Disclosure
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. Such a diagnosis carries important prognostic information, guides selection of antiepileptic medication, and suggests when to discontinue medication. 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 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, and myoclonic 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 myoclonic epilepsy (JME). 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. Patient Education For excellent patient education resources, visit eMedicine's Procedures Center and Brain and Nervous System Center. Also, see eMedicine's patient education articles Electroencephalography (EEG) and Epilepsy.
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:
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 Image 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 Image 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. |
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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.
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 Image 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. Typically, an initial positive component is followed by one or more negative components and then a negative slow wave. They are frontally dominant (see Image 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 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.
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 between 3 and 10 years old. 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 Image 7). The electrical field of epileptiform discharges is not distributed widely. Frequently, the dipole is located tangentially, with positivity in the frontal regions. 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 Image 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 at around age 15 years.
BRE appears to be a dominantly inherited condition with variable penetrance. The reader should keep in mind that 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. 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 the majority of patients by age 16 years, BPEOP may persist in 20% of patients after age 20 years.
JME is the most common epilepsy syndrome presenting with generalized tonic-clonic seizures between ages 12 and 30 years in a patient who is otherwise neurologically normal. It may account for as many as 10% of all patients with epilepsy. Imaging study findings are normal. In susceptible persons, sleep deprivation often precipitates seizures. 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, as the drugs used in this entity differ from those used in most other seizure types.
EEG
Interictal EEG shows a normal background with frequent generalized polyspike and wave discharges that may be anteriorly dominant or diffuse (see Image 9). Polyspike and wave discharges by definition have at least 3 spikelike components in them.
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 often may outlast the duration of the photic stimulation or even induce a seizure (see Image 10).
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