How is an electroencephalogram (EEG) used to diagnose seizures?

Role of EEG in Diagnosing Seizures

EEG is the definitive test for detecting ongoing electrical seizure activity, particularly when clinical manifestations are subtle or absent, making it essential in the diagnosis and classification of epileptic seizures and syndromes. 1

Diagnostic Value of EEG

• EEG provides critical information about brain electrical activity and is essential for detecting seizures, monitoring their duration, and evaluating response to therapy 2
• EEG helps differentiate seizures from other causes of paroxysmal events that may or may not be associated with loss of consciousness 3
• Standard EEG recording should include 19 electrodes of the 10-20 International System, with a recording duration of 20-30 minutes to capture variations in vigilance levels 1
• Routine EEG (typically 20-30 minutes) will miss nonconvulsive seizures in approximately half of patients with seizures when compared to prolonged monitoring 2

Types of EEG Findings in Seizure Diagnosis

• Electrographic seizures are characterized by rhythmic discharges with evolution in frequency, amplitude, and spatial distribution 1
• Findings suggestive of autoimmune encephalitis include focal slowing/seizures, lateralized periodic discharges, and extreme delta brush (occasionally seen in NMDAR-antibody encephalitis) 2
• Generalized Rhythmic Delta Activity (GRDA) itself is not epileptiform but may coexist with epileptiform activity in some cases 4
• Interictal epileptiform discharges such as spikes and sharp waves support the diagnosis of epilepsy when a habitual seizure is not captured 5

Continuous vs. Routine EEG Monitoring

• Continuous EEG (cEEG) monitoring is more resource-intensive than intermittent EEG but may detect more seizures, particularly in patients with cyclic seizure patterns 2, 1
• cEEG is valuable for detecting nonconvulsive seizures in critically ill patients with brain injuries, whose brains are often in a particularly vulnerable state 6
• The impact of continuous over routine EEG recordings on mortality outcomes remains unclear, though it increases detection of seizure activity 1

Specific Clinical Scenarios for EEG Use

• Urgent EEG is recommended in patients with convulsive status epilepticus who do not return to functional baseline within 60 minutes after seizure medication 2
• EEG is recommended during therapeutic hypothermia and within 24 hours of rewarming to exclude nonconvulsive seizures in all comatose patients after cardiac arrest 2
• EEG is suggested in comatose ICU patients without an acute primary brain condition but with unexplained impairment of mental status to exclude nonconvulsive seizures, particularly in those with severe sepsis or renal/hepatic failure 2
• EEG can help detect delayed cerebral ischemia in comatose subarachnoid hemorrhage patients when neurological examination is unreliable 2

Limitations of EEG in Seizure Diagnosis

• EEG itself neither proves nor excludes a diagnosis of seizures with 100% certainty but refines the probability of the condition 3
• A normal EEG does not exclude autoimmune encephalitis, but it can support primary psychiatric disorders when investigating patients with isolated new psychiatric symptoms 2
• The optimal montage and number of electrodes to record EEG in the ICU remains uncertain, especially considering the practical challenges of placing many electrodes in an electrophysiologically unfriendly environment 2
• Quantitative EEG algorithms have been developed to support the time-consuming expert review of cEEG recordings, but data do not support the use of bispectral index score (BIS) measurements as an EEG quantification tool in brain-injured patients 2

Practical Considerations

• Neurologic consultation before obtaining an EEG is reasonable given the specialized nature of EEG interpretation 1
• Average response time for emergent EEG is approximately 3 hours, which may delay diagnosis and treatment 1
• Automated seizure detection systems based on feature extraction from EEG signals are being developed to improve diagnostic accuracy 7