How do you differentiate seizure activity from artifact on electroencephalogram (EEG)?

Differentiating Seizure Activity from Artifact on EEG

Accurate differentiation between true seizure activity and artifacts on EEG requires skilled interpretation of specific electrographic patterns and characteristics according to standardized criteria established by the American Clinical Neurophysiology Society. 1

Key Definitions and Criteria for Seizure Activity

Electrographic Seizure Patterns

• Epileptiform discharges averaging >2.5 Hz for ≥10 seconds (>25 discharges in 10 seconds) 1
• Pattern evolution - any pattern with definite evolution lasting ≥10 seconds 1

Electroclinical Seizure Patterns

• EEG pattern with clinical correlate time-locked to the pattern (any duration) 1
• EEG and clinical improvement with parenteral antiseizure medication 1

Electroclinical Status Epilepticus

• Electroclinical seizure for ≥10 continuous minutes OR
• Total duration ≥20% of any 60-minute recording period OR
• ≥5 continuous minutes if convulsive (bilateral tonic-clonic activity) 1

Distinguishing Features of Seizure Activity vs. Artifacts

Spatial Distribution and Field Characteristics

• Seizure activity: Shows physiologic electrical field distribution across the brain
• Artifacts: Often shows non-physiologic distribution or abrupt field changes

Morphology and Evolution

• Seizure activity:
  • Shows evolution in frequency, amplitude, or morphology
  • Typically has stereotyped patterns
  • Often begins with low-amplitude fast activity that evolves
• Artifacts:
  • Usually maintains consistent frequency and morphology
  • Often appears/disappears abruptly without evolution
  • May show perfect rhythmicity (mechanical artifacts)

Reactivity and Context

• Seizure activity: Typically not affected by external stimuli or patient movement
• Artifacts: Often changes with patient movement, electrode manipulation, or environmental factors

Common Artifacts Mimicking Seizure Activity

1. Muscle artifact:

   • High frequency (>20 Hz)
   • Often seen in frontal/temporal regions
   • Changes with patient movement/tension
   • Can mimic fast seizure activity

2. Eye movement artifacts:

   • Vertical eye movements appear in-phase on EOG
   • Horizontal eye movements appear out-of-phase
   • Most prominent in anterior channels 1

3. Electrode artifacts:

   • Abrupt onset/offset
   • Often affects single or adjacent electrodes
   • May show non-physiologic potentials

4. Pulse/ECG artifact:

   • Regular rhythm corresponding to heart rate
   • Often seen near temporal electrodes

5. Movement artifacts:

   • Abrupt, high-amplitude changes
   • Often affects multiple channels simultaneously

Technical Considerations for Accurate Differentiation

Optimal EEG Montage Selection

• Recommended montage: References frontal, central, and occipital electrodes to contralateral mastoid 1
• Alternative montage: Uses Fz-Cz, Cz-Oz derivations 1
• Video EEG: Correct categorization requires skilled interpretation of video EEG 1

Electrode Placement Considerations

• Frontal derivations capture K-complexes and slow wave activity
• Occipital derivations capture alpha rhythm
• Central derivations best capture sleep spindles and vertex waves 1

Advanced Techniques for Difficult Cases

1. Continuous EEG monitoring:

   • Increases sensitivity for detecting episodic seizure activity
   • More effective than brief intermittent recordings for detecting unpredictable patterns 1
   • Particularly valuable in post-cardiac arrest patients 1

2. Quantitative analysis:

   • Spectral power analysis
   • Phase-lag-index-based connectivity assessment 2
   • Machine learning approaches using time-frequency domain features 3

3. Peak-to-Peak Amplitude Fluctuation (PPAF):

   • Assesses amplitude variability within waveforms
   • Particularly useful in frontal (Fp2) and parietal (Cz, Pz) regions 4

Common Pitfalls and Caveats

1. Misinterpreting normal variants:

   • Benign epileptiform transients of sleep
   • Wicket spikes
   • Rhythmic mid-temporal theta of drowsiness

2. Overinterpretation of rhythmic patterns:

   • Patterns on the ictal-interictal continuum may not represent true seizures 1
   • Generalized periodic discharges (0.5-2.5 Hz) without evolution may not require treatment 1

3. Failing to correlate with clinical state:

   • Some myoclonic movements occur without EEG correlate (subcortical myoclonus) 1
   • Some patterns may be part of the ictal-interictal continuum rather than definite seizures 1

4. Relying solely on automated detection:

   • While machine learning approaches show promise (91-99% sensitivity) 5, expert review remains essential

Remember that skilled interpretation of EEG requires understanding of both the technical aspects of recording and the clinical context of the patient. The differentiation between seizure activity and artifact is crucial for appropriate treatment decisions and avoiding misdiagnosis of epilepsy.