EEG in Toxic-Metabolic Encephalopathy

EEG in toxic-metabolic and infectious encephalopathies provides one of the most clinically useful applications of the modality. Unlike epilepsy, where EEG often confirms a clinical diagnosis, encephalopathy EEG can be the first objective marker of brain dysfunction in a patient with altered mental status. Specific patterns — triphasic waves, generalized periodic discharges, FIRDA, posterior dominant rhythm slowing — narrow the differential and track treatment response in real time. The patterns are non-specific for cause but are sensitive to severity. This page covers the major encephalopathy patterns, their differential, and the clinical utility of EEG in tracking recovery.

Spectrum of Encephalopathy on EEG

Mild Encephalopathy

  • PDR slowing (e.g., 8 Hz instead of 10 Hz baseline).
  • Decreased reactivity to eye opening.
  • Mild generalized theta intermixed.
  • Background still organized.

Moderate Encephalopathy

  • PDR slows further (5–7 Hz) or fragments.
  • Generalized theta-delta predominates.
  • Intermittent rhythmic delta activity (FIRDA, OIRDA, generalized RDA).
  • Background poorly reactive.

Severe Encephalopathy

  • Continuous generalized delta.
  • Triphasic waves (in metabolic).
  • Generalized periodic discharges.
  • Burst-suppression.
  • Suppression.
  • Coma EEG patterns (see dedicated coma page).

Triphasic Waves

Morphology

  • Three phases: initial sharp negative, large positive, terminal slow negative.
  • Anterior-posterior gradient: anterior maximum, propagating to posterior with measurable lag.
  • Frequency: 1.5–2.5 Hz.
  • Bilateral synchronous, often continuous.

Differential

  • Hepatic encephalopathy: classic, often prominent.
  • Uremic encephalopathy: common.
  • Hyponatremic encephalopathy: especially during correction.
  • Hypocalcemic, hypomagnesemic encephalopathy: occasional.
  • Anoxic encephalopathy: can resemble triphasic waves.
  • Septic encephalopathy: common in ICU.
  • Drug intoxication: lithium, baclofen, others.
  • Degenerative diseases: occasionally CJD, advanced dementia.

Distinguishing Triphasic Waves from Generalized Periodic Discharges (GPDs)

  • Triphasic waves: anterior maximum, smoother morphology, characteristic 3-phase shape, anterior-posterior lag, larger amplitude.
  • GPDs: more variable morphology, no clear AP lag, often associated with acute brain injury (post-cardiac arrest).
  • Treatment implications: triphasic waves are metabolic and reversible; GPDs may indicate ictal-interictal continuum.

Response to Treatment

  • Triphasic waves resolve with correction of underlying metabolic cause.
  • Resolution may take days; lag behind clinical improvement.
  • Persistent triphasic waves after metabolic correction: investigate alternative causes.

Rhythmic Delta Activity Patterns

FIRDA (Frontal Intermittent Rhythmic Delta Activity)

  • Bilateral synchronous rhythmic delta at 1.5–2.5 Hz over frontal regions.
  • Intermittent (occurs in bursts).
  • Found in diffuse encephalopathy, increased ICP, deep midline lesions (rare).
  • Non-specific for cause.
  • ACNS 2021: now classified as GRDA (generalized rhythmic delta activity) with frontal maximum.

OIRDA (Occipital Intermittent Rhythmic Delta Activity)

  • Similar pattern in occipital regions.
  • More commonly seen in children with absence epilepsy.
  • In adults, can occur in encephalopathy or specific syndromes.

GRDA (Generalized Rhythmic Delta Activity)

  • Diffuse rhythmic delta, often with frontal predominance.
  • ACNS terminology replaces FIRDA in formal reporting.
  • Indicates diffuse cortical dysfunction.

Specific Encephalopathies

Hepatic Encephalopathy

  • Spectrum: mild PDR slowing → triphasic waves → burst-suppression with severe stages.
  • West-Haven staging:
    • Grade 1: subtle changes; mild PDR slowing.
    • Grade 2: confusion; theta-delta predominance.
    • Grade 3: stupor; triphasic waves common.
    • Grade 4: coma; burst-suppression or low-amplitude EEG.
  • EEG correlates with ammonia level and clinical grade.
  • Reverses with treatment (lactulose, rifaximin).

