Skip to Main Content

We have a new app!

Take the Access library with you wherever you go—easy access to books, videos, images, podcasts, personalized features, and more.

Download the Access App here: iOS and Android. Learn more here!


January 8, 2020


A seizure (from the Latin sacire, “to take possession of”) is a transient occurrence of signs or symptoms due to abnormal excessive or synchronous neuronal activity in the brain. Depending on the distribution of discharges, this abnormal brain activity can have various manifestations, ranging from dramatic convulsive activity to experiential phenomena not readily discernible by an observer. Although a variety of factors influence the incidence and prevalence of seizures, ~5–10% of the population will have at least one seizure, with the highest incidence occurring in early childhood and late adulthood.

The meaning of the term seizure needs to be carefully distinguished from that of epilepsy. Epilepsy describes a condition in which a person has a risk of recurrent seizures due to a chronic, underlying process. This definition implies that a person with a single seizure, or recurrent seizures due to correctable or avoidable circumstances, does not necessarily have epilepsy (although a single seizure associated with particular clinical or electroencephalographic features may establish the diagnosis of epilepsy). Epilepsy refers to a clinical phenomenon rather than a single disease entity, because there are many forms and causes of epilepsy. However, among the many causes of epilepsy there are various epilepsy syndromes in which the clinical and pathologic characteristics are distinctive and suggest a specific underlying etiology.

Using the definition of epilepsy as two or more unprovoked seizures, the incidence of epilepsy is ~0.3–0.5% in different populations throughout the world, and the prevalence of epilepsy has been estimated at 5–30 persons per 1000.


Determining the type of seizure that has occurred is essential for focusing the diagnostic approach on particular etiologies, selecting the appropriate therapy, and providing potentially vital information regarding prognosis. The International League Against Epilepsy (ILAE) Commission on Classification and Terminology provided an updated approach to classification of seizures in 2017 (Table 418-1). This system is based on the clinical features of seizures and associated electroencephalographic findings. Other potentially distinctive features such as etiology or cellular substrate are not considered in this classification system, although this will undoubtedly change in the future as more is learned about the pathophysiologic mechanisms that underlie specific seizure types.

|Download (.pdf)|Print
TABLE 418-1 Classification of Seizures*
  1. Focal Onset

    (Can be further described as having intact or impaired awareness, motor or nonmotor onset, or evolve from focal to bilateral tonic clonic)

  2. Generalized Onset

    1. Motor


       Other motor (e.g., atonic, myoclonic)

    2. Nonmotor (absence)

  3. Unknown Onset

    (Can be further described as motor or nonmotor, or unclassified)

*Based on the new 2017 International League Against Epilepsy classification of seizure types (RS Fisher et al: Epilepsia 58: 522, 2017).

A fundamental principle is that seizures may be either focal or generalized. Focal seizures originate within networks limited to one brain region ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.