Status epilepticus — continuous or rapidly recurring seizure activity — is the most acute neurologic emergency in clinical neurophysiology. The ACNS 2021 Standardized Critical Care EEG Terminology provides a precise vocabulary for describing the EEG patterns seen in status, the periodic and rhythmic patterns on the ictal-interictal continuum (IIC), and the response to treatment. This page covers the EEG patterns of convulsive and non-convulsive status, the ACNS terminology, the ictal-interictal continuum, and the principles for monitoring response to anesthetic infusion.

Definitions

  • Status epilepticus: continuous seizure activity ≥5 minutes OR recurrent seizures without recovery between.
  • Refractory SE: continues despite first and second-line treatment.
  • Super-refractory SE: continues for ≥24 hours despite anesthetic infusion.
  • Non-convulsive SE (NCSE): continuous electrographic seizure activity without prominent motor manifestations.
  • Subclinical SE: electrographic SE without obvious clinical correlate.

Convulsive Status Epilepticus

Clinical

  • Continuous tonic-clonic activity or recurrent generalized convulsions.
  • Often progresses to subtle motor activity then to non-convulsive state if untreated.

EEG

  • Continuous high-amplitude spike-and-wave or rhythmic activity during convulsive phase.
  • Often obscured by movement artifact.
  • As convulsions subside, may continue as subtle motor SE or transition to NCSE.
  • Continuous EEG essential after convulsions stop to detect ongoing electrographic seizure.

Non-Convulsive Status Epilepticus (NCSE)

Categories

  • Absence status (typical or atypical): prolonged generalized spike-and-wave; confusional state.
  • Complex partial status: continuous or rapidly recurring focal seizures with altered consciousness.
  • Simple partial status: focal seizures without altered consciousness (preserved awareness).
  • Subclinical status: electrographic seizures without clinical correlate; common in ICU.

Clinical Presentation

  • Altered consciousness, confusion, behavioral change, “encephalopathy.”
  • Subtle eyelid flutter, facial twitching, automatisms.
  • Often unrecognized clinically — diagnosis requires EEG.
  • High prevalence in obtunded ICU patients (10–35%).

EEG Diagnostic Criteria (Salzburg Criteria)

For NCSE in patients without prior epilepsy:

  • Continuous rhythmic discharges >2.5 Hz; OR
  • Continuous rhythmic discharges 0.5–2.5 Hz with one of:
    • Clinical improvement with antiepileptic drug.
    • Clear evolution in frequency, location, or morphology.
    • Subtle ictal clinical phenomenon.

ACNS Standardized Critical Care EEG Terminology (2021)

The ACNS terminology provides reproducible categorization of ICU EEG patterns. The core structure uses a hierarchical naming convention:

Main Pattern Types

  • RDA (Rhythmic Delta Activity): monomorphic rhythmic delta <4 Hz.
  • PD (Periodic Discharges): discrete spike/sharp/polyspike discharges with consistent inter-discharge interval.
  • SW (Spike-and-Wave or Sharp-and-Wave): combinations of spike/sharp with following slow wave.
  • BS (Burst-Suppression): alternating bursts and suppressions.

Localization Prefixes

  • G: Generalized.
  • L: Lateralized.
  • BI: Bilateral Independent.
  • UI: Unilateral Independent (between hemispheres).
  • M: Multifocal.

Common Patterns Named

  • GPD: Generalized Periodic Discharges (formerly “GPEDs”).
  • LPD: Lateralized Periodic Discharges (formerly “PLEDs”).
  • BIPD: Bilateral Independent Periodic Discharges (formerly “BIPLEDs”).
  • GRDA: Generalized Rhythmic Delta Activity.
  • LRDA: Lateralized Rhythmic Delta Activity.
  • BIRDA: Bilateral Independent Rhythmic Delta Activity.
  • FIRDA: Frontal Intermittent Rhythmic Delta Activity (now considered GRDA in ACNS but historically distinct).

Modifiers

  • +F: with embedded fast activity.
  • +R: with rhythmic features.
  • +S: with sharp/spike features.
  • +FR, +FS, +RS: combinations.
  • Higher modifier load (e.g., GPD+F) suggests higher seizure risk.

Prevalence Categories

  • Continuous: >90% of recording.
  • Abundant: 50–89%.
  • Frequent: 10–49%.
  • Occasional: 1–9%.
  • Rare: <1%.

