Encephalopathic Patterns

Lynn Liu, MD, MS and J. Craig Henry, MD
Revisions: Zulfi Haneef, MD

When a patient is unresponsive without lateralizing exam findings and the etiology of the coma is in question, an EEG can be helpful to further assess the degree of the coma and perhaps give insight into the etiology or localization of the underlying pathology. With repeated EEGs, just like with repeated physical examinations, the evolution of the EEG pattern can help determine prognosis.

In general, there is a continuum of EEG findings that correspond to the severity of a diffuse clinical encephalopathy. Extremes of normalcy or abnormality at both ends of this continuum are fairly easy to recognize, and what lies in between can be categorized as mild, moderate, or severe based on some basic reproducible EEG findings.

·       Mild: As the level of arousal becomes clouded there is corresponding mild slowing of the EEG. It may begin with slowing of the posterior dominant rhythm, but as the depth of encephalopathy increases, there are increasing amounts of theta and then delta activity intermixed with the background. These changes are independent of the etiology of the encephalopathy.

·       Moderate: With worsened cerebral function, there is increased theta and delta range slowing. Background organizational features such as anterior-posterior gradients and the posterior dominant rhythm become difficult recognize, but the EEG background retains some variability in response to stimulation and there is usually evidence of discernible state changes.

·       Severe: Eventually there is loss of normal state changes that control wakefulness and sleep, and the EEG becomes monotonous in appearance and unresponsive to external or internal stimulation. At the far extreme of a severe diffuse encephalopathy, there is a loss of all cortical activity and the EEG shows electrocerebral silence.

 Within these general patterns of encephalopathy, there can be more specific findings that suggest particular etiologies.

Degree of Encephalopathy

Degree	Organization	Sleep	Reactivity
Normal	Yes	Yes	Yes
Mild	Retained	Yes	Yes
Mild to Moderate	Recognizable	Rudimentary	Yes
Moderate	No	Rudimentary	Yes
Moderate to Severe	No	No	Yes
Severe	No	No	No

 

Additional features if there is burst suppression:

Localization of Bursts	G / L / BI / UI / Mf
Highly Epileptiform Bursts	Present or Absent
Identical Bursts	Present or Absent

 

Systematic Assessment of the EEG Background

Feature	Categories / criteria
Symmetry	Symmetric; mildly asymmetric (<50% voltage asymmetry or 0.5–1 Hz frequency difference); markedly asymmetric (≥50% voltage asymmetry or >1 Hz frequency difference)
Predominant frequency	Beta, alpha, theta, delta
Posterior dominant rhythm (PDR)	Present (specify frequency), absent, or unclear
Continuity	Continuous (<1% suppression/attenuation); nearly continuous (1–9%); discontinuous (10–49%); burst-suppression/burst-attenuation (50–99%); suppressed (>99%)
Reactivity	Reactive, unreactive, SIRPIDs only, unclear
State changes	Present with normal N2 sleep transients; present with abnormal/no N2 transients; absent
CAPE	Present, absent, unclear
Voltage	High (≥150 µV); normal (≥20 to <150 µV); low (10 to <20 µV); suppressed (<10 µV)
Anterior-posterior gradient	Present, absent, reverse
Breach effect	Present, absent, unclear

Adapted from: 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.

 

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Generalized Periodic Discharges with Triphasic Morphology, previously known as Triphasic Waves are a periodic pattern of waveforms that have three phases and are generally most prominent in the anterior leads. These waveforms often have a subtle time lag between anterior and posterior regions. Their presence tends to be associated with metabolic encephalopathy, often of hepatic or renal origin, but they are nonspecific and may be seen in a variety of toxic-metabolic encephalopathies.

Generalized Rhythmic Delta Activity (GRDA) frontally predominant – previously known as Frontal Intermittent Rhythmic Delta Activity (FIRDA): This is an intermittent pattern of rhythmic monomorphic delta waves seen with mild to moderate diffuse encephalopathies. It tends to occur with subcortical white matter disease more than cortical disease, but the etiology is usually nonspecific. It becomes more pronounced as the patient becomes drowsier.

 

Dominant Frequency is in the alpha range with loss of reactivity previously known as Alpha Coma This monomorphic pattern of diffuse alpha frequencies that are more prominent in the anterior than the posterior regions of the brain typically occurs in the setting of hypoxic-ischemic injury. It looks like a normally organized background turned upside-down (or more accurately, frontside-back). The prognosis for independent living following development of this EEG pattern is poor. It can also be seen with anesthetic sedation so the context matters.

