Revised Operational Classification of Seizure Types

In April 2017, the International League Against Epilepsy (ILAE), which is the parent body of over 100 national professional epilepsy societies, published a revised operational classification of seizure types^1–4 and epilepsies.⁵ The seizure classification was a revision of the classification established in 1981⁶ and partially updated in 2010.⁷ The 1981 classification, which included the terms simple partial and complex partial seizures, had some limitations. There was no specific mention of several focal motor seizure types, such as atonic, myoclonic, or tonic, which could only appear as generalized seizures. Seizures, for example tonic-clonic, could not be classified if the onset was unknown. Some important generalized onset seizure types were missing. Certain terms, such as partial, simple, complex, and psychic, were not clear to the public and even to some physicians. Using consciousness to classify seizures was sometimes confusing, since unconsciousness or impaired consciousness meant different things to different people. In some studies, more than half of the people with epilepsy misclassified their seizure type.⁸

Figure 1 illustrates the expanded ILAE 2017 classification of seizures. A basic version can be constructed by eliminating the small font subcategories. This classification is empirical and based upon an extension of the 1981 system, since we do not yet know enough to classify according to the underlying science of epilepsy.

Partial seizures are renamed focal seizures. Seizures are classified as focal with onset in one hemisphere, versus generalized with onset in both hemispheres or seizures of unknown onset. For focal seizures, the next optional classifier is by level of awareness, which serves as a surrogate for consciousness. A seizure is focal aware (previously ‘simple partial’) if awareness is retained and focal impaired awareness (previously ‘complex partial’) if awareness is disturbed for any part of the seizure. The next optional categorization is into motor and nonmotor focal seizures, with specific types listed in Figure 1. The specific focal seizure subclassification derives from the first manifestation of a seizure, even if that is not the most prominent behavior, because the first manifestation marks the onset zone (seizure focus or network). For example, an event with chills and piloerection, followed by impaired awareness and tonic stiffening of the right arm is a focal impaired awareness autonomic seizure. It is always acceptable to add other descriptive terms (a standard set is suggested2), but these do not change the classification. A ‘secondarily generalized tonic-clonic seizure’ is renamed as focal to bilateral tonic-clonic seizure.

Generalized onset seizures rarely associate with retained awareness, so the subcategorization is directly into motor and nonmotor/absence. Types are similar to those of the 1981 classification, but a few new types have been added. The unknown onset category is a placeholder, so an unknown onset seizure may be revised to focal or generalized onset if more information becomes available. Behavior is the key to classification, but ancillary information, particularly electroencephalography (EEG), can aid classification when such information is available.

Also available is a new classification of the epilepsies,⁵ which accounts for whether the patient has focal, generalized, or mixed seizure types; etiologies; neuroimaging; immunologic or genetic findings; epilepsy syndromes; various prognoses and comorbidities; and other important factors in the overall clinical picture. The net result of the new classifications, once past a learning and adoption period, should be greater clarity of understanding by the medical community, public, and patients about their seizures and epilepsy.