The Ictal-Interictal Continuum (IIC)

Some patterns are clearly ictal (high-frequency rhythmic discharges); some are clearly interictal (sporadic spikes). The IIC describes intermediate patterns where ictal status is uncertain. Examples:

  • Periodic discharges at 2 Hz with modifiers (+F, +S): possibly ictal.
  • Rhythmic delta activity at 2 Hz with sharp features: possibly ictal.
  • BIPDs in ICU patient: possibly ictal contribution.

Considerations for Treating IIC Patterns

  • Patient clinical state (alert vs comatose).
  • Underlying brain injury (acute stroke, TBI, hypoxic-ischemic).
  • Response to anti-seizure medication trial.
  • Trend over time (worsening or improving).

Trial of Antiseizure Medication

  • For IIC patterns of uncertain significance, an ASM trial can clarify.
  • Administer benzodiazepine (e.g., lorazepam 2 mg IV) or levetiracetam loading dose.
  • Watch for both clinical improvement and EEG change.
  • Improvement on both supports ictal contribution.
  • No change argues against ictal status.

Refractory Status Epilepticus

EEG During Anesthetic Infusion

  • Target: burst-suppression with 50–80% suppression.
  • Monitor continuously with cEEG.
  • Track burst-suppression ratio (qEEG).
  • Adjust anesthetic infusion to maintain target.

Anesthetics Used

  • Midazolam infusion: 0.05–2 mg/kg/hr.
  • Propofol: 1–10 mg/kg/hr. Watch for propofol infusion syndrome (especially prolonged use, high doses, children).
  • Pentobarbital: 1–5 mg/kg/hr. Significant cardiovascular depression.
  • Ketamine: alternative for refractory; NMDA antagonist; less cardiovascular depression.

Weaning Protocol

  • After 24+ hours of seizure freedom on anesthetic.
  • Gradually reduce anesthetic while monitoring EEG.
  • If seizures recur, return to anesthetic and re-escalate.
  • Add additional ASMs during weaning.

Super-Refractory Status Epilepticus

  • Continues for >24 hours despite anesthetic infusion.
  • Approaches:
    • Multiple anesthetic combination.
    • Ketogenic diet.
    • Hypothermia.
    • Vagus nerve stimulation.
    • ECT.
    • Resective surgery (for clearly focal cases).
    • Magnesium infusion.
    • Immunotherapy if autoimmune encephalitis suspected.
  • Investigate for autoimmune encephalitis (anti-NMDA receptor antibody, others) — autoimmune cases respond to immunotherapy.

NORSE and FIRES

NORSE (New-Onset Refractory Status Epilepticus)

  • Previously healthy patient develops refractory SE without identified cause.
  • Most commonly autoimmune (anti-NMDA receptor encephalitis), infectious, or cryptogenic.
  • Empirical immunotherapy (steroids + IVIG or plasmapheresis) while investigation proceeds.

FIRES (Febrile Infection-Related Epilepsy Syndrome)

  • Pediatric form of NORSE.
  • Refractory SE following febrile illness in previously healthy child.
  • Often pharmacoresistant; long-term epilepsy and cognitive sequelae common.
  • Ketogenic diet, immunotherapy, anakinra (IL-1 antagonist) emerging treatments.

Subclinical Seizures in ICU

  • 10–35% of ICU patients with unexplained altered mental status have subclinical seizures.
  • Risk factors: acute brain injury (stroke, TBI, hemorrhage), prior epilepsy, prior convulsive status, sepsis-induced encephalopathy.
  • ACNS recommends ≥24 hours of cEEG for any unexplained altered consciousness.
  • Detection rate at 30 minutes: ~50%; at 24 hours: ~85%; at 48 hours: ~95%.

Special Patterns

Triphasic Waves

  • Diffuse rhythmic triphasic morphology.
  • Classic of metabolic encephalopathy (hepatic, uremic, hyponatremia).
  • Can also occur in degenerative diseases, drug intoxication.
  • Distinguish from PD: triphasic waves are anteriorly maximal, smoother morphology, no clear interdischarge interval.
  • Reversible with treatment of underlying cause.

Periodic Lateralized Epileptiform Discharges (PLED) → LPD

  • Lateralized periodic complexes at 0.5–2 Hz.
  • Often after focal stroke, hemorrhage, herpes encephalitis, tumor.
  • Significance debated: ictal or post-ictal?
  • Recent evidence: LPDs often part of IIC; ASM trial may be helpful.
  • “PLEDs plus” with embedded fast activity: more likely ictal.

