What are EEG waves and their role in diagnosing neurological conditions?

EEG Waves: Definition and Clinical Applications

What EEG Waves Are

EEG (electroencephalography) measures electrical activity generated by synchronized neuronal firing in the brain, recorded as voltage fluctuations (brain waves) from electrodes placed on the scalp, with frequencies ranging from 1-100 Hz and temporal resolution under 1 millisecond. 1

The primary frequency bands include:

• Delta waves (1-4 Hz): Abnormally prominent delta rhythms in resting state indicate brain dysfunction 1
• Theta waves (4-7 Hz): Small amplitude in healthy adults; prominent theta suggests pathology 1
• Alpha waves (8-12 Hz): Dominant posterior rhythms during eyes-closed rest; high amplitude low-frequency alpha (8-10 Hz) reflects low arousal and attention levels 1
• Beta waves (12-30 Hz): Associated with active cognitive processing; high-frequency alpha (10-13 Hz) and low-frequency beta reflect perceptual and memory processes 1
• Gamma waves (30-70 Hz): Fastest rhythms associated with sensorimotor and cognitive events 1

Critical Diagnostic Applications

Seizure Detection and Status Epilepticus

EEG is essential for detecting seizures, including nonconvulsive seizures that are frequently missed on clinical examination alone, with continuous EEG (cEEG) detecting seizures in approximately 50% more patients than routine 30-minute recordings. 1

• All patients with acute brain injury and unexplained persistent altered consciousness should receive EEG (strong recommendation) 1
• Urgent EEG is recommended in patients with convulsive status epilepticus who do not return to functional baseline within 60 minutes 1
• Routine EEG will miss nonconvulsive seizures in approximately half of patients compared to prolonged monitoring 1

Metabolic Encephalopathy Assessment

Progressive EEG slowing with increasing amplitude changes, appearance of triphasic waves, and eventual discontinuous patterns characterize worsening hepatic encephalopathy severity. 1

• Triphasic waves are characteristic of metabolic encephalopathies (hepatic, uremic, hyponatremia) and certain drug intoxications (lithium, valproate, baclofen), though not specific to any single condition 1, 2
• Critical distinction: Triphasic waves reflect diffuse cortical dysfunction of metabolic origin, NOT epileptic activity, though they can be difficult to distinguish from nonconvulsive status epilepticus 2
• Serial EEG recordings help monitor treatment response and disease progression in hepatic encephalopathy 3

Post-Cardiac Arrest Prognostication

In comatose patients treated with targeted temperature management (TTM), absence of EEG reactivity after rewarming predicts poor neurologic outcome with 0% false positive rate (95% CI: 0-3%). 1

For patients treated with TTM:

• Burst suppression after rewarming predicts poor outcome (FPR 0%; 95% CI: 0-5%) 1
• Absence of EEG reactivity during TTM predicts poor outcome (FPR 2%; 95% CI: 1-7%) 1
• Status myoclonus combined with other diagnostic tests at ≥72 hours helps predict poor outcomes (FPR 0%; 95% CI: 0-4%) 1

For patients not treated with TTM:

• EEG grades 4-5 at ≤72 hours predict poor outcome (FPR 0%; 95% CI: 0-8%) 1
• Burst suppression at 72 hours predicts poor outcome (FPR 0%; 95% CI: 0-11%) 1

Ischemia Detection in Neurocritical Care

EEG can detect ischemia in patients with subarachnoid hemorrhage or acute ischemic stroke, though no published studies demonstrate enhanced outcomes from treatment strategies directed solely by EEG-based ischemia monitoring. 1

Technical Considerations

Recording Standards

• Standard diagnostic EEG uses the 19-electrode 10-20 International System and typically lasts 20-30 minutes 3, 4
• Continuous monitoring may extend for days, weeks, or months depending on clinical indication 4, 5
• Quantitative EEG analysis improves reliability of assessment in hepatic encephalopathy 3

Advantages and Limitations

EEG provides unique real-time assessment of brain function, making it essential for studying rapidly changing activities like epileptic phenomena, triphasic waves, and cortical reactivity. 1

Advantages:

• High temporal resolution (<1 ms) 1
• Relatively resistant to environmental electrical noise and anesthesia levels compared to other monitoring 1
• Non-invasive and repeatable 6

Limitations:

• Low to moderate spatial resolution (few centimeters) due to volume conduction effects 1
• Activity at one electrode may reflect sources distributed over large regions (>10 cm²) 1
• Requires artifact-free recordings for accurate interpretation 1

Common Pitfalls

Bispectral index (BIS) measurements should NOT be used for brain-injured patients in the ICU due to large intra- and inter-individual variability and interferences. 1

• Low-voltage EEG is unreliable because technical factors can affect amplitude 1
• Lack of standardized EEG terminology remains a major limitation in research and practice 1
• Single routine EEG has limited diagnostic value, showing interictal epileptiform discharges in only a small percentage of cases 7