Who Should Undergo EEG After a Seizure
All patients with altered consciousness after a seizure, suspected nonconvulsive status epilepticus, or persistent unexplained neurological deficits should undergo emergent EEG, ideally within 16 hours of seizure onset for maximum diagnostic yield. 1, 2, 3
Emergent EEG Indications (Order Immediately)
The following clinical scenarios require urgent EEG within hours:
- Altered consciousness persisting after motor seizures - approximately 25% of patients have ongoing electrical seizures despite cessation of visible convulsive activity 1, 2
- Suspected nonconvulsive status epilepticus - found in 8% of comatose ICU patients without clinical seizure activity 4, 2
- Subtle convulsive status epilepticus - clinical manifestations may be minimal while electrical seizures continue 1, 2
- Patients not returning to functional baseline within 60 minutes after seizure medication 1
- Comatose patients after cardiac arrest - perform within 24 hours of rewarming to exclude nonconvulsive seizures 1, 4
- Patients who received long-acting paralytics - cannot assess clinical seizure activity 1, 2
The average response time for emergent EEG is approximately 3 hours, so order early to stay within the critical window 2.
Standard EEG Indications (Within 16-24 Hours)
First unprovoked seizure patients should undergo EEG within 16 hours of seizure onset for optimal diagnostic yield:
- Epileptiform discharges are detected in 52.1% of recordings performed before 16 hours versus only 20.2% after 16 hours 3
- Early EEG (within 48 hours) is the method of choice for accurate diagnosis and allows definition of epilepsy syndrome in two-thirds of patients 5
- EEG is recommended as part of the standard neurodiagnostic evaluation for first unprovoked seizures 6
High-Risk Populations Requiring EEG
Certain patient populations have significantly higher risk of electrographic seizures and warrant lower threshold for EEG:
- Comatose patients (odds ratio 7.7) - require longer monitoring as 20% have first seizure detected after >24 hours 7
- Pediatric patients <18 years (odds ratio 6.7) 7
- History of epilepsy (odds ratio 2.7) 7
- Convulsive seizures prior to monitoring (odds ratio 2.4) 7
- Comatose ICU patients with unexplained impaired mental status, particularly those with severe sepsis or renal/hepatic failure 1
Duration of EEG Monitoring
Continuous EEG monitoring is superior to routine EEG for detecting nonconvulsive seizures:
- Routine EEG misses nonconvulsive seizures in approximately 50% of patients compared to prolonged monitoring 1, 4
- 88% of seizures are detected within the first 24 hours of continuous monitoring 7
- An additional 5% are detected on day 2, and 7% after 48 hours 7
- For first seizure evaluation, 24-hour video EEG increases detection of epileptiform discharges by 32% compared to standard 60-minute recordings 8
Critical Pitfalls to Avoid
Do not rely solely on routine outpatient EEG when nonconvulsive seizures are suspected - this misses approximately half of cases 1, 4. In one prospective study, 37% of patients referred for emergency EEG had combined EEG and clinical evidence of status epilepticus that would have gone undetected without EEG 9.
Do not delay EEG ordering - neurologic consultation before obtaining EEG is reasonable given the specialized interpretation required, but this should not delay ordering the study in time-critical situations 2. Detection of ongoing electrical seizures fundamentally changes management, indicating need for escalation to second-line agents 2.
Do not assume clinical cessation equals electrical cessation - ongoing electrical seizures cause neuronal injury even without motor manifestations 2.