EEG in Common Epilepsy Syndromes
Author: Raj D Sheth, MD, Division Chief, Division of Pediatric Neurology, Department of Pediatrics, Nemours Alfred I duPont Hospital for Children
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Updated: Sep 7, 2008
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[pic][pic]Role of EEG in Epilepsy Syndromes
EEG is an essential component in the evaluation of epilepsy. The EEG provides important information about background EEG and epileptiform discharges and is required for the diagnosis of specificelectroclinical syndromes.1 Such a diagnosis carries important prognostic information, guides selection of antiepileptic medication, and suggests when to discontinue medication. Neurologic examination and imaging in the essential idiopathic, typically genetic, epilepsies are usually normal.2
EEG background frequencies
Following a seizure (ie, during the postictal period) the EEG background maybe slow. However, interictal background EEG frequencies that are slower than normal for age usually suggest a symptomatic epilepsy (ie, epilepsy secondary to brain insult). Normal background suggests primary epilepsy (ie, idiopathic or possibly genetic epilepsy). Thus, EEG background offers important prognostic and classification information.
These help separategeneralized from focal (ie, partial) seizures.
• Symptomatic epilepsy - Seizures resulting from an identifiable cerebral disorder
• Cryptogenic epilepsy - Seizures occurring without identifiable cause in a patient with cognitive impairment or with neurological deficits (Examples of cryptogenic epilepsy include Lennox-Gastaut syndrome (LGS), infantile spasms, andmyoclonic astatic epilepsy of Doose.)
• Idiopathic epilepsy - Seizures occurring without an identifiable cause in a patient with entirely normal findings on neurologic examination and of normal intelligence (Examples of idiopathic epilepsy include benign partial epilepsy of childhood with centrotemporal spikes, benign partial epilepsy of childhood with occipital paroxysms, and juvenile myoclonicepilepsy.)
EEG characteristics of these specific electroclinical epilepsy syndromes are discussed in this article. Roles of EEG in temporal lobe epilepsy and frontal lobe epilepsy, among others, are not addressed here.
For related information, see Medscape's Epilepsy Resource Center.
Generalized seizures are rare in neonates. Many of the so-called subtle, generalizedtonic, and multifocal myoclonic seizures do not have an EEG correlate. These movements in the severely affected infant may represent brain stem release phenomena. Focal seizures, particularly clonic seizures, are highly associated with EEG changes. Thus, EEG plays a crucial role in the evaluation of neonatal seizures. The EEG changes significantly with gestational age; therefore, calculation ofgestational age and familiarity with age-specific norms is crucial in interpretation of the EEG in infants.
Two well-defined EEG seizure patterns are seen in neonates. They include the following:
• Seizures with focal low-frequency electrographic correlates: These may occur at 1-1.5 Hz frequency and generally are seen in severe cerebral insults, such as severe hypoxic-ischemic encephalopathy.• Seizures with focal high-frequency electrographic correlates: These typically evolve over 10-20 seconds and usually are seen with focal cerebral insults, such as strokes. Strokes in the neonate, unlike in the older individual, typically are associated with porencephalic cysts. Porencephalic cysts result from strokes that involve large portions of the cerebral parenchyma (ie, loss of both...