What characterizes the onset of a generalized tonic-clonic seizure on electroencephalogram (EEG)?

Electroencephalographic Characteristics of Generalized Tonic-Clonic Seizure Onset

The onset of a generalized tonic-clonic seizure on EEG is characterized by an abrupt appearance of epileptiform discharges averaging >2.5 Hz for ≥10 seconds or any pattern with definite evolution lasting ≥10 seconds, often beginning with generalized polyspikes followed by rhythmic activity that increases in amplitude and decreases in frequency during the tonic phase.

Initial EEG Changes at Seizure Onset

• Pre-ictal period: Some patients may show brief focal features or asymmetry at onset, even in idiopathic generalized epilepsy 1
• Seizure initiation: Characterized by sudden appearance of:
  • Generalized polyspike discharges
  • High-amplitude, high-frequency (>13 Hz) activity
  • Bilateral synchronous discharges, though they may be asymmetric initially

Progression of EEG Patterns During Seizure

Tonic Phase

• Initial fast activity (>13 Hz)
• Progressive slowing with increasing amplitude
• Generalized muscle artifact often obscures EEG during this phase
• Usually symmetric in idiopathic generalized epilepsy, may be asymmetric in focal-onset seizures 2

Clonic Phase

• Rhythmic spike-and-wave complexes
• Progressive slowing of frequency (typically 2-3 Hz)
• Increasing inter-discharge intervals as seizure progresses
• May show asymmetry or asynchrony, particularly in secondarily generalized seizures 2

Terminal Phase

• Postictal generalized EEG suppression (PGES)
• Gradual recovery of background activity
• May show postictal slowing

Diagnostic Criteria According to Guidelines

According to the American Clinical Neurophysiology Society criteria cited in the 2024 American Heart Association guidelines 3, an electrographic seizure is defined as:

1. Epileptiform discharges averaging >2.5 Hz for ≥10 seconds (>25 discharges in 10 seconds), OR
2. Any pattern with definite evolution (at least 2 unequivocal, sequential changes in frequency, morphology, or location) lasting ≥10 seconds

Distinguishing Features from Other Conditions

Generalized vs. Focal-Onset Seizures

• Primary generalized seizures typically show bilateral synchronous discharges from onset
• Secondarily generalized seizures (focal to bilateral tonic-clonic) may show:
  • Initial focal discharge
  • Asymmetric evolution during tonic and clonic phases 4
  • More frequent asymmetry during the terminal clonic phase 2

Seizure vs. Syncope on EEG

• Syncope shows diffuse slowing rather than epileptiform discharges
• EEG in syncope is typically normal between episodes 3
• Seizures show characteristic epileptiform patterns during and sometimes between events

Clinical Correlation

The EEG findings should be interpreted in conjunction with clinical manifestations:

• Tonic phase correlates with sustained muscle contraction
• Clonic phase correlates with rhythmic jerking movements
• Postictal EEG suppression correlates with impaired consciousness after seizure

Potential Pitfalls

• Artifact contamination: Muscle artifact during the tonic phase may obscure underlying EEG activity
• Misinterpretation: Brief focal features at onset of generalized seizures may lead to incorrect classification 1
• Missed diagnosis: Non-convulsive status epilepticus may be overlooked without EEG monitoring 5
• Overinterpretation: Not all rhythmic patterns represent seizures; patterns on the ictal-interictal continuum require careful interpretation 3

Monitoring Recommendations

• Continuous EEG monitoring is valuable for patients with altered consciousness after seizures 3
• Video-EEG correlation improves diagnostic accuracy, particularly for distinguishing between different seizure types 5
• EEG should be interpreted by experienced clinicians familiar with seizure patterns and artifacts

Understanding these EEG characteristics is crucial for accurate diagnosis, classification, and management of generalized tonic-clonic seizures, ultimately improving patient outcomes related to morbidity, mortality, and quality of life.