Generalized Periodic Discharges (GPDs)

  • Generalized at 0.5–3 Hz periodicity.
  • Causes: post-cardiac arrest, advanced toxic-metabolic encephalopathy, CJD.
  • Often associated with poor prognosis after cardiac arrest.
  • +F modifier (with fast activity) suggests higher seizure risk.

Stimulus-Induced Rhythmic Periodic or Ictal Discharges (SIRPIDs)

  • Periodic/rhythmic patterns evoked by external stimulation (e.g., suctioning, examination).
  • Common in ICU.
  • Ictal status uncertain — often treated as IIC.

🔍 Did You Know?

The ACNS Standardized Critical Care EEG Terminology, first published in 2013 and updated in 2021, has transformed ICU EEG interpretation by replacing imprecise terms (“PLEDs,” “GPEDs,” “triphasic waves”) with a structured, reproducible vocabulary. Before standardization, the same EEG pattern might be called “GPEDs” at one institution, “GRDA with sharp features” at another, and “encephalopathic background” at a third — with no consistent treatment implications. The ACNS terminology forces precise specification: pattern type (PD, RDA, SW, BS), localization (G, L, BI, M), prevalence (continuous, abundant, frequent, occasional, rare), morphology, and modifiers (+F for fast activity, +S for sharp features). This precision matters clinically because specific patterns predict seizure risk: GPDs with +F modifier in a TBI patient predict 30–50% risk of evolving to seizures within 24 hours; pure GRDA without modifiers predicts only 5–10% risk. The lesson is that language shapes thinking: when neurophysiologists use the same terms for the same patterns, treatment decisions become evidence-based rather than expert opinion. For practicing intensivists and neurologists, the practical implication is that learning the ACNS terminology is no longer optional for ICU EEG — it’s the lingua franca of modern critical care neurology, and it determines what gets treated and what gets observed.

Pitfalls and Pearls

  • SE definition: ≥5 minutes continuous or recurrent without recovery.
  • NCSE in ICU: 10–35% of unexplained altered mental status; needs cEEG to detect.
  • Salzburg criteria: rhythmic discharges >2.5 Hz OR slower with evolution/clinical improvement.
  • ACNS terminology: PD, RDA, SW, BS; G, L, BI, M; modifiers +F, +S, +R; prevalence categories.
  • GPD with +F: high seizure risk; consider treatment.
  • Pure GRDA: relatively low seizure risk.
  • Ictal-interictal continuum: intermediate patterns; ASM trial helpful.
  • Anesthetic infusion goal: 50–80% burst-suppression.
  • Propofol infusion syndrome: prolonged use, high doses, children; lactic acidosis, rhabdomyolysis.
  • Super-refractory SE: ketogenic diet, hypothermia, surgery, immunotherapy.
  • NORSE: empirical immunotherapy while investigating.
  • FIRES: pediatric NORSE; anakinra, ketogenic diet.
  • Triphasic waves: classic metabolic encephalopathy; anteriorly maximal; reversible.
  • LPDs: lateralized periodic; often after stroke or encephalitis; IIC contribution.
  • SIRPIDs: stimulus-induced periodic patterns in ICU; IIC.
  • cEEG ≥24 hours: for any unexplained altered consciousness in ICU.
  • Subclinical seizure detection: 50% at 30 min, 85% at 24 hr, 95% at 48 hr.
  • Burst-suppression target during treatment: quantifiable via qEEG.

References

  1. Hirsch LJ, Fong MWK, Leitinger M, et al. American Clinical Neurophysiology Society’s Standardized Critical Care EEG Terminology: 2021 Version. J Clin Neurophysiol. 2021;38(1):1-29.
  2. Trinka E, Leitinger M. Which EEG patterns in coma are nonconvulsive status epilepticus? Epilepsy Behav. 2015;49:203-222.
  3. Leitinger M, Beniczky S, Rohracher A, et al. Salzburg Consensus Criteria for Non-Convulsive Status Epilepticus. Epilepsy Behav. 2015;49:158-163.
  4. Brophy GM, Bell R, Claassen J, et al. Guidelines for the evaluation and management of status epilepticus. Neurocrit Care. 2012;17(1):3-23.
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