortex.
By the time a clear pattern appears on scalp EEG — or by the time dramatic symptoms appear — the electrical activity may already have moved away from the true onset zone. The scalp recording can therefore appear temporal or frontal even when the seizure began deeper.
Why stereo-EEG changed the picture
Stereo-electroencephalography (SEEG) uses depth electrodes placed in selected brain regions during specialist presurgical investigation. Because these electrodes record directly from the brain, they have helped reveal seizures originating in structures that scalp EEG may sample poorly, including the insula, cingulate cortex and operculum.
This has changed how clinicians interpret seizure semiology. A symptom may reflect the region a seizure has spread into rather than the site where it started. Modern localisation therefore combines the sequence of symptoms, scalp EEG, MRI and other imaging, and — in selected drug-resistant cases — intracranial recordings.
What this does not mean
It does not mean scalp EEG is unreliable or unimportant. It means every test has a detection limit. Scalp EEG is extremely useful for many epilepsy syndromes and focal epilepsies, but a normal or apparently localising recording has to be interpreted in clinical context.
Sources
Jobst et al. — The Insula and Its Epilepsies
Tufenkjian & Lüders — Seizure semiology, localisation and seizure classification
Tao et al. — Cortical substrates of scalp EEG epileptiform discharges