Uremic Encephalopathy

  • Diffuse slowing.
  • Triphasic waves common in moderate-severe.
  • Reverses with dialysis.

Sepsis-Associated Encephalopathy

  • Common in ICU.
  • Continuous slow theta-delta background.
  • May progress to GPDs or non-convulsive seizures.
  • Increasingly recognized as marker of brain involvement in sepsis.

Hyponatremic Encephalopathy

  • Slow background with intermittent generalized slowing.
  • Triphasic waves during acute correction (central pontine myelinolysis preceding).
  • Important: rapid Na correction in chronic hyponatremia → osmotic demyelination → severe encephalopathy with poor recovery.

Hypoglycemic Encephalopathy

  • Generalized slowing first.
  • Can progress rapidly to coma EEG patterns.
  • EEG may continue to show abnormality even after glucose correction.
  • Severe prolonged hypoglycemia can cause neuronal injury and persistent EEG changes.

Wernicke Encephalopathy / Thiamine Deficiency

  • Mild diffuse slowing.
  • Can have triphasic waves or focal patterns.
  • Treatment: IV thiamine before glucose; rapid clinical and EEG response.

Drug Intoxication / Withdrawal

  • Benzodiazepine intoxication: prominent fast beta activity superimposed on slowed background; high doses → burst-suppression.
  • Lithium toxicity: triphasic waves, generalized slowing, sometimes mimicking CJD.
  • Baclofen toxicity: triphasic waves.
  • Cefepime toxicity: GPDs, sometimes mimicking non-convulsive status.
  • Anticholinergic toxicity: variable, sometimes focal slowing.
  • Alcohol withdrawal: high-amplitude fast activity, sometimes paroxysmal slow waves.
  • Anesthetic-induced burst-suppression: dose-dependent, reversible.

Infectious Encephalopathies

Herpes Simplex Encephalitis

  • Focal temporal slowing, often unilateral.
  • Periodic lateralized discharges (LPDs) at 1–3 Hz over the affected temporal region.
  • Classic of HSV — empirical acyclovir warranted when LPDs + altered consciousness + fever.

Bacterial Meningitis

  • Generalized slowing.
  • Focal abnormalities if cerebritis or abscess develops.

Other Viral Encephalitis

  • Diffuse slowing.
  • Multifocal patterns occasionally.
  • Anti-NMDA receptor encephalitis: “extreme delta brush” pattern — rhythmic delta with superimposed beta brushes; characteristic but not pathognomonic.

Degenerative Encephalopathies

Creutzfeldt-Jakob Disease (CJD)

  • Generalized periodic discharges at ~1 Hz.
  • Often biphasic morphology.
  • Prominent in middle stages of disease.
  • ~67% sensitivity for sporadic CJD.
  • Combined with myoclonus, dementia, and CSF biomarkers (14-3-3, RT-QuIC) supports diagnosis.

Alzheimer Disease

  • PDR slowing, decreased alpha, increased theta and delta.
  • Not specific.
  • qEEG measures may help with staging.

Dementia with Lewy Bodies

  • PDR slowing.
  • Fluctuating background — may correlate with cognitive fluctuations.
  • FIRDA or rhythmic delta sometimes prominent.

Subacute Sclerosing Panencephalitis (SSPE)

  • Periodic high-amplitude complexes every 4–15 seconds.
  • Often associated with myoclonic jerks.
  • Children/adolescents with late measles complications.

Hashimoto Encephalopathy

  • Generalized slowing.
  • Can have triphasic waves or focal patterns.
  • Anti-thyroid antibodies.
  • Responds to corticosteroids.

Tracking Recovery

  • EEG sensitively tracks encephalopathy severity over hours to days.
  • Reappearance of PDR is an early sign of recovery.
  • Resolution of triphasic waves correlates with metabolic improvement.
  • Continuous EEG monitoring useful in evolving encephalopathies.

