What is the role of electroencephalogram (EEG) in diagnosing and managing encephalopathy in patients of any age with suspected seizures or altered mental status?

Role of EEG in Encephalopathy

EEG should be performed in all patients with encephalopathy to exclude non-convulsive seizures and status epilepticus, which occur in 8-37% of encephalopathic patients and can only be diagnosed with EEG. 1, 2, 3

Primary Indications for EEG in Encephalopathy

Detecting Subclinical Seizure Activity

• Order EEG immediately for any patient who does not follow commands after a seizure or has unexplained altered mental status, as this represents a Class 1 recommendation with the highest level of evidence. 2
• Non-convulsive status epilepticus (NCSE) occurs in 8% of comatose ICU patients with encephalopathy who show no overt seizure activity. 1, 4
• In prospective studies, 37% of patients with persistent unexplained altered consciousness had combined EEG and clinical evidence of status epilepticus that would have been undetected without EEG. 3
• Approximately 28% of electrographic seizures are detected only after prolonged monitoring, not on brief routine EEG. 2

Differentiating Encephalopathy Etiologies

• EEG can provide evidence of focal or multifocal brain abnormality when MRI is negative, supporting encephalitis over metabolic encephalopathy. 1
• In septic encephalopathy, EEG is more sensitive than clinical criteria for detecting cerebral dysfunction and correlates with mortality risk even within the same clinical severity group. 5
• EEG patterns can suggest specific diagnoses: triphasic waves support hepatic encephalopathy, lateralized periodic discharges suggest HSV encephalitis, and extreme delta brush indicates anti-NMDAR encephalitis. 1, 6

Timing and Duration of EEG

When to Order

• Perform EEG within 24 hours for patients suspected of having non-convulsive seizures, with an average response time of approximately 3 hours from request to preliminary reading. 2
• In post-cardiac arrest patients who remain comatose, obtain EEG within 24 hours of rewarming. 2

Duration of Monitoring

• Standard diagnostic EEG should last 20-30 minutes to capture variations in vigilance levels. 1, 2
• Continuous EEG monitoring for at least 24 hours is superior to routine EEG, as it detects seizures in approximately 50% more patients compared to brief intermittent recordings. 2, 4
• Several days of continuous monitoring may be needed since epileptiform activity can emerge late in the clinical course. 2

Technical Specifications

Recording Parameters

• Use 19 electrodes of the 10-20 International System for diagnostic purposes. 1, 2
• For ICU monitoring, simplified montages with 4-10 electrodes are acceptable alternatives, though they may miss some findings. 1, 2
• Record both eyes-closed and eyes-open conditions when possible; photic stimulation is not necessary. 1

Interpretation Framework

• Always report the basic background frequency, as this provides information on encephalopathy severity independent of patient cooperation. 1
• Use American Clinical Neurophysiology Society criteria to classify findings, including electrographic seizure, electrographic status epilepticus, and ictal-interictal continuum patterns. 2

Specific Encephalopathy Types

Autoimmune Encephalitis

• EEG confirms focal or multifocal brain abnormality and rules out subclinical seizures. 1
• Findings include focal slowing/seizures, lateralized periodic discharges, and extreme delta brush (occasionally seen in NMDAR-antibody encephalitis). 1
• Frequent subclinical seizures are commonly identified in LGI1-antibody encephalitis, though patients may have normal EEG including those with faciobrachial dystonic seizures. 1
• A normal EEG does not exclude autoimmune encephalitis but can support primary psychiatric disorders when investigating isolated new psychiatric symptoms. 1

Hepatic Encephalopathy

• EEG provides information on severity (minimal to severe) independent of patient cooperation. 1
• Triphasic waves are highly suggestive of hepatic encephalopathy in confused/stuporous patients. 1
• In acute liver failure, EEG is the only method for diagnosing non-convulsive seizures and monitoring treatment response. 1
• EEG monitoring is essential in ICU patients with acute liver failure to detect seizure activity that can increase intracranial pressure. 1

Septic Encephalopathy

• EEG is more sensitive than clinical criteria for detecting encephalopathy and shows well-defined categories that correlate with mortality. 5
• EEG patterns range from excessive theta (mild) to predominant delta, triphasic waves, and suppression/burst suppression (severe) in ascending order of severity. 5

Viral Encephalitis

• EEG should be considered in all patients with undiagnosed encephalopathy to detect non-convulsive status epilepticus, which can only be diagnosed with EEG. 1
• Subtle motor status epilepticus often follows overt convulsive seizures and requires EEG for diagnosis. 1

Treatment Implications

When EEG Detects Seizures

• Treat non-convulsive seizures detected by EEG with standard antiseizure medications such as levetiracetam, valproate, or phenytoin/fosphenytoin (Class 2a recommendation). 2
• Continue EEG for at least 24 hours if the patient does not return to baseline neurologic function after treatment. 2
• In post-cardiac arrest patients, lorazepam is significantly superior to phenytoin for overt status epilepticus. 2

Monitoring Treatment Response

• EEG is essential for monitoring patients who require pentobarbital coma for refractory status epilepticus. 3
• Breakthrough seizures detectable only by EEG occurred in more than half of patients with refractory status epilepticus treated with continuous IV midazolam. 4

Common Pitfalls and Caveats

Interpretation Limitations

• EEG is influenced by sedative drugs, electric noise, and when suppressed in severe coma, cannot reliably provide information on residual cortical or subcortical activity. 1
• Patient temperature should be considered in EEG interpretation, particularly when evaluating evoked potentials. 1
• Correct categorization of EEG patterns requires skilled interpretation by board-certified epileptologists. 2

False Negatives

• Routine outpatient EEG may miss approximately 50% of non-convulsive seizures compared to prolonged monitoring. 2
• A normal EEG does not exclude epilepsy or autoimmune encephalitis, as seizures may occur intermittently. 1, 4

Resource Considerations

• Continuous EEG is labor-intensive and adds significant cost, with controversial cost-effectiveness, but is more likely to detect abnormal electrical activity than standard EEG in critically ill patients. 2, 4
• Consider neurologic consultation before obtaining EEG given the specialized nature of interpretation. 2