Excerpt from EEG Atlas: Generalized Epilepsies

Synonyms, Key Words, and Related Terms: EEG atlas, spikes and sharp waves, spike-wave complexes, SWC, 3-Hz SWC, slow SWC, polyspikes, generalized epileptiform discharges, hypsarrhythmia, idiopathic generalized epilepsies, childhood absence epilepsy, juvenile myoclonic epilepsy, epilepsy with grand mal seizures, symptomatic generalized epilepsies, West syndrome, cryptogenic generalized epilepsy, Lennox-Gastaut syndrome, LGS

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The International Classification of Epileptic Syndromes and Epilepsies (International League Against Epilepsy [ILAE], 1989) classifies the epilepsies along 2 dichotomies: partial (ie, localization-related) versus generalized, and idiopathic versus cryptogenic or symptomatic. This double dichotomy conveniently allows the epilepsy classification system to be presented in a simple table, as follows:

Classification of the Epilepsies*

Generalized Localization-related

Idiopathic
(genetic) Childhood absence epilepsy
Juvenile absence epilepsy
Juvenile myoclonic epilepsy
Epilepsy with grand-mal seizures on awakening
Other idiopathic generalized epilepsies
Benign focal epilepsy of childhood (2 types)
ADNFLE**
Primary reading epilepsy

Symptomatic
or cryptogenic West syndrome
Lennox-Gastaut syndrome
Other symptomatic generalized epilepsies Mesiotemporal lobe epilepsy
Neocortical focal epilepsy

*Adapted from Tich and Pereon (1999)

**ADNFLE - Autosomal dominant nocturnal frontal lobe epilepsy

The term "idiopathic" often is misunderstood in this setting and requires clarification. Whereas the term idiopathic in medicine usually means "of unknown cause," idiopathic epilepsies are not truly of "unknown cause" (this confusing terminology will most likely be corrected in the upcoming ILAE classification system [Engel, 1998]). In epilepsy, idiopathic seizures are genetically determined and have no apparent structural cause, with seizures as the only manifestation of the condition. Findings of the neurologic examination and neuroimaging studies are normal, and EEG findings also are normal other than the epileptiform abnormalities. In some syndromes, the genetic substrate has even been identified.

Most idiopathic epilepsies are generalized, but a few genetic epilepsies are focal. Nonidiopathic epilepsies are by definition not genetic (although some may be associated with a minor genetic predisposition), but are the result of a brain insult or lesion. If the damage is focal, it results in a localization-related epilepsy; if it is diffuse, it results in a generalized epilepsy. The difference between symptomatic and cryptogenic is subtle: symptomatic means that the etiology is known, while cryptogenic means that an underlying etiology is apparent but cannot be documented objectively. Thus, the boundary between the 2 is largely dependent on our diagnostic and imaging techniques.

This review discusses EEG findings in the generalized epilepsies.

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