Classification of Seizures
Primary Classification Framework
Seizures are classified into three fundamental categories based on onset: focal onset, generalized onset, or unknown onset, as established by the International League Against Epilepsy (ILAE) 2017 classification system 1, 2, 3.
This operational classification system prioritizes clinical semiology (symptoms and signs) as the foundation for grouping seizures, rather than underlying mechanisms which remain incompletely understood 1, 2.
Focal Onset Seizures
Focal seizures arise within networks limited to a single cerebral hemisphere and may remain localized or subsequently spread 1, 4.
Key classification features for focal seizures:
Level of awareness: Focal seizures are subdivided into focal aware seizures (previously "simple partial") or focal impaired awareness seizures (previously "complex partial") based on whether consciousness is preserved 4, 2, 3.
Motor vs. nonmotor features: Focal seizures are further characterized by their most prominent manifestations as either motor onset or nonmotor onset 1, 2.
Secondary generalization: Focal seizures that evolve to engage both hemispheres are classified as focal to bilateral tonic-clonic seizures (previously "secondarily generalized") 4, 2, 3.
Focal epilepsies account for the majority of epilepsy cases in both children and adults 4.
Generalized Onset Seizures
Generalized seizures rapidly engage bilateral networks and affect both cerebral hemispheres from the onset, even when caused by a focal lesion 1, 2.
Generalized seizures are subdivided into:
Motor seizures: Including tonic-clonic seizures and other motor manifestations (tonic, clonic, myoclonic, atonic, epileptic spasms) 1, 2, 3.
Nonmotor seizures: Primarily referring to absence seizures (typical, atypical, myoclonic, eyelid myoclonia) 2, 3.
Unknown Onset Seizures
When the initial manifestations of a seizure are missed, unclear, or unwitnessed, the seizure is classified as unknown onset 2, 3.
Unknown onset seizures can be further classified as:
- Motor (tonic-clonic, epileptic spasms) or nonmotor manifestations 2.
- Behavior arrest 2.
- These can be reclassified as focal or generalized once additional information becomes available 3.
Etiologic Classification: Provoked vs. Unprovoked
Beyond seizure type, a critical distinction exists between provoked (acute symptomatic) and unprovoked seizures 1, 5, 6.
Provoked Seizures
Provoked seizures occur at the time of or within 7 days of an acute neurologic, systemic, metabolic, or toxic insult 1, 5, 6.
Common causes include:
Metabolic derangements: Hyponatremia, hypocalcemia, hypomagnesemia, hypernatremia, hypoglycemia, hyperglycemia, uremia 1, 7, 6.
Toxic exposures: Cocaine, tramadol, tricyclic antidepressants, antihistamines, theophylline 7, 6.
Acute CNS insults: Encephalitis, meningitis, CNS mass lesions, acute stroke, traumatic brain injury, intracranial hemorrhage 1, 6.
Unprovoked Seizures
Unprovoked seizures occur without acute precipitating factors 1, 5, 6.
Categories include:
Remote symptomatic seizures: Resulting from CNS or systemic insults occurring more than 7 days in the past (prior stroke, remote traumatic brain injury) 1, 5, 6.
Idiopathic seizures: No identifiable structural or metabolic cause 6.
Genetic epilepsy syndromes: Particularly in pediatric populations 1, 6.
Structural lesions: Malformations of cortical development, vascular malformations, tumors 6, 4.
Epilepsy Classification
Epilepsy is defined as: (1) at least two unprovoked seizures occurring more than 24 hours apart, (2) one unprovoked seizure with probability of recurrence similar to two seizures over 10 years, or (3) diagnosis of an epilepsy syndrome 1, 5.
Epilepsies are classified as:
Focal epilepsy: Seizures originating from one hemisphere 4, 3.
Generalized epilepsy: Bilateral network involvement from onset 3.
Combined generalized and focal epilepsy: A newly recognized category 3.
Unknown epilepsy: When classification cannot be determined 3.
Etiologic subcategories:
- Idiopathic: No identifiable cause 8.
- Cryptogenic: Presumed symptomatic but cause unknown 8.
- Symptomatic: Known structural, metabolic, genetic, infectious, or immune etiology 8, 4.
Pediatric-Specific Considerations
In children, focal epilepsies are subdivided into three groups 4:
Self-limited focal epilepsy syndromes: Such as self-limited epilepsy with centrotemporal spikes 4.
Focal epilepsy of unknown cause: Not meeting criteria for self-limited syndromes 4.
Focal epilepsy of known cause: Structural lesions (developmental or acquired) 4.
Neonatal seizures have distinct etiologies:
- Hypoxic ischemic injury is the most common cause (46-65%), with approximately 90% occurring within 2 days of birth 6.
- Intracranial hemorrhage and perinatal ischemic stroke account for 10-12% 6.
- An underlying cause can be identified in approximately 95% of neonatal seizures 6.
Clinical Pitfalls
Terminology evolution: The 2017 ILAE classification replaced "simple partial" with "focal aware," "complex partial" with "focal impaired awareness," and "secondarily generalized" with "focal to bilateral tonic-clonic" 4, 2, 3.
Distinguishing seizures from syncope: Lateral tongue biting, prolonged confusion with muscle aching, tonic-clonic movements lasting >1 minute, and coarse rhythmic synchronous jerking strongly suggest seizure rather than syncope 5.
Urinary incontinence occurs in both seizures and syncope and should not be used as a distinguishing feature 5.
Approximately 8% of first seizure patients have correctable metabolic abnormalities, necessitating comprehensive metabolic evaluation 5, 6.
Focal neurologic findings have 97% correlation with symptomatic seizures, requiring urgent neuroimaging 6.