Continuous EEG & ACNS Standardized Critical Care Terminology

Continuous EEG & ACNS Standardized Critical Care Terminology

Continuous electroencephalography (cEEG) has transitioned from an elective diagnostic modality into an indispensable real-time monitor of cerebral cortical function in the neurocritical care unit. Because clinical examination in comatose or deeply sedated patients cannot reliably distinguish secondary ischemic deterioration, metabolic encephalopathy, or occult epileptic activity, cEEG provides continuous bedside functional neuro-monitoring.

Indications for Continuous EEG in the Neuro-ICU

Use cEEG when nonconvulsive seizures are suspected, consciousness does not recover as expected after convulsive status, or high-risk injury and an unreliable examination warrant surveillance. Monitoring should begin promptly when the indication is present; one universal admission deadline or prevalence estimate does not apply to every ICU coma. At least 24 hours is often needed, with longer recordings for coma, ongoing treatment, or high-risk patterns. Duration is individualized from the evolving findings.

During continuous anesthetic treatment of status epilepticus, cEEG verifies electrographic seizure control because paralysis and sedation can hide clinical activity. The target may be seizure cessation or selected burst suppression under a defined protocol. A fixed number of bursts per screen is not a universal validated target. Review raw EEG with trends and bedside events rather than treating a quantitative display as the diagnosis.

The 2021 ACNS Standardized Critical Care EEG Terminology

To eliminate ambiguous subjective descriptors (e.g., "triphasic waves," "PLEDs," "subclinical seizures"), the American Clinical Neurophysiology Society (ACNS) established an objective, highly reproducible nomenclature updated in 2021. Rhythmic and periodic patterns (RPPs) are classified by pairing a Main Term 1 (spatial localization) with a Main Term 2 (pattern morphology), supplemented by Plus Modifiers.

Spatial Localization (Main Term 1)

• Generalized (G): Bilaterally synchronous, symmetric, and uniform discharge across both hemispheres.
• Lateralized (L): Unilateral, asymmetric, or focal discharge predominantly involving one hemisphere or a specific lobar distribution.
• Bilateral Independent (BI): Synchronous or asynchronous discharges occurring independently and simultaneously in both hemispheres.
• Multifocal (Mf): At least three distinct, independent foci of discharges across both hemispheres.

Core Morphology (Main Term 2)

• Periodic Discharges (PDs): Waveforms of relatively uniform morphology repeating at regular intervals with intervening flat or lower-amplitude inter-discharge baseline segments.
• Rhythmic Delta Activity (RDA): Continuous, rhythmic waves repeating without an intervening flat baseline, exhibiting a regular, sinusoidal or rhythmic delta ($<4\text{ Hz}$) frequency.
• Spike-and-Wave / Sharp-and-Wave (SW): Polyspike, spike, or sharp-wave components persistently followed by a slow-wave deflection in a repetitive rhythmic cycle.

Plus (+) Modifiers

Plus modifiers designate specific features that increase the intrinsic epileptogenicity of an underlying rhythmic or periodic pattern:

• +F (Fast activity): Superimposed fast frequencies (beta or gamma bands) occurring over the main rhythmic or periodic waveform.
• +R (Rhythmic activity): Rhythmic background activity interlaced with periodic complexes.
• +S (Sharp or Spiky): Spiky or sharp morphology embedded within otherwise sinusoidal rhythmic slow activity.

The Ictal-Interictal Continuum & Salzburg Criteria for NCSE

The Ictal-Interictal Continuum (IIC) represents a neurophysiological twilight zone. Patterns on the IIC do not meet strict electrographic seizure criteria but reflect marked neuronal hypersynchrony driven by intense metabolic demand. On advanced multimodal monitoring, IIC patterns frequently trigger profound neurovascular uncoupling, marked elevations in the cerebral microdialysis lactate-to-pyruvate ratio ($LPR > 40$), and critical reductions in brain tissue oxygenation ($PbtO_2 < 15\text{ mmHg}$), culminating in secondary metabolic exhaustion.

ACNS Seizure, Status, and IIC Definitions

An electrographic seizure is epileptiform discharges averaging more than 2.5 Hz for at least ten seconds, or a pattern with definite evolution lasting at least ten seconds. Electrographic status epilepticus is a seizure lasting at least ten continuous minutes or seizure activity occupying at least 20% of any 60-minute period. A ten-second electrographic seizure is not automatically status epilepticus.

The ictal–interictal continuum (IIC) is an electrographic description, not a diagnosis by itself. Under ACNS 2021, applicable patterns include PDs or spike-and-wave above 1 and up to 2.5 Hz; PDs or spike-and-wave at 0.5–1 Hz with a plus modifier or fluctuation; and lateralized RDA above 1 Hz with a plus modifier or fluctuation. Generalized RDA is not included simply because it is rhythmic. Patterns already meeting seizure criteria are classified accordingly rather than remaining on the IIC.

Clinical correlation, the underlying lesion, metabolic and drug effects, pattern burden, and response to treatment guide management. A time-locked clinical correlate can establish an electroclinical seizure even with a brief pattern. In a diagnostic medication trial, EEG improvement alone after a sedative does not prove that an encephalopathy was ictal; clinical improvement and the complete criteria matter. Salzburg criteria provide another structured clinical framework and should not be conflated with a rule that any ten-second fast pattern is status.

A 2-Hz LPD pattern with superimposed fast activity may lie on the IIC and carry substantial seizure risk. In contrast, a clearly evolving 20-second pattern can meet seizure criteria. The distinction affects monitoring and treatment but does not imply that all IIC patterns require anesthetic coma. Correct a toxic or metabolic trigger, assess clinical effects, and balance treatment benefit against hypotension, ventilation, and loss of examination.

Background Architecture, Reactivity, and Suppression

ACNS describes continuity by the proportion of suppression or attenuation: continuous below 1%, nearly continuous 1–9%, discontinuous 10–49%, and burst attenuation/suppression 50–99%. Suppression involves activity below 10 microvolts; attenuation and suppression are distinct descriptors. Background voltage, symmetry, state changes, and medication exposure must be evaluated together.

Reactivity is a reproducible background change with stimulation. Test using an appropriate protocol and avoid mistaking muscle or movement artifact for cerebral response. A reactive background can be reassuring in context, but absent reactivity alone is not a reliable standalone poor-outcome predictor. Sedatives, temperature, technical factors, and injury affect interpretation.

Spontaneous suppression or burst suppression after arrest can be concerning, particularly at delayed, confounder-free assessment. It does not supply a universal zero-error prediction or a histologic diagnosis of cortical necrosis. Drug-induced burst suppression during seizure or ICP treatment has a different context. Use the dedicated neuroprognostication framework rather than transferring an early pattern directly into a withdrawal decision.

Quantitative EEG (qEEG) Bedside Trends

• Alpha–delta ratio: A sustained decline can raise concern for ischemia in aSAH, but changes also reflect sedation, sleep, seizures, artifact, or other illness. Review raw EEG and correlate with examination, TCD, imaging, and other monitoring. No single percentage decline reliably diagnoses every DCI event.