Seizure Classification According to the International League Against Epilepsy

Basic Classification Framework

Seizures are classified based on their origin in the brain, not on fundamental mechanisms, as the basic mechanism of epileptic seizures has not been fully elucidated 1
Correct seizure classification is crucial for proper diagnosis, treatment selection, and accurate prognostication 1

Focal onset seizures arise within networks of a single cerebral hemisphere and may remain localized or become more widely distributed 1
Generalized onset seizures rapidly affect both hemispheres and both sides of the body from onset 1

The International League Against Epilepsy defines epilepsy as having at least two unprovoked seizures occurring more than 24 hours apart, one unprovoked seizure with high recurrence risk, or diagnosis of an epilepsy syndrome 1, 2
The American College of Radiology recommends that epilepsy classification should emphasize the etiology of seizures and epilepsies, although this fact is from the Journal of the American College of Radiology, the guideline society is not specified, however the International League Against Epilepsy is the one that defines epilepsy 1, 2

The term "generalized" implies diffuse involvement of the brain, differing from focal seizures with secondary generalization (now known as focal to bilateral tonic-clonic), which start focally and then propagate to both hemispheres 3

Primary generalized epilepsy syndromes include juvenile myoclonic epilepsy and childhood absence epilepsy 3

MRI is rarely indicated in evaluation of a neurologically normal patient presenting with generalized seizures because the rate of positive intracranial findings is low, given their genetic underpinnings 3
CT has limited role in evaluation of generalized seizures, with only about 6% of CT examinations showing positive findings for generalized seizures 3

The American College of Emergency Physicians notes that seizures can be categorized by their relationship to precipitating factors, including provoked (acute symptomatic) seizures that occur at the time of or within 7 days of an acute neurologic, systemic, metabolic, or toxic insult 4
Provoked seizures can be caused by factors such as hyponatremia, other electrolyte abnormalities, alcohol or drug withdrawal, toxic ingestions, encephalitis, or acute CNS mass lesions, as reported in the Annals of Emergency Medicine 4
Unprovoked seizures, on the other hand, occur without acute precipitating factors, more than 7 days after any insult, and include idiopathic seizures and remote symptomatic seizures (from prior stroke, traumatic brain injury, or other past events) 4
Recurrent unprovoked seizures define epilepsy, according to the clinical context provided in the Annals of Emergency Medicine 4

Idiopathic generalized epilepsy is the most common source of generalized epileptiform discharges, driven by strong genetic factors. (American College of Radiology) 5
True focal seizures with secondary generalization begin focally and then spread bilaterally, distinguishing them from primary generalized seizures. (American College of Radiology) 5

Generalized periodic discharges (0.5–2.5 Hz) are present in approximately 80 % of patients who develop epileptiform activity after cardiac arrest. (Observational cohort) [6][7]8
These patterns occupy the “ictal‑interictal continuum,” and their clinical significance remains uncertain. (Observational cohort) [6][7]
It is unclear whether the discharges are markers of severe brain injury or represent true epileptic activity that requires antiseizure treatment. (Observational cohort) [6][7]
The TELSTAR randomized trial found no overall outcome benefit from treating post‑arrest generalized periodic discharges, although a small subgroup with documented electrographic seizures may derive benefit. (Randomized trial) [6][7]8

In neurologically normal children with typical clinical features of genetic generalized epilepsy, brain MRI reveals structural abnormalities in only about 2 % of cases, underscoring the primarily genetic etiology. (American College of Radiology) [9][5]
Computed tomography (CT) identifies a structural cause in merely 6 % of generalized seizure presentations, compared with ~50 % in focal epilepsy, indicating limited utility of CT for generalized epileptiform discharges. (American College of Radiology) [9][5]