EEG Can Show Positive Results Without Active Seizures
Yes, an electroencephalogram (EEG) can show abnormal electrical activity even when a patient is not experiencing active clinical seizures. This is supported by multiple lines of evidence from clinical guidelines and research.
Types of EEG Abnormalities Without Clinical Seizures
- Electrographic seizures without clinical manifestations can be detected on EEG, particularly in comatose patients in intensive care settings 1
- Breakthrough seizures detectable only by EEG (without clinical manifestations) occurred in more than half of patients with refractory status epilepticus treated with continuous IV midazolam 1
- Nonconvulsive status epilepticus was found in 8% of comatose ICU patients who did not exhibit clinical seizure activity 1
- EEG-only seizure activity was found in approximately 11% (5 of 46) of comatose ICU patients who did not exhibit clinical seizure activity 1
Specific Clinical Scenarios
- In post-cardiac arrest patients, persistent electrographic seizures were found in 48% of patients with continuous EEG monitoring, many without clinical manifestations 1
- Epileptiform discharges on EEG during therapeutic temperature management (TTM) can occur without clinical seizures, though their presence may predict poor outcomes 1
- In patients with absence epilepsy, ambulatory EEG monitoring revealed that many generalized epileptic discharges did not exhibit clinical seizure patterns despite being of long duration 2
- In patients who appeared to have clinical resolution of absence seizures after valproate therapy, ambulatory EEG revealed the persistence of epileptiform discharges 2
Diagnostic Value of EEG in Seizure Detection
- EEG is the definitive test for detecting ongoing electrical seizure activity, particularly when clinical manifestations are subtle or absent 3
- Routine EEG may miss nonconvulsive seizures in approximately half of patients with seizures when compared to prolonged monitoring 3
- Findings suggestive of autoimmune encephalitis include focal slowing/seizures, lateralized periodic discharges, and extreme delta brush, which may be present without clinical seizures 1
Limitations of EEG Interpretation
- A normal EEG does not exclude the possibility of epilepsy, as seizures may occur intermittently 1, 4
- EEG findings should be interpreted in the context of the overall clinical picture, as epileptiform patterns may be present in patients without clinical seizures 5
- Benign epileptiform variants unassociated with seizures can be present in the EEG, including "14 & 6" positive bursts, small sharp spikes, wicket spikes, 6-Hz spike-and-wave discharges, and rhythmic temporal theta activity 5
Clinical Implications
- Continuous EEG monitoring is more likely to detect abnormal electrical activity than standard EEG, particularly in critically ill patients 3, 6
- In drug overdose cases, EEG may show dysrhythmias even without clinical seizures 7
- Emergent EEG should be considered in patients with altered consciousness after motor seizures, suspected nonconvulsive status epilepticus, or in patients who have received long-acting paralytics or are in drug-induced coma 3, 8
In conclusion, EEG can definitely show abnormal electrical activity consistent with seizures even when a patient is not experiencing active clinical seizures. This is particularly important in critically ill patients, those with altered consciousness, and in the evaluation of certain epilepsy syndromes where subclinical seizures may be common.