Approach to Reading an Encephalopathy EEG

  1. Background frequency and reactivity (PDR Hz, response to eye opening).
  2. Predominant frequency in wake (alpha, theta, delta).
  3. Symmetry and lateralization.
  4. Presence of paroxysmal patterns (triphasic waves, GPDs, LPDs).
  5. Reactivity to stimulation.
  6. Track over time (serial recordings).
  7. Specify ACNS terminology when applicable.
  8. Correlate with clinical state and metabolic/toxic/infectious context.

🔍 Did You Know?

Cefepime-induced encephalopathy with generalized periodic discharges (GPDs) has emerged as one of the most common reversible causes of ICU EEG patterns mimicking non-convulsive status epilepticus. Cefepime, widely used for hospital-acquired infections, is renally cleared; in patients with even modest renal impairment (eGFR <60), cefepime can accumulate and cause encephalopathy with prominent GPDs (sometimes with sharp/spike features), myoclonus, and altered consciousness. The pattern often looks identical to true non-convulsive status epilepticus, leading clinicians to treat with anti-seizure medications without effect. The key clinical clues are: recent cefepime initiation, renal dysfunction, GPDs without clear ictal evolution, and incomplete response to benzodiazepines. Recognition is critical because discontinuation of cefepime produces rapid clinical and EEG improvement — often within 24–48 hours — without need for prolonged ASM treatment or escalation to anesthetic infusion. The same caution applies to other neurotoxic antibiotics (other cephalosporins, imipenem, fluoroquinolones) in renally impaired patients. The lesson for ICU clinicians and neurophysiologists is that medication review is part of every encephalopathy EEG interpretation, and “GPDs in a renally impaired patient on cefepime” is a recognized syndrome that warrants medication discontinuation before considering aggressive seizure treatment.

Pitfalls and Pearls

  • Triphasic waves: anterior maximum, AP gradient, 1.5–2.5 Hz; classic hepatic/uremic/hyponatremic encephalopathy.
  • Distinguish triphasic from GPDs: triphasic = anterior, AP lag, smoother; GPDs = often acute brain injury context.
  • FIRDA / GRDA: rhythmic delta over frontal regions; diffuse encephalopathy.
  • PDR slowing <8 Hz: almost always pathologic in adults.
  • Hepatic encephalopathy: spectrum from PDR slowing to burst-suppression; correlates with West-Haven grade.
  • Cefepime toxicity: GPDs in renally impaired; discontinue before treating as NCSE.
  • Lithium toxicity: triphasic waves; can mimic CJD.
  • HSV encephalitis: temporal LPDs; empirical acyclovir warranted.
  • Anti-NMDA receptor encephalitis: “extreme delta brush” pattern.
  • CJD: ~1 Hz GPDs; with myoclonus + dementia + CSF biomarkers.
  • SSPE: periodic complexes every 4–15 sec with myoclonus.
  • Drug effects: benzo (fast beta), barbiturate (burst-suppression at high dose), alcohol withdrawal (high-amplitude fast).
  • Wernicke: IV thiamine before glucose; rapid EEG response.
  • Hashimoto encephalopathy: steroid-responsive; check anti-thyroid antibodies.
  • Serial EEG: tracks evolution of encephalopathy and recovery.
  • Reappearance of PDR: early sign of recovery.
  • EEG findings non-specific for cause: integrate with clinical context.

References

  1. Brenner RP. The interpretation of the EEG in stupor and coma. Neurologist. 2005;11(5):271-284.
  2. Kaplan PW, Sutter R. Seeing more clearly through the fog of encephalopathy. J Clin Neurophysiol. 2013;30(5):431-434.
  3. 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.
  4. Sutter R, Stevens RD, Kaplan PW. Clinical and imaging correlates of EEG patterns in hospitalized patients with encephalopathy. J Neurol. 2013;260(4):1087-1098.
  5. Wesselink R, van Roij C, van Gils MJ, et al. Cefepime-induced neurotoxicity: a systematic review. Crit Care. 2017;21(1):276.
  6. Hirsch LJ, Brenner RP. Atlas of EEG in Critical Care. Wiley-Blackwell; 2010.