You are in: eMedicine Specialties > Neurology > Electroencephalography and Evoked Potentials EEG in Common Epilepsy SyndromesArticle Last Updated: Sep 7, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 10
  Author: Raj D Sheth, MD, Division Chief, Division of Pediatric Neurology, Department of Pediatrics, Nemours Alfred I duPont Hospital for Children Raj D Sheth is a member of the following medical societies: American Academy of Neurology, American Academy of Pediatrics, American Epilepsy Society, American Neurological Association, and Child Neurology Society Editors: Sydney Louis, MD, Emeritus Professor, Department of Neurology, Brown University School of Medicine; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Norberto Alvarez, MD, Assistant Professor, Department of Neurology, Harvard Medical School; Consulting Staff, Department of Neurology, Boston Children's Hospital; Selim R Benbadis, MD, Professor, Director of Comprehensive Epilepsy Program, Departments of Neurology and Neurosurgery, University of South Florida School of Medicine, Tampa General Hospital; Selim R Benbadis, MD, Professor, Director of Comprehensive Epilepsy Program, Departments of Neurology and Neurosurgery, University of South Florida School of Medicine, Tampa General Hospital Author and Editor Disclosure Synonyms and related keywords: absence epilepsy, neonatal seizures, symptomatic epilepsy, cryptogenic epilepsy, idiopathic epilepsy, benign partial epilepsy of childhood with occipital paroxysms, benign rolandic epilepsy, hypsarrhythmia, infantile spasms, juvenile absence epilepsy, juvenile myoclonic epilepsy, West syndrome, Lennox-Gastaut syndrome ROLE OF EEG IN EPILEPSY SYNDROMESSection 2 of 10
  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 specific electroclinical 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 may be 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. Epileptiform discharges These help separate generalized from focal (ie, partial) seizures. Epilepsy syndromes
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. NEONATAL SEIZURESSection 3 of 10
Generalized seizures are rare in neonates. Many of the so-called subtle, generalized tonic, 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 of gestational 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:
INFANTILE SPASMS AND WEST SYNDROMESection 4 of 10
West syndrome is a triad of infantile spasms, developmental retardation or regression, and hypsarrhythmia on EEG. The syndrome presents in infants aged between 6 and 18 months. Presence of a hypsarrhythmic EEG confirms the diagnosis of infantile spasms (see Media file 1). EEG patterns may evolve over a period; they initially appear in the sleep EEG record and subsequently present during the awake state. Hypsarrhythmia is seen in 75% of patients with West syndrome. Hypsarrhythmia consists of diffuse giant waves (high voltage, >400 microvolts) with a chaotic background of irregular, multifocal spikes and sharp waves and very little synchrony between the cerebral hemispheres. During sleep, the EEG may display bursts of synchronous polyspikes and waves. A pseudoperiodic pattern may be evident. Persistent slowing or epileptiform discharges in the hypsarrhythmic background may be present and may represent an area of focal dysfunction. Several variations to the hypsarrhythmic pattern, which are referred to as hypsarrhythmic variants, may be noted. Clinical spasms are associated with a marked suppression of the background that lasts for the duration of the spasm. This characteristic response is called the electrodecremental response (see Media file 2). EEG is useful in judging successful treatment of West syndrome. Typically, shortly after treatment with adrenocorticotropic hormone (ACTH) or vigabatrin is initiated, the spasms stop and hypsarrhythmia disappears. Hypsarrhythmia rarely persists beyond the age of 24 months. It may evolve into the slow spike and wave discharges seen in LGS. LENNOX-GASTAUT SYNDROMESection 5 of 10
Lennox-Gastaut Syndrome (LGS) is a childhood (onset age 3-5 years) epileptic encephalopathy that manifests with atonic seizures, tonic seizures, and atypical absence seizures associated with mental retardation and a characteristic EEG pattern. Infantile spasms and West syndrome frequently transform into LGS. Unlike West syndrome, LGS tends to be a lifelong epileptic encephalopathy. EEG shows an abnormally slow background and diffuse slow spike and slow wave (<2.5 Hz) activity (see Media files 3-4). The slow spike and wave activity serves to differentiate (poor-prognosis) LGS from benign absence epilepsy, in which diffuse 3-Hz spike and wave activity is seen, and from some of the more benign myoclonic types of epilepsy characterized by fast spike and wave (>2.5 Hz) activity, which carries a dramatically better prognosis than LGS. Many other epilepsy syndromes overlap with LGS, however, including myoclonic astatic epilepsy of Doose and other severe myoclonic epilepsies. EEG features of LGS may be divided into interictal and ictal.
