EEG (Electroencephalogram) Explained: Seizures, Encephalopathy and Sleep
A routine EEG is short and often normal in epilepsy. Learn indications, sleep-deprived EEG, and video EEG monitoring.
The routine EEG records surface brain electrical activity over 20–40 minutes. It supports (but does not confirm) an epilepsy diagnosis — a single routine EEG has only 25–50% sensitivity for interictal epileptiform activity. A normal EEG does not exclude epilepsy. Sleep-deprived and video EEG monitoring improve yield.
EEG is essential in epilepsy diagnosis but often misused when clinical diagnosis alone is warranted. NICE NG217 (2022) recommends EEG in patients with a clinical diagnosis of epilepsy to support classification and identify a syndrome — not to confirm the diagnosis itself. A single routine EEG has 25–50% sensitivity for epileptiform activity in known epilepsy; sleep-deprived EEG raises yield to ~80% and 24-h ambulatory / video EEG monitoring is used when diagnosis is uncertain, seizure classification is needed, or pre-surgical evaluation is planned. Key findings: interictal epileptiform discharges (IEDs — spikes, sharp waves, spike-wave complexes) suggest epileptic tendency but are not diagnostic; generalised 3-Hz spike-and-wave supports typical absence; hypsarrhythmia in West syndrome; PLEDs in acute focal brain lesions. Encephalopathy patterns: triphasic waves (hepatic, uraemic, hypoxic), diffuse slowing (metabolic, sedative, dementia), burst suppression (deep coma, sedation, hypothermia). EEG is required to confirm brain death in some jurisdictions (isoelectric EEG). Emerging: quantitative EEG in ICU seizure detection, home EEG devices for sleep and neurofeedback.
- Duration (routine)
- 20–40 min
- Sensitivity (routine EEG)
- 25–50%
- Sleep-deprived
- Sensitivity ~80%
- Video EEG
- For diagnosis + presurgical assessment
- Normal EEG
- Does NOT exclude epilepsy
EEG modalities
| Modality | When to use |
|---|---|
| Routine EEG | First epilepsy classification |
| Sleep-deprived EEG | Increase yield after normal routine |
| Ambulatory 24-h EEG | Recurrent uncertain episodes |
| Video EEG monitoring | Diagnosis uncertain / pre-surgical |
| ICU continuous EEG | Non-convulsive status detection |
When EEG changes management
Classifying focal vs generalised epilepsy (drug choice differs).
Diagnosing childhood absence, JME, or lennox-gastaut.
Diagnosing non-convulsive status epilepticus in altered consciousness.
Pre-surgical assessment of drug-resistant epilepsy.
- Unexplained persistent altered consciousness — non-convulsive status suspected.
- First seizure + focal neurology — urgent imaging + EEG.
- Drug-resistant epilepsy — refer for video EEG.
Seizure workup
- 1First seizure?Clinical assessment + brain imaging (usually MRI) + EEG.
- 2Routine EEG normal but strong clinical suspicion?Sleep-deprived EEG.
- 3Uncertain events?Video EEG monitoring.
- 4ICU altered consciousness?Continuous EEG.
Related questions people ask
Frequently asked questions
- EEG supports classification, not diagnosis, of epilepsy.
- Normal EEG does not exclude epilepsy.
- Sleep-deprived and video EEG increase yield.
- Non-convulsive status needs urgent EEG.
- Pre-surgical epilepsy assessment requires video EEG.
References
2 sources- NICE NG217Epilepsies in children, young people and adults
UK national guideline.
nice.org.uk
- ILAEPosition papers
International League Against Epilepsy.
ilae.org
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