EEG Diagnostic Criteria for Non-Convulsive Status Epilepticus (NCSE)
According to the American Clinical Neurophysiology Society criteria, NCSE is diagnosed when electrographic seizure patterns persist for ≥10 continuous minutes or for a total duration of ≥20% of any 60-minute period of EEG monitoring. 1
Primary Diagnostic Criteria
Electrographic Seizure Patterns
- Definite electrographic seizures (any of these qualify):
- Epileptiform discharges averaging >2.5 Hz for ≥10 seconds
- Any pattern with definite evolution (at least 2 unequivocal, sequential changes in frequency, morphology, or location) lasting ≥10 seconds 1
Electrographic Status Epilepticus
- Any pattern qualifying as an electrographic seizure that persists for:
- ≥10 continuous minutes, OR
- A total duration of ≥20% of any 60-minute period of monitoring 1
Ictal-Interictal Continuum (Possible NCSE)
Patterns that don't meet strict seizure criteria but may represent NCSE include:
- Any periodic discharges or spike/sharp-wave pattern averaging >1.0 and ≤2.5 Hz over 10 seconds
- Any periodic discharges or spike/sharp-wave pattern averaging ≥0.5 Hz and ≤1.0 Hz over 10 seconds with either:
- Plus modifier (superimposed fast activity, rhythmic activity, or sharp waves/spikes)
- Fluctuation (≥3 changes within 1 minute in frequency, morphology, or location)
- Lateralized rhythmic delta activity averaging >1 Hz over 10 seconds with either plus modifier or fluctuation 1, 2
Additional Criteria for Borderline Cases
For patterns in the ictal-interictal continuum with frequency <2.5 Hz, at least one of these additional criteria is needed to diagnose NCSE:
- Subtle clinical ictal phenomena
- Typical spatiotemporal evolution
- Response to antiseizure medication 2
EEG Monitoring Approach
Duration and Electrode Configuration
- Continuous EEG monitoring is reasonable for patients who do not follow commands after return of spontaneous circulation (ROSC) 1
- If continuous EEG is not available, standard EEG with extended recording time (>30 minutes) should be performed 3
- Brief "spot" EEGs may miss seizure activity due to the episodic nature of epileptiform patterns 1, 3
- Full 21-electrode montage is preferred, though simplified 6-10 electrode montages may be used in emergent situations 1
Timing of EEG
- For patients with altered consciousness or suspected ongoing seizure activity, emergent EEG should be obtained within 60 minutes 3
- EEG should be promptly performed and interpreted for diagnosis of seizures in patients who do not follow commands after ROSC 1
Clinical Considerations
Common Patterns in Post-Cardiac Arrest Patients
- Burst-suppression patterns during the first 24-48 hours after ROSC may be compatible with neurological recovery, but persisting beyond 72 hours are consistently associated with poor outcomes 1
- Status epilepticus during therapeutic hypothermia or immediately after rewarming is almost invariably associated with poor outcomes, especially with unreactive or discontinuous EEG background 1
Pitfalls and Caveats
- Myoclonus may occur with or without EEG correlates (subcortical vs. cortical) 1
- Sedative medications can affect EEG patterns and interpretation
- Classical coma patterns (diffuse polymorphic delta activity, spindle coma, alpha/theta coma, low output voltage, burst suppression) do not reflect NCSE 2
- The distinction between nonconvulsive seizures and NCSE in critically ill patients can be somewhat arbitrary 2
Treatment Implications
- Treatment of nonconvulsive seizures diagnosed by EEG is reasonable in adult survivors of cardiac arrest 1
- A therapeutic trial of a nonsedating antiseizure medication may be reasonable for patients with EEG patterns on the ictal-interictal continuum 1
- Detection of ongoing seizure activity necessitates adjustment of antiseizure medications 3
By applying these criteria systematically, clinicians can accurately diagnose NCSE and initiate appropriate treatment to prevent further neurological injury.