Role of EEG in Epilepsy Syndromes
Electroencephalography (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. Neurologic examination and imaging in the essential idiopathic, typically genetic, epilepsies are usually normal. 
EEG background frequencies and epileptiform discharges
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 help clinicians to separate generalized from focal (ie, partial) seizures.
Epilepsy syndromes include symptomatic, cryogenic, and idiopathic epilepsy. Symptomatic epilepsy is defined as seizures resulting from an identifiable cerebral disorder. Cryptogenic epilepsy consists of seizures that occur without an identifiable cause in a patient with cognitive impairment or with neurologic deficits (eg, Lennox-Gastaut syndrome (LGS), infantile spasms [see the first image below], and myoclonic astatic epilepsy of Doose.)
Idiopathic epilepsy consists of seizures that occur without an identifiable cause in a patient with entirely normal findings on neurologic examination and of normal intelligence (eg, benign partial epilepsy of childhood with centrotemporal spikes [BECTS], benign partial epilepsy of childhood with occipital paroxysms [BPEOP], and juvenile myoclonic epilepsy [see the second image below]).
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 more information, see the following:
Generalized seizures are rare in neonates. Many of the so-called subtle, generalized tonic, and multifocal myoclonic seizures do not have an electroencephalographic (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, as follows:
Seizures with focal low-frequency electrographic correlates: These patterns may occur at 1-1.5 Hz frequency and are generally seen in severe cerebral insults, such as severe hypoxic-ischemic encephalopathy.
Seizures with focal high-frequency electrographic correlates: These patterns typically evolve over 10-20 seconds and are usually seen with focal cerebral insults, such as strokes. Strokes in the neonate, unlike in the older individual, are typically associated with porencephalic cysts. Porencephalic cysts result from strokes that involve large portions of the cerebral parenchyma (ie, loss of both gray and white matter leading to a communication between the subarachnoid space and the cerebral ventricles).
For more information, see Benign Neonatal Convulsions.
Infantile Spasms and West Syndrome
West syndrome is a triad of infantile spasms, developmental retardation or regression, and hypsarrhythmia on electroencephalogram (EEG). The syndrome presents in infants aged between 6 and 18 months.
The presence of a hypsarrhythmic EEG confirms the diagnosis of infantile spasms (see the following image). EEG patterns may evolve over a time 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 [µV]) 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 the image below).
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 Lennox-Gastaut syndrome.
Lennox-Gastaut syndrome (LGS) is a childhood (onset, age 3-5 y) epileptic encephalopathy that manifests with atonic seizures, tonic seizures, and atypical absence seizures associated with mental retardation and a characteristic electroencephalographic (EEG) pattern. Infantile spasms and West syndrome frequently transform into Lennox-Gastaut syndrome. However, unlike West syndrome, Lennox-Gastaut syndrome tends to be a lifelong epileptic encephalopathy.
The EEG in affected patients shows an abnormally slow background and diffuse slow spike and slow wave (< 2.5 Hz) activity (see the images below). The slow spike and wave activity serves to differentiate (poor prognosis) Lennox-Gastaut syndrome 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 Lennox-Gastaut syndrome. Many other epilepsy syndromes overlap with Lennox-Gastaut syndrome, however, including myoclonic astatic epilepsy of Doose and other severe myoclonic epilepsies.
EEG features of Lennox-Gastaut syndrome may be divided into interictal and ictal.
Characteristic interictal EEG features consist of background slowing and diffuse slow spike and wave activity that last from several minutes to a near continuous state. The duration of the epileptiform discharges tends to correlate with epilepsy control, with shorter durations occurring in patients with better control of seizures. Spikes, or more commonly sharp waves, are typically 200 milliseconds in duration and are followed by slow waves. Polyspike discharges are seen in those epilepsy variants with prominent myoclonic seizures or during non–rapid eye movement (REM) sleep.
Ictal EEG features have varying electrographic accompaniment with the seizure type.
For more information, see Lennox-Gastaut Syndrome.
Childhood Absence Epilepsy
Childhood absence epilepsy (CAE) presents between ages 3 and 5 years and usually remits by ages 10-12 years. Unlike juvenile absence epilepsy, childhood absence epilepsy is usually not associated with tonic-clonic seizures. Electroencephalography (EEG) shows a normal background for age and 3-Hz generalized spike and wave discharges (see the following image).
The 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 the image below). The duration of the discharges is typically 3-25 seconds.
The discharges are not truly bisynchronous; usually a millisecond difference is noted between the left and right cerebral hemispheres. Eye opening does not alter the discharges. However, the discharges are state dependent; their frequency increases with non–rapid eye movement (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 in childhood absence epilepsy 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 vs childhood absence epilepsy
Absence status epilepticus occurs in about 10% of patients with childhood absence epilepsy. 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 seizure vs atypical absence seizure
Absence should be differentiated from atypical absence seizures, which usually are seen in patients with Lennox-Gastaut syndrome. The EEG in atypical absence seizures shows a less abrupt onset and offset than in typical absence seizures. Furthermore, the EEG background is slow, and the duration of discharges is shorter.
For more information, see Absence Seizures.
Benign Partial Epilepsies
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 [BECTS] 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 benign rolandic epilepsy 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 thus are simple partial seizures. During sleep, patients may have generalized tonic-clonic convulsions.
The electroencephalographic (EEG) features of benign rolandic epilepsy include frequent spike and wave discharges in the centrotemporal region (see the image below). 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 (µV), 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, and they 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 the following image). Non–rapid eye movement (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. 
Benign rolandic epilepsy appears to be a dominantly inherited condition with variable penetrance.  This condition is a syndromic diagnosis, with the EEG forming an important component of the diagnosis. However, epileptiform discharges in the rolandic region do not necessarily mean that the patient has benign rolandic epilepsy.
Benign partial epilepsy of childhood with occipital paroxysms
Gastaut described a partial epilepsy that was analogous to benign rolandic epilepsy, 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 µV) 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. The complexes may be unilateral or bilateral and may occur independently on each side.
As in benign rolandic epilepsy, the occipital spike-wave index is activated prominently with non-REM sleep. Generalized spike and wave discharges may be present in 10% of children. Unlike benign rolandic epilepsy, which remits in most patients by age 16 years, BPEOP may persist in 20% of patients after age 20 years.
For more information, see Benign Childhood Epilepsy.
Juvenile Myoