What is evoked EEG in a patient with suspected seizures or altered mental status and a history of encephalopathy?

What is Evoked EEG?

Evoked potentials (EPs) are neurophysiological recordings that measure the brain's electrical response to specific sensory stimuli, distinct from standard EEG which records spontaneous brain activity. 1

Core Definition and Mechanism

Evoked potentials are generated through either:

• Passive reception of sensory stimuli (called "exogenous" EPs) 1
• Cognitive processing of sensory stimuli (called "endogenous" or "cognitive" EPs) 1

Unlike standard EEG which only assesses cortical function, evoked potentials can evaluate both brainstem and cerebral cortex function. 1

Types of Evoked Potentials

EPs are classified by the stimulus type used:

• Visual Evoked Potentials (VEP) - using visual stimuli 1
• Auditory Evoked Potentials (AEP) - using auditory stimuli 1
• Somatosensory Evoked Potentials (SEP) - using tactile/sensory stimuli 1

They can also be classified by analysis time window:

• Short-latency EPs 1
• Middle-latency EPs 1
• Long-latency EPs 1

Clinical Applications in Encephalopathy

In patients with encephalopathy and altered mental status, evoked potentials provide quantitative assessment of brainstem and cortical function that complements standard EEG. 1

Specific Brainstem Assessment

• Brainstem Auditory Evoked Potentials (BAEPs) assess medullary, pontine, and midbrain function 1
• Short-latency Somatosensory Evoked Potentials (SSEPs) evaluate brainstem conduction 1
• These can detect brainstem conduction deficits due to edema that may be insensitive to hepatic encephalopathy itself 1

Cortical Function Assessment

• Long-latency VEP, SEP, and AEP assess cerebral cortex function 1
• Can provide quantitative assessment of cognitive processes 1

Key Advantages Over Standard EEG

Evoked potentials have relative resistance to environmental electrical noise and levels of anesthesia, which may completely obscure standard EEG in the ICU environment. 1

Additional advantages include:

• Provide straightforward brainstem assessment not available with standard EEG 1
• Can be summarized into quantitative indices for easier communication and follow-up 1
• Remain interpretable in patients under muscle blockade where clinical examination is not feasible 1

Quantitative Indices

Two summary indices can be derived from evoked potentials:

• Index of Global Cortical Function (IGCF) - derived from flash visual and cortical SSEPs, graded 0-4, correlates with Glasgow Coma Score 1
• Index of Brainstem Conduction (IBSC) - derived from subcortical somatosensory and BAEPs, reflects medullary, pontine, and/or midbrain dysfunction 1

Important Limitations

The major limitation of evoked potentials is that they summate functional status over the time needed for averaging, unlike EEG which provides real-time assessment. 1

This makes EPs unsuitable for:

• Studying rapidly changing activities 1
• Detecting epileptic phenomena 1
• Monitoring triphasic waves or periodic patterns 1
• Assessing cortical reactivity to stimulation 1

Clinical Context for Use

In patients with suspected seizures or altered mental status with encephalopathy history:

• Standard EEG remains the primary tool for detecting seizures and should be performed urgently 1, 2
• Evoked potentials serve as a complementary technique when brainstem function assessment is needed 1
• EPs can help differentiate structural from metabolic causes when combined with standard EEG 1
• In hepatic encephalopathy specifically, cortical evoked potentials worsen in parallel with increasing severity 1

Common Pitfalls

• Do not rely on evoked potentials alone to detect seizures - standard EEG is required 1
• Evoked potentials cannot provide real-time monitoring of changing brain states 1
• Many abnormal EP patterns are non-specific and may reflect various pathophysiological events 1
• Advances in neuroimaging have limited EP application in many ICUs, though they remain useful in select patients for outcome prediction 1