What should be looked for when interpreting an electroencephalogram (EEG)?

What to Look for in an EEG

When interpreting an EEG, systematically assess the background activity, look for epileptiform discharges (spikes, sharp waves, periodic discharges), identify seizure activity, evaluate reactivity and continuity, and recognize specific patterns that suggest particular etiologies. 1

Standard Recording Parameters

• Use 19 electrodes of the 10-20 International System for diagnostic purposes 2, 1
• Record for 20-30 minutes to capture variations in vigilance levels 2, 1
• Include both eyes-closed and eyes-open recordings when possible 2
• Consider polygraphic recording to understand unusual EEG patterns 2

Background Activity Assessment

Evaluate the dominant background frequency and voltage as your first step:

• Normal adult background shows 8-13 Hz posterior dominant rhythm (alpha) when awake with eyes closed 3
• Assess whether background is continuous, nearly continuous, or discontinuous 2
• Determine if voltage is normal or low 2
• Note any diffuse slowing (dominant frequency <8 Hz indicates encephalopathy) 2
• Check for reactivity to eye opening/closing and external stimuli 2

Epileptiform Patterns to Identify

Look for interictal epileptiform discharges that support epilepsy diagnosis:

• Spikes and sharp waves (the hallmark of epilepsy) 4
• Generalized spike-wave complexes (bisynchronous, symmetric in idiopathic generalized epilepsy) 5
• Polyspikes and polyspike-wave discharges 5
• Periodic lateralized epileptiform discharges (PLEDs) - seen in HSV encephalitis but not pathognomonic 2
• Generalized periodic discharges 2

Critical distinction: Benign variants can appear epileptiform but occur in healthy individuals without epilepsy - understanding these prevents overtreatment 3

Seizure Activity Detection

EEG is the definitive test for detecting ongoing electrical seizure activity, especially when clinical manifestations are subtle or absent: 1, 6

• Electrographic seizures show evolving rhythmic or repetitive patterns with clear beginning, middle, and end 1
• Approximately 25% of patients have ongoing electrical seizures despite cessation of visible convulsive activity 6
• Nonconvulsive status epilepticus occurs in 8% of comatose ICU patients without clinical seizure activity 6
• Routine EEG misses nonconvulsive seizures in approximately 50% of patients compared to prolonged monitoring 1, 6

Pattern-Specific Findings Suggesting Etiology

Certain EEG phenotypes indicate specific pathophysiology when interpreted in clinical context: 7

• Lateralized periodic discharges suggest HSV-1 encephalitis 7
• Generalized periodic discharges suggest sporadic Creutzfeldt-Jakob disease 7
• Extreme delta brush suggests anti-NMDA receptor autoimmune encephalitis 8, 7
• Triphasic waves are highly suggestive of hepatic encephalopathy in confused/stuporous patients 2
• Occipital intermittent rhythmic delta activity and photoparoxysmal response characterize idiopathic generalized epilepsy 5

Post-Cardiac Arrest Prognostication

For comatose patients after cardiac arrest, specific EEG patterns predict neurological outcome:

• Continuous or nearly continuous, normal-voltage background without seizures or abundant/generalized periodic discharges predicts good outcome with high specificity (>80%) 2
• Use American Clinical Neurophysiology Society (ACNS) terminology to classify EEG patterns 2
• Perform EEG within 24 hours of rewarming to exclude nonconvulsive seizures 1
• Assess background continuity, voltage, presence of discharges, and reactivity during first 5 days after return of spontaneous circulation 2

Critical Clinical Scenarios Requiring Urgent EEG

Order emergent EEG in these situations: 1, 6

• Altered consciousness after motor seizures 1
• Suspected nonconvulsive status epilepticus 1, 6
• Subtle convulsive status epilepticus 1, 6
• Patients who received long-acting paralytics 1, 6
• Drug-induced coma 1
• Convulsive status epilepticus patients not returning to functional baseline within 60 minutes after medication 1
• Comatose ICU patients with unexplained impairment of mental status, particularly with severe sepsis or renal/hepatic failure 1

Distinguishing Organic from Psychiatric Causes

• EEG is abnormal in more than 80% of patients with acute viral encephalitis 2
• Perform EEG for patients with mildly altered behavior when uncertain whether psychiatric or organic cause 2
• Normal EEG does not exclude autoimmune encephalitis but supports primary psychiatric disorders when investigating isolated new psychiatric symptoms 1
• Encephalopathic changes on EEG help distinguish organic from psychiatric etiology 2

Common Pitfalls to Avoid

• Do not rely solely on routine outpatient EEG when nonconvulsive seizures are suspected - continuous monitoring detects significantly more seizures 1
• Recognize that PLEDs occur in many conditions beyond HSV encephalitis, including other viral encephalitides and non-infectious conditions 2
• Understand that focal, irregular, and "fragments" of discharges are not uncommon in idiopathic generalized epilepsy despite the classic description of generalized discharges 5
• Be aware that sedative drugs influence EEG findings, and suppressed EEG in severe coma cannot reliably provide information on residual cortical or subcortical activity 2
• Consider that increased intracranial pressure may cause bilateral loss of somatosensory evoked potential N20 due to midbrain compression, which may be reversible unlike post-anoxic causes 2