Table 2 Development of ILAE classifications

Generalized seizures are those in which the first clinical and electroencephalographic changes indicate initial involvement of both cerebral hemispheres.

Generalized seizures are conceptualized as originating at some point within and rapidly engaging bilaterally distributed networks. Such bilateral networks can include cortical and subcortical structures, but do not necessarily include the entire cortex. Generalized seizures can be asymmetric.

Focal seizures (previously defined as partial) are those in which the first clinical and electroencephalographic changes indicate initial activation of a system of neurons limited to a part of one cerebral hemisphere.

Focal seizures are conceptualized as originating at some point within networks limited to one hemisphere. They may be discretely localized or more widely distributed. Focal seizures may originate in subcortical structures. For each seizure type, ictal onset is consistent from one seizure to another, with preferential propagation patterns that can involve the contralateral hemisphere. The Glossary of Ictal Semiology [51] Provides an initial vocabulary which, while in need of revision and expansion, is an example of the type of “dictionary” needed for discussing seizure semiology. This not only allows but requires greater precision in seizure description. Terms such as hypermotor, akinetic, versive, hemiconvulsion, and retained responsiveness or awareness communicate much more information about a patient’s seizure manifestations than do the terms complex and simple partial.

Idiopathic epilepsy: there is no underlying cause other than a possible hereditary predisposition.

Genetic epilepsy: the epilepsy is, as best as understood, the direct result of a known or presumed genetic defect(s) in which seizures are the core symptom of the disorder. This attribution must be supported by specific forms of evidence (e.g. specific molecular genetic studies or family studies).

Genetic: The epilepsy is a direct result of a genetic cause. Ideally, a gene and the mechanisms should be identified; however, this term would also apply to electroclinical syndromes for which twin or family segregation studies reproducibly show clinical evidence of a genetic basis (e.g., in the case of the genetic generalized epilepsies). At this time, channelopathies are the best example of genetic epilepsies. Ultimately, we expect causes of epilepsies to be identified by the mechanisms involved (i.e., channelopathies, mitochondrial respiratory chaindefects, etc.).

Symptomatic epilepsy: this type of epilepsy is the consequence of a known or suspected disorder of the central nervous system.

Structural and metabolic epilepsy: this type of epilepsy is the secondary result of a distinct structural or metabolic condition. These structural or metabolic disorders may be of acquired or genetic origin (as is the case for malformations of cortical development and certain metabolic disorders).

Structural-Metabolic: The epilepsy is the secondary result of a separate structural or metabolic condition. Structural and metabolic were combined to separate the concept from genetic and also because the two are often inseparable. Note that structural brain lesions, including many malformations of cortical development, often have genetic causes and most metabolic disorders are also of genetic origin. The distinction between “genetic epilepsy” and epilepsy due to a structural/metabolic cause is far from perfect, but we anticipate more specific characterizations of cause in the upcoming years.

Cryptogenic (probable symptomatic) epilepsy: this refers to a disorder whose cause is hidden or occult. Cryptogenic epilepsies are presumed to be symptomatic.

Unknown epilepsy: the nature of the underlying cause is as yet unknown; it may have a fundamental genetic basis (e.g., a previously unrecognized channelopathy) or it may be the consequence of an unrecognized structural or metabolic disorder not yet identified.

Unknown: Plain and direct, this label simply and accurately indicates ignorance and that further investigation is needed to identify the cause of the epilepsy. Unlike cryptogenic (presumed symptomatic), it makes no presumptions and requires no explanation or reinterpretation.