Recognition of interictal and ictal discharges on EEG. Focal vs generalized epilepsy

The differentiation between focal and generalized epilepsies based on clinical and electroencephalographic features is difficult and sometimes confusing.

To review the EEG findings in patients with focal epilepsy.

An extensive literature review was done. We used the following Pubmed and Medline descriptors alone and in different combinations for database searching: focal, partial, epilepsy, electroencephalographic findings, and EEG. Additional filters included review, original articles, and language limited to Spanish and English. Using the above criteria, a total of 69 articles showed the interictal and ictal EEG findings in focal epilepsy.

Focal epileptiform discharges and persistence of focal abnormalities, characterize the interictal EEG findings in focal epilepsies. To distinguish SBS from primary generalized spike waves are required to note: (a) a lead-in time of at least 2 s, (b) the morphology of the focal triggering spikes clearly differ from that of the bisynchronous epileptiform paroxysms, and (c) the morphology of triggering spikes resemble that of other focal spikes from the same region. Focal and Generalized Epilepsy can coexist. Delayed Lateralization on EEG with inconclusive onset and bizarre semiology confusing semiology should not be confused with generalized onset seizures with focal evolution.

A close attention to localization and morphology of epileptiform discharges, the correct interpretation of secondary bilateral synchrony, and provocative maneuvers help to correctly identify the EEG findings leading to diagnose focal epilepsies. The presence of generalized epileptiform activity does not rule out the existence of a focal epilepsy.