Classifying Epileptic Seizures and the Epilepsies
The first classification of epileptic seizures was proposed by Henri Gastaut in 1964, with modifications by the Commission on Classification and Terminology of The International League Against Epilepsy (ILAE) in 1981 and 1989. The original purpose of classifying seizures and epilepsy, as stated by Engel, was to “provide a universal vocabulary that not only facilitated communication among clinicians, but also established a taxonomic foundation for performing quantitative clinical and basic research on epilepsy.” The classification was based on expert opinion of the electroclinical features of seizures. Gastaut and colleagues recognized the imperfection of their system due to limited knowledge of the underlying pathophysiology of epilepsy. With advances in neuroimaging, neurophysiology, genetics, and neuroimmunology, classification needed to evolve further.
The International League Against Epilepsy (ILAE) organization of the epilepsies in 2010 was a major update of terminology to incorporate scientific advances. The term organization, rather than classification, was proposed, as the new term enables epilepsies to be organized by different parameters such as seizure type, age at onset, electroencephalogram (EEG), or neuroimaging. This new system, with its limitations, is “a work in progress” that will continue to develop as knowledge of the underlying pathophysiology and etiologies of epilepsies evolves.
Generalized and focal seizures
In all classification schemes, the distinction between focal and generalized seizures is critical, since this distinction determines possible etiologies (Chapter 3) and choice of medical and surgical treatments (Chapters 11 and 27). In the updated nomenclature (2010), generalized seizures (Chapter 24) originate
at some point within and rapidly engage bilaterally distributed networks. Such networks can include cortical and subcortical structures but do not necessarily involve the entire cortex. Although individual seizure onsets can appear localized, the location and lateralization are not consistent from one seizure to another. Generalized seizures can be asymmetric.
The subtypes are summarized in Table 2.1, with the main changes from the 1981 classification being the addition of subtypes of absence and myoclonic seizures.
Focal seizures originate “at some point within networks limited to one hemisphere. Focal seizures may originate within subcortical structures.” Focal seizures may be classified as focal without impairment of consciousness (clonic, autonomic, and hemiconvulsive), focal with subjective sensory or psychic phenomena (aura specific), focal dyscognitive with impairment of consciousness, and focal evolving to a bilateral convulsive seizure.
|Tonic–clonic (in any combination)|
|Absence with special features|
The terms simple partial, complex partial, and partial seizures with secondary generalization have been embedded in the epilepsy lexicon for decades. There is considerable resistance to letting go of these terms. However, simple (without alteration of awareness) and complex (with altered awareness) are often used incorrectly. Complex partial has been replaced by the term focal dyscognitive, describing seizures with disturbed cognition as the prominent feature. The term secondarily generalized seizure is replaced by focal seizure evolving to a bilateral convulsive seizure.
Neonatal seizures (Chapter 20) are no longer regarded as a separate entity. Seizures in neonates can be classified within the new scheme.
Epileptic spasms (Chapter 21) were not acknowledged in the 1981 classification. Epileptic spasms is preferred to infantile spasms because they may continue or begin after the first year of life. Because there is insufficient knowledge to classify these seizures as focal, generalized, or both, they have been placed in their own group, unknown. In some patients, there is evidence that epileptic spasms can arise from surgically treatable focal brain lesions.
Generalized and focal epilepsies
Many patients can be classified as having focal or generalized epilepsy based on clinical features (Chapter 5), EEG (Chapter 7), and MRI (Chapter 8). Generalized epilepsies are associated with generalized spike wave discharges on EEG while focal epilepsies are associated with focal slowing or epileptiform discharges and sometimes focal structural abnormalities (Chapters 7 and 24).
However, some patients do not fit exactly into the generalized or focal epilepsy categories and instead have features of both. Children with Dravet syndrome are an example.
Simple and complex partial seizures
Evolving to bilateral convulsive seizure
Secondarily generalized seizure