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, and identification of normal versus abnormal patterns to accurately assess brain function and diagnose neurological conditions.

Basic Setup and Recording Parameters

• For diagnostic purposes, use the standard 19 electrodes of the 10-20 International System; for monitoring purposes, four electrodes (e.g., P3, P4, F3, F4) may be sufficient 1
• Recording duration should be 20-30 minutes to capture variations in vigilance levels, though some experts consider 5-10 minutes adequate 1
• Include both eyes-closed and eyes-open recordings whenever possible 1
• Consider polygraphic recording to better understand unusual EEG patterns 1
• When possible, maintain consistent recording conditions (same time of day, similar feeding conditions) 1

Systematic Interpretation Approach

1. Assess Technical Quality

• Check electrode impedance and identify artifacts (muscle, eye movement, electrical interference) 2
• Verify appropriate filter settings and montage selection 2
• Ensure adequate recording duration to capture both wakefulness and drowsiness 1

2. Evaluate Background Activity

• Determine the dominant frequency (alpha: 8-13 Hz, beta: >13 Hz, theta: 4-7 Hz, delta: <4 Hz) 1
• Assess symmetry between hemispheres and anteroposterior gradients 1
• Note reactivity to eye opening/closing and other stimuli 1
• Identify sleep stages if present 2

3. Identify Abnormal Patterns

• Epileptiform discharges: spikes, sharp waves, spike-and-wave complexes 2
• Focal slowing: suggests localized dysfunction 2
• Generalized slowing: indicates diffuse encephalopathy 1
• Triphasic waves: often seen in metabolic encephalopathies, particularly hepatic 1
• Periodic patterns: may suggest specific etiologies (e.g., herpes encephalitis) 3

4. Recognize Normal Variants

• Benign epileptiform transients of sleep (BETS) 2
• Wicket spikes, 14 and 6 Hz positive bursts, rhythmic temporal theta of drowsiness 2
• Mu rhythm (central areas) and lambda waves (occipital areas) 2

Special Considerations for Specific Clinical Scenarios

Epilepsy Evaluation

• Identify interictal epileptiform discharges (IEDs) which support a clinical diagnosis of epilepsy 2
• Document electrographic seizures when present (evolution in frequency, amplitude, and spatial distribution) 2
• Note that only when an electrographic seizure is recorded is the diagnosis confirmed 2

Encephalopathy Assessment

• Evaluate for generalized slowing, which correlates with severity of encephalopathy 1
• In hepatic encephalopathy, look for triphasic waves and increased theta/delta activity 1
• Quantitative EEG analysis may improve reliability in assessing encephalopathy severity 1

Post-Cardiac Arrest Prognostication

• A continuous or nearly continuous normal-voltage EEG background without periodic discharges or seizures within 72 hours from ROSC suggests good outcome 1
• Avoid using low-voltage or discontinuous EEG background alone for prognostication 1
• Use the American Clinical Neurophysiology Society (ACNS) terminology to classify EEG patterns 1

Advanced EEG Analysis Techniques

• Quantitative EEG (qEEG): spectral analysis to determine frequency band power distribution 4, 5
• Source localization: algorithms to identify the neural generators of scalp EEG signals 6, 5
• Connectivity analysis: measures of functional relationships between brain regions 5
• Microstate analysis: identification of quasi-stable topographical patterns 5

Common Pitfalls to Avoid

• Misinterpreting normal variants as epileptiform discharges 2
• Over-interpreting artifacts (muscle, movement, electrode) as cerebral activity 2
• Failing to consider medication effects on EEG patterns 1
• Using EEG-derived indices (BIS, cerebral recovery index) alone for prognostication 1
• Not recognizing non-convulsive status epilepticus in encephalopathic patients 3

Specialized Applications

• Continuous EEG monitoring: for detection of nonconvulsive seizures in critically ill patients 3, 4
• Evoked potentials: for assessment of sensory pathways and cognitive function 1
• Intraoperative monitoring: to detect changes in cerebral function during surgery 7
• Brain-computer interfaces: for communication and control in severely disabled individuals 5