CHILDHOOD ABSENCE EPILEPSYSection 6 of 10
Childhood absence epilepsy (CAE) presents between ages 3 and 5 years and usually remits by ages 10-12 years. Unlike juvenile absence epilepsy, CAE usually is not associated with tonic-clonic seizures. EEG shows a normal background for age and 3-Hz generalized spike and wave discharges (see Media file 5). Frequency of the spike-wave complexes is usually 4 Hz at the onset of the absence seizures and may slow to 2.5 Hz at the end of a seizure.3 Typically, an initial positive component is followed by one or more negative components and then a negative slow wave. They are frontally dominant (see Media file 6). Duration of discharges is typically 3-25 seconds. Discharges are not truly bisynchronous; usually a millisecond difference is noted between left and right cerebral hemispheres. Eye opening does not alter the discharges. However, discharges are state dependent. Their frequency increases with non-REM sleep, although the duration of the discharges is reduced. During REM sleep, the frequency of discharges resembles that seen in wakefulness. Some patients display occipital intermittent rhythmic delta discharges (OIRDA), which is thought to be a favorable prognostic indicator. Generalized discharges are ictal in nature. They may be so brief that no obvious clinical movements are seen, although typically minor eyelid fluttering or subtle rhythmic contractions of the mouth are seen. These minor motor accompaniments occur in 85% of patients with absence epilepsy. Absence status epilepticus occurs in about 10% of patients with CAE. Typically, a child with staring spells is misdiagnosed as having partial complex seizures and is treated with carbamazepine. In fact, carbamazepine can precipitate absence status, which is a nonconvulsive status epilepticus in which patients appear to be in a "twilight state." They are able to answer questions intermittently, although at times they are confused. EEG is crucial in the diagnosis. It shows near-continuous generalized spike and wave discharges. Absence should be differentiated from atypical absence seizures, which usually are seen in patients with LGS. EEG in atypical absence seizures shows a less abrupt onset and offset than in typical absence seizures. Furthermore, EEG background is slow, and duration of discharges is shorter. BENIGN PARTIAL EPILEPSIESSection 7 of 10
Two benign partial epilepsy syndromes of childhood have been well defined: (1) benign rolandic epilepsy (BRE), also called benign partial epilepsy of childhood with centrotemporal spikes and (2) benign partial epilepsy of childhood with occipital paroxysms (BPEOP). Other less well-defined syndromes include frontal and parietal partial epilepsy syndromes. Benign rolandic epilepsy Patients with BRE are typically aged 3-10 years. They may present with a history of orobuccal numbness on one side of the mouth or with a tingling sensation on one side of the face. These seizures are associated with preserved mentation and are thus simple partial seizures. During sleep, patients may have generalized tonic-clonic convulsions. EEG features of BRE include frequent spike and wave discharges in the centrotemporal region (see Media file 7). The electrical field of epileptiform discharges is not distributed widely. Frequently, the dipole is located tangentially, with positivity in the frontal regions.4 The negative pole is 150-300 microvolts, and the entire spike and wave complex lasts for 80-120 milliseconds. Characteristically, the spike is triphasic and blends into the after-coming slow wave. Commonly, epileptiform discharges occur in runs. Discharges may be bilateral in 30% of patients; when they occur bilaterally, the discharges are independent and asynchronous. Unilateral discharges are more common. Activating movements or eye opening does not block the discharges. Sleep, however, has a prominent activation on the epileptiform discharges (see Media file 8). Non-REM sleep, in particular, may show a 400-500% increase in the spike-wave index. Over time, the epileptiform discharges decrease, and they finally disappear around age 15 years. At times, the EEG, in addition to displaying centrotemporal spikes, can show generalized or multifocal spike wave discharges.5 BRE appears to be a dominantly inherited condition with variable penetrance.4 BRE is a syndromic diagnosis with the EEG forming an important component of the diagnosis. Epileptiform discharges in the rolandic region do not necessarily mean that the patient has BRE. Benign partial epilepsy of childhood with occipital paroxysms Gastaut described a partial epilepsy that was analogous to BRE, although the 2 syndromes have important differences.6 In BPEOP, for example, epileptiform discharges are located in the posterior head region, most prominently in the occipital region. Typical phenomena include interictal high-voltage (200-300 microvolts) EEG spike and wave complexes occurring in runs with a degree of rhythmicity and a frequency of 1-3 Hz. Typically, they are blocked or prominently attenuated with eye opening. They may be unilateral or bilateral and may occur independently on each side. Like that in BRE, the occipital spike-wave index is activated prominently with non-REM sleep. Generalized spike and wave discharges also may be present in 10% of children. Unlike BRE, which remits in most patients by age 16 years, BPEOP may persist in 20% of patients after age 20 years. JUVENILE MYOCLONIC EPILEPSYSection 8 of 10
Juvenile myoclonic epilepsy (JME) is the most common epilepsy syndrome presenting with generalized tonic-clonic seizures in a patient aged 12-30 years who is otherwise neurologically normal. It may account for as many as 10% of all patients with epilepsy. Imaging findings are normal. In susceptible persons, sleep deprivation often precipitates seizures. Typically, the patient may experience myoclonic jerks in the morning, although many patients do not mention that they are having myoclonic seizures until asked specifically about body jerks. Approximately 15% of patients have associated juvenile absence epilepsy or generalized tonic-clonic seizures upon awakening. Often the diagnosis is not made in a definitive fashion, which is unfortunate since a correct diagnosis helps guide management, which, in turn, affects prognosis because the drugs used in this entity differ from those used in most other seizure types.7 EEG Interictal EEG shows a normal background with frequent generalized polyspike and wave discharges that may be anteriorly dominant or diffuse (see Media file 9). Polyspike and wave discharges by definition have at least 3 spikelike components in them.8 Photosensitivity is present in at least 30% of patients. Photic stimulation, commonly at a frequency of 10-20 Hz, will elicit a photoparoxysmal response that may often outlast the duration of the photic stimulation or even induce a seizure (see Media file 10). MULTIMEDIASection 9 of 10
REFERENCESSection 10 of 10
EEG in Common Epilepsy Syndromes excerpt Article Last Updated: Sep 7, 2008 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
