What is an Electroencephalogram (EEG)?

What is an Electroencephalogram (EEG)

An electroencephalogram (EEG) is a non-invasive neurophysiological technique that records the electrical activity of the brain through electrodes placed on the scalp, providing real-time assessment of cortical function and brain dynamics that is essential for detecting seizures, evaluating altered consciousness, and monitoring brain function in various neurological conditions. 1

Basic Principles and Recording Technique

EEG measures the electrical potentials generated by synchronized activity of neuronal populations, primarily reflecting cortical activity. The recording process involves:

• Electrode Placement: Typically using the standardized 10-20 International System with 19 electrodes for diagnostic purposes, though monitoring may use fewer electrodes (e.g., P3, P4, F3, F4) 1
• Signal Acquisition: Recording microvolt-sized signals (typically 10-100 μV) that result from synchronized neuronal activity 2
• Recording Duration: Standard recordings last 20-30 minutes to capture variations in vigilance levels, though shorter 5-10 minute recordings may be sufficient in some cases 1
• Recording Conditions: Recordings with eyes closed and eyes open are recommended when possible 1

Clinical Applications

EEG has several important clinical applications that directly impact patient morbidity and mortality:

1. Seizure Detection and Epilepsy Management

• Essential for diagnosing seizures, particularly nonconvulsive seizures that have no visible manifestations
• Helps classify epilepsy syndromes and select appropriate antiepileptic medications
• Critical in monitoring status epilepticus and response to treatment 3, 4

2. Evaluation of Altered Consciousness

• Strongly recommended for all patients with acute brain injury and unexplained altered consciousness 1
• Can detect nonconvulsive seizures in approximately half of cases that would be missed by routine "spot" EEG 1
• Helps differentiate between metabolic, structural, and epileptic causes of encephalopathy 5

3. Post-Cardiac Arrest Evaluation

• Recommended during therapeutic hypothermia and within 24 hours of rewarming to exclude nonconvulsive seizures in comatose patients after cardiac arrest 1
• Assists in prognostication after hypoxic-ischemic brain injury

4. Monitoring in Critical Care

• Continuous EEG (cEEG) monitoring is valuable for detecting cyclic seizure patterns and nonconvulsive status epilepticus in ICU patients 1
• Can identify patterns associated with specific conditions (e.g., triphasic waves in hepatic encephalopathy) 1

EEG Patterns and Interpretation

EEG interpretation requires understanding various patterns:

• Normal Rhythms: Alpha (8-13 Hz), beta (>13 Hz), theta (4-7 Hz), and delta (<4 Hz) waves
• Abnormal Patterns:
  • Progressive slowing of background activity correlates with worsening encephalopathy
  • Epileptiform discharges (spikes, sharp waves) suggest seizure susceptibility
  • Triphasic waves often seen in metabolic encephalopathies
  • Periodic patterns may indicate specific pathologies (e.g., periodic lateralized epileptiform discharges in herpes encephalitis) 5

Advanced EEG Applications

• Quantitative EEG (qEEG): Computer-assisted analysis that can improve reliability of EEG assessment 1
• EEG Source Imaging: Combines high-density EEG with sophisticated algorithms to localize generators of EEG activity, improving spatial resolution 6
• Wearable EEG: Emerging technology for prolonged monitoring that may enable days to weeks of continuous recording 2

Limitations and Considerations

• EEG primarily assesses cortical function and may not directly reflect deeper brain structures 1
• Environmental electrical noise can interfere with recordings, particularly in ICU settings
• Interpretation requires expertise and experience to distinguish normal variants from pathological patterns 3
• Quantitative EEG algorithms can support interpretation but should not replace expert review 1

Comparison with Other Neurophysiological Techniques

Unlike evoked potentials (EPs) which assess specific neural pathways through stimulation, EEG provides:

• Continuous, real-time assessment of spontaneous brain activity
• Superior ability to detect rapidly changing activities (e.g., seizures, triphasic waves)
• Assessment of cortical reactivity to stimulation 1

EEG remains a cornerstone of neurological evaluation despite advances in neuroimaging because of its excellent temporal resolution, ability to directly measure neuronal activity, and capacity for continuous bedside monitoring.