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  • Updated 04.26.2021
  • Released 06.10.2014
  • Expires For CME 04.26.2024

Focal onset generalized absence seizures

Introduction

This article includes discussion of focal onset generalized absence seizures and secondarily generalized absence seizures. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.

Overview

Although absences are considered a paradigm of primarily generalized seizures, they may occasionally be of focal onset (focal onset generalized absence seizures), usually arising from focal brain pathology. Focal onset generalized absence seizures may manifest either as typical (with > 2.5 Hz spike-wave discharges) or atypical (with < 2.5 Hz spike-wave discharges) absence seizures. The most classic focal onset generalized absence seizures are those in patients with focal structural, mainly frontal lobe, brain abnormalities. Frontal absences are characterized by speech and behavioral arrest, staring, reduced to complete loss of consciousness (sometimes with minor head and eye turning), and simple automatisms. There do not appear to be a preceding aura. Absences can be very short, less than 5 seconds, but may continue for 30 seconds or more. In addition, approximately one-third of patients with Lennox-Gastaut syndrome manifest with atypical absence seizures due to secondary bilateral synchrony, which refers to bilateral and synchronous 2 to 4 Hz EEG discharges generated by a unilateral cortical focus. Prognosis is variable and depends on the underlying disorder. Prophylactic treatment is usually with broad spectrum antiepileptic drugs, such as valproate, levetiracetam, lamotrigine, and topiramate. In this article, the author details nomenclature, classification, clinical manifestations, pathophysiology, EEG, neuroimaging, and optimal management of patients with focal onset generalized absence seizures.

Key points

• Focal onset generalized absence seizures are of focal onset and may manifest as either typical (with > 2.5 Hz spike-wave discharges) or atypical (with < 2.5 Hz spike-wave discharges) absence seizures.

• The most classic focal onset generalized absence seizures are those manifested in patients with focal structural, mainly frontal, brain abnormalities.

• Approximately one-third of patients with Lennox-Gastaut syndrome manifest with atypical absence seizures due to secondary bilateral synchrony—that is bilateral and synchronous 2 to 4 Hz EEG discharges generated by a unilateral cortical focus.

• EEG and MRI are the main modes of investigations.

• Prognosis is variable depending on the underlying cause.

• Broad spectrum antiepileptic drugs (valproate, levetiracetam, lamotrigine, topiramate) alone, but more commonly in combination, are the main prophylactic treatment.

Historical note and terminology

Although absences are considered a paradigm of primarily generalized seizures, they may occasionally be of focal onset (focal onset generalized absence seizures), usually arising from focal, mainly frontal lobe, brain pathology (09; 17; 46; 08; 10).

The term “secondarily generalized seizures” was first introduced in the 1970 International League Against Epilepsy (ILAE) classifications of seizures and was maintained in the 1981 revision (20; Commission of Classification and Terminology of the International League Against Epilepsy 1981). The prefix “secondarily” denotes generalized seizures that are of focal onset, as opposed to “primarily,” which are generalized epileptic seizures from onset. Secondarily generalized seizures rapidly evolve from a focal discharge or a focal seizure.

According to the more recent ILAE definitions, generalized seizures “originate at some point within, and rapidly engage, bilaterally distributed networks. Such bilateral networks can include cortical and subcortical structures, but do not necessarily include 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.” Conversely, focal seizures “originate within networks limited to one hemisphere, which may be discretely localized or more widely distributed. Focal seizures may originate in subcortical structures” (05; Commission on Classification and Terminology of the International League Against Epilepsy 2014).

Secondarily generalized seizures (of focal onset) start within networks limited to 1 hemisphere and either discretely localize or more widely distribute, but do not remain localized. They spread and trigger a generalized seizure. Secondarily generalized seizures usually refer to convulsive seizures (tonic, clonic, tonic-clonic), which are the most common progression of a focal seizure. However, though less common, it is well-known that focal discharges can rapidly evolve to an absence and/or a generalized spike-wave discharge. Typical of this situation is frontal absence seizures. Secondarily generalized absence seizures evolving from focal seizures have been introduced by the ILAE Task Force on Classification (16).

In the new ILAE reports on classification, the term “secondarily” generalized seizures has been discarded (05; Commission on Classification and Terminology of the International League Against Epilepsy 2014; 18; 19). It is replaced with “focal to bilateral convulsive seizures (tonic, clonic or tonic and clonic components in any order).” Furthermore, secondarily generalized absence seizures evolving from focal seizures are not recognized in these reports though well documented as described in this clinical article. More relevant to secondarily (or focal onset) generalized absence seizures is “secondary bilateral synchrony,” which refers to bilateral and synchronous 2 to 4 Hz EEG discharges generated by a unilateral cortical focus. Contrary to secondary bilateral synchrony, primary bilateral synchrony manifests with more rapid symmetrical and synchronous generalized spike-wave discharges caused by a generalized epileptic process independent of any focal hemispheric lesion. The term “secondary bilateral synchrony” was coined by Tükel and Jasper (49).

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