Dominant Frequency is in the beta range but it is usually intermixed delta with loss of reactivity previously known as Beta Coma. This pattern of diffuse beta frequently is associated with a drug overdose. These faster frequencies seen can be seen with sedatives such as barbiturates and benzodiazepines. The overall prognosis is better compared to other coma patterns if the patient is supported through the overdose period assuming there is no other cerebral injury.

 

 

Dominant frequency is a beta delta pattern previously known as a Spindle Delta Coma In contrast to the beta predominant pattern, there are bursts of beta activity riding over large slow polymorphic delta waves that are maximal centrally and can be seen in encephalopathy from a variety of etiologies. It is a nonspecific pattern.

Burst Suppression Pattern is an extreme pattern that can be seen in severe coma states or can be induced iatrogenically by anesthesia or for the treatment of status epilepticus. This is an alternating pattern of bursts of higher voltage activity separated by periods of relatively suppressed cerebral activity. This pattern is typically described by its ratio of burst period to suppression period. When the bursts are lost and the suppression periods become continuous, then the pattern is called background suppression with no apparent cerebral activity over 2 microvolts.

 

Electrocerebral Silence or Inactivity These studies, also called brain death studies, are now rarely used. They were frequently requested in transplant centers as a confirmatory test to document brain death in association with the clinical examination. The principal limitation EEG for the purpose of establishing a diagnosis of brain death is that it only records cortical activity and cannot reveal useful information about brainstem activity. Other tests such nuclear medicine test for cerebral perfusion or evoked potentials have largely supplanted the usefulness of the EEG. In addition, there are no EEG criteria for the very young, and the presence of small populations of cortex that cannot generate sufficient voltages above 2 microvolts cannot be revealed by an EEG.

When other ancillary tests are unavailable, an EEG for the purpose of establishing electrocerebral inactivity (ECI) is still sometimes performed. Reversible factors that suppress cerebral activity need to be excluded, and the study must be performed according to a strict criteria:

Exclusion Criteria:

• Overdose of CNS depressants
• barbiturates, benzodiazepines, methaquinolone and meprobamate
• Hypothermia (T < 32.2C)
• Cardiovascular shock (BP < 80mmHg)
• Severe metabolic or endocrine disorders

Technical requirements of a brain death study.

• At least 8 scalp electrodes and reference electrodes
• Interelectrode impedance between 100-10,000 ohms
• Document testing integrity of entire system
• Interelectrode distances of > 10 cm
• Sensitivity < 2mV/mm
• Time constant 0.3-0.4 second
• EKG channel
• Test EEG reactivity to external stimulation
• Recording time at least 30 minutes
• Qualified technologist (rEEG,T)

Terminology note: Critical care EEG terminology in this chapter follows the American Clinical Neurophysiology Society (ACNS) Standardized Critical Care EEG Terminology, 2021. Tables have been reorganized and adapted by EEGmaster for educational purposes and are not reproductions of the ACNS refer

Adapted from: 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. doi:10.1097/WNP.0000000000000806.

 

Table 1. Sporadic Epileptiform Discharges: Prevalence

Prevalence	Definition
Abundant	≥1 discharge per 10 seconds, but not periodic
Frequent	≥1 discharge per minute but <1 per 10 seconds
Occasional	≥1 discharge per hour but <1 per minute
Rare	<1 discharge per hour

Source: Based on the ACNS Standardized Critical Care EEG Terminology, 2021.

Table 2. Constructing the Name of a Rhythmic or Periodic Pattern

Step	Choose	Abbreviation	Example
1. Distribution	Generalized	G	GPDs
	Lateralized	L	LPDs
	Bilateral independent	BI	BIPDs
	Unilateral independent	UI	UIPDs
	Multifocal	Mf	MfPDs
2. Pattern	Periodic discharges	PDs	GPDs
	Rhythmic delta activity	RDA	LRDA
	Spike-and-wave / sharp-and-wave / polyspike-and-wave	SW	GSW
3. Add modifiers	Add applicable major, minor, or “plus” modifiers	—	e.g., LPDs+R

Naming principle: Distribution + Pattern + Modifiers

The ACNS 2021 terminology explicitly structures RPP names using two main terms: Main Term 1 describes localization/distribution and Main Term 2 describes the pattern.

Table 3. Distribution of Rhythmic and Periodic Patterns

Term	Abbreviation	Practical description
Generalized	G	Bilaterally synchronous and symmetric
Lateralized	L	Predominantly involving one hemisphere; may be unilateral or bilateral asymmetric/asynchronous
Bilateral independent	BI	Two independent patterns involving the two hemispheres
Unilateral independent	UI	Two independent patterns within one hemisphere
Multifocal	Mf	At least three independent patterns involving at least three distinct brain regions

Source: Adapted for educational presentation from ACNS Standardized Critical Care EEG Terminology, 2021.

Table 4. Prevalence of Rhythmic and Periodic Patterns

Category	Percentage of record/epoch
Continuous	≥90%
Abundant	50–89%
Frequent	10–49%
Occasional	1–9%
Rare	<1%

This table is worth keeping separate from Table 1 because RPP prevalence and sporadic epileptiform-discharge prevalence use different definitions.

Table 5. Duration of Rhythmic and Periodic Patterns

Category	Typical duration
Very long	≥1 hour
Long	10–59 minutes
Intermediate duration	1–9.9 minutes
Brief	10–59 seconds
Very brief	<10 seconds

Note that the 2021 terminology changed intermediate from 1–4.9 to 1–9.9 minutes, and long accordingly begins at 10 minutes.

Table 6. Major Modifiers of Rhythmic and Periodic Patterns

Instead of reproducing the densely packed ACNS modifier matrix, I would make this a straightforward teaching table.

Feature	Categories / values to report
Prevalence	Continuous, abundant, frequent, occasional, rare
Duration	Very brief, brief, intermediate, long, very long
Frequency	Report typical frequency and range in Hz
Number of phases	1, 2, 3, or >3
Sharpness	Spiky, sharp, sharply contoured, blunt
Absolute voltage	Very low, low, medium, high
Relative voltage	Relative to background
Stimulus relationship	Stimulus-induced, stimulus-terminated, spontaneous only, unknown
Evolution	Evolving, fluctuating, or static

Source: Adapted for educational presentation from ACNS Standardized Critical Care EEG Terminology, 2021.

Table 7. Absolute Voltage of RPPs

Category	Voltage
High	≥150 µV
Medium	50–149 µV
Low	20–49 µV
Very low	<20 µV

The 2021 revision changed the medium/high boundary to 150 µV.

Table 8. Minor Modifiers

Feature	Categories
Onset	Sudden / gradual
Triphasic morphology	Yes / no
Anterior-posterior lag	A→P / P→A / no lag
Polarity	Negative / positive / dipole / unclear

Table 9. “Plus” Modifiers

Modifier	Meaning
+F	Superimposed fast activity
+R	Superimposed rhythmic activity
+S	Superimposed sharp waves or spikes, or sharply contoured activity
+FR	Both +F and +R
+FS	Both +F and +S

A short note underneath can explain that not every plus modifier applies to every RPP type.

Table 10. Systematic Assessment of the EEG Background

Feature	What to assess
Symmetry	Symmetric, mildly asymmetric, markedly asymmetric
Predominant frequency	Beta, alpha, theta, delta
Posterior dominant rhythm	Present with frequency specified, absent, or unclear
Continuity	Continuous, nearly continuous, discontinuous, burst suppression/attenuation, suppressed
Reactivity	Reactive, unreactive, SIRPIDs only, unclear
State changes	Present with normal N2 transients, present with abnormal/no N2 transients, absent
CAPE	Present, absent, unclear
Voltage	High, normal, low, suppressed
Anterior-posterior gradient	Present, absent, reverse
Breach effect	Present, absent, unclear

Adapted from: 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. doi:10.1097/WNP.0000000000000806.

Periodic Discharges These discharges occur with a regular periodicity throughout the EEG and can be generalized, regional/lateralized, or bilateral and independent. Generalized periodic discharges (GPDs) are more frequently associated with severe encephalopathy and can been seen after a severe hypoxic/ischemic event. Lateralized Periodic Discharges (LPDs) are seen in a variety of clinical settings including herpes encephalitis, acute stroke, tumor, or as the aftermath of nonconvulsive status epilepticus. Bilateral independent lateralized discharges are rare and suggest that multifocal injury to the brain such as occurs in the setting of shower emboli resulting in multifocal strokes or aggressive metastatic brain cancer. While Periodic Discharges are not an epileptogenic pattern per se, they are associated with pathological processes where seizures often occur.

 

Generalized Periodic Discharges (GPDs) in the setting of a hypoxic ischemic injury after cardiac arrest

Right Temporal Lateralized Periodic Discharges (LPDs) in the setting of herpes encephalitis; LPD+, LRDA

 

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. doi:10.1097/WNP.0000000000000806.