How do you interpret an electroencephalogram (EEG)?

How to Interpret an Electroencephalogram (EEG)

EEG interpretation requires a systematic approach focusing on electrode placement, background activity, waveform patterns, and abnormal findings to accurately assess brain function and detect neurological disorders.

Standard Recording Setup

Electrode Placement

• Use the standardized 10-20 International System with 19 electrodes for diagnostic purposes 1
• For monitoring purposes, fewer electrodes may be sufficient (e.g., P3, P4, F3, F4) 2
• Two main montage options:
  • Recommended montage: Referential derivations with frontal, central, and occipital electrodes referenced to contralateral mastoid 2
  • Alternative montage: Fz-Cz, Cz-Oz (Mayo Clinic approach) 2

Recording Parameters

• Record for 20-30 minutes to capture variations in vigilance levels 2, 1
• Include both eyes-closed and eyes-open recordings when possible 2, 1
• Consider recording at the same time of day and under similar feeding conditions for serial EEGs 2

Systematic Interpretation Process

1. Assess Technical Quality

• Check for adequate signal quality and absence of artifacts
• Identify common artifacts:
  • Muscle activity (high-frequency)
  • Eye movements (slow waves in frontal leads)
  • ECG artifact (regular spikes synchronized with heartbeat)
  • Movement artifacts (abrupt high-amplitude deflections)
  • 60 Hz electrical interference (regular oscillations)

2. Evaluate Background Activity

• Determine the dominant posterior rhythm (alpha rhythm: 8-12 Hz)
• Assess symmetry between hemispheres
• Note reactivity to eye opening (alpha attenuation)
• Evaluate organization and continuity of background activity

3. Identify Normal Rhythms

• Alpha (8-12 Hz): Posterior dominant rhythm, attenuates with eye opening
• Beta (>13 Hz): Faster activity, often frontal predominance
• Theta (4-7 Hz): Normal in drowsiness and children
• Delta (<4 Hz): Normal in deep sleep and young children

4. Detect Abnormal Patterns

• Focal slowing: Suggests structural lesion
• Generalized slowing: Indicates diffuse encephalopathy
• Epileptiform discharges:
  • Spikes (20-70 ms)
  • Sharp waves (70-200 ms)
  • Spike-and-wave complexes
• Triphasic waves: Often seen in metabolic encephalopathies, particularly hepatic encephalopathy 2
• Periodic patterns: May indicate encephalitis or prion disease

5. Assess Sleep Architecture (if sleep recorded)

• Identify sleep stages:
  • NREM 1: Vertex waves, slow eye movements
  • NREM 2: Sleep spindles, K-complexes
  • NREM 3: High-amplitude slow waves (>20% of epoch)
  • REM: Sawtooth waves, low-amplitude mixed frequency activity 1

Special Considerations

Epileptiform Activity

• Distinguish between:
  • Focal epileptiform discharges: Localized to specific region
  • Generalized epileptiform discharges: Bilateral, synchronous activity
  • Nonconvulsive seizures: Rhythmic activity with evolution in frequency, amplitude, or distribution 1, 3

Critical Care EEG

• Monitor for:
  • Nonconvulsive status epilepticus
  • Cerebral ischemia
  • Changes in background activity reflecting neurological deterioration 3
• Continuous EEG monitoring is essential for detecting nonconvulsive seizures in comatose patients 1, 3

Quantitative EEG (qEEG)

• Computer-assisted analysis can improve reliability of assessment 2, 1
• Useful applications:
  • Screening for epileptic activity in long-term monitoring 4
  • Continuous monitoring in ICU/OR settings 4
  • Topographic mapping for presurgical evaluation 4

Common Pitfalls to Avoid

1. Misinterpreting normal variants as abnormal:

   • Benign epileptiform transients of sleep (BETS)
   • Wicket spikes
   • 14 and 6 Hz positive bursts
   • Rhythmic mid-temporal theta of drowsiness (RMTD)

2. Overlooking drug effects:

   • Sedatives/hypnotics: Increased beta activity
   • Antiepileptic drugs: Background slowing
   • Anesthetics: Burst suppression patterns

3. Environmental interference:

   • Electrical noise can mimic cerebral activity
   • Always verify suspicious patterns in multiple channels

4. Overinterpretation of asymmetries:

   • Minor asymmetries can be normal variants
   • Consider skull defects or electrode placement issues

5. Missing subtle seizure patterns:

   • Nonconvulsive seizures may present with subtle changes
   • Evolution in frequency, amplitude, or distribution is key

Advanced Interpretation Techniques

• Source localization: Estimating neural generators of EEG activity 5
• Functional connectivity analysis: Assessing communication between brain regions 2
• Artificial intelligence assistance: AI models can achieve expert-level performance in routine EEG interpretation 6

EEG interpretation requires both technical knowledge and clinical experience. While guidelines provide a framework, developing pattern recognition skills through supervised practice is essential for accurate interpretation.