What does an electroencephalogram (EEG) show?

What an EEG Shows

An EEG (electroencephalogram) records the electrical activity of the brain and primarily shows brain function through wave patterns, detecting seizures (including non-convulsive seizures), assessing severity of encephalopathy, identifying specific abnormal patterns that suggest particular diagnoses, and evaluating cortical function in patients with altered consciousness. 1, 2

Primary Information Provided by EEG

Brain Electrical Activity and Function

• EEG measures neuronal electrical activity across the cortex, displaying this as waveforms characterized by frequency (alpha 8-13 Hz, beta >13 Hz, theta 4-7 Hz, delta <4 Hz) and amplitude 3, 4
• The background rhythm and its symmetry between hemispheres provides real-time assessment of cortical function 3
• EEG reactivity to stimulation (eye opening/closing) indicates preserved cortical responsiveness 3

Seizure Detection and Epilepsy

• EEG is essential for detecting seizures, including their duration and response to therapy 1
• Non-convulsive seizures occur in 8-37% of encephalopathic patients and can only be diagnosed with EEG 2
• Interictal epileptiform discharges support a clinical diagnosis of seizures, but only when an electrographic seizure is recorded is the diagnosis confirmed 4
• Continuous EEG monitoring for at least 24 hours detects seizures in approximately 50% more patients compared to brief intermittent recordings 2

Encephalopathy Severity Assessment

• EEG provides information on the severity of encephalopathy (minimal to severe), independent of patient cooperation 1
• Progressive encephalopathy shows: initial slowing of background rhythm → increased then decreased amplitude → discontinuous pattern → isoelectric EEG 1
• The degree of generalized slowing correlates with encephalopathy severity 3

Specific Diagnostic Patterns

Pattern Recognition for Etiology

• Triphasic waves are highly suggestive of hepatic encephalopathy in confused/stuporous patients, though also seen in other metabolic encephalopathies (uremic, hyponatremia) and drug intoxications (lithium, valproate, baclofen) 1, 3
• Lateralized periodic discharges suggest HSV encephalitis 2, 5
• Extreme delta brush indicates anti-NMDAR encephalitis 2, 5
• Generalized periodic discharges may indicate Creutzfeldt-Jakob disease 5

Focal vs. Diffuse Abnormalities

• EEG can provide evidence of focal or multifocal brain abnormality when MRI is negative, supporting encephalitis over metabolic encephalopathy 2
• Focal slowing or asymmetry suggests structural lesions 3

What EEG Does NOT Show Well

Important Limitations

• EEG only assesses cortical function and does not directly evaluate brainstem structures (unlike evoked potentials which assess brainstem pathways) 1, 6
• When suppressed in severe coma, EEG cannot reliably provide information on residual cortical or subcortical activity 1, 2
• EEG is influenced by sedative drugs and electrical noise in the ICU environment 1
• EEG does not provide structural anatomic information like neuroimaging 5

Clinical Context and Timing

When to Obtain EEG

• Recommended in all patients with acute brain injury and unexplained persistent altered consciousness 1
• Urgent EEG (within 60 minutes) is recommended for patients with convulsive status epilepticus who do not return to functional baseline within 60 minutes after seizure medication 1
• Should be performed within 24 hours for patients suspected of having non-convulsive seizures 2

Technical Specifications

• Standard diagnostic EEG uses 19 electrodes of the 10-20 International System 1, 2, 3
• Recording duration of 20-30 minutes captures variations in vigilance levels 1, 3
• Four electrodes may be sufficient for monitoring purposes 1, 3

Common Pitfalls to Avoid

• Routine brief EEG will miss non-convulsive seizures in approximately half of patients with seizures compared to prolonged monitoring 1
• Failing to recognize that medication effects (especially sedatives and anesthetics) can significantly alter EEG patterns 2, 3
• Using quantitative EEG indices (like BIS) alone without expert visual interpretation, particularly in brain-injured patients 1
• Misinterpreting benign variants and artifacts as epileptiform patterns without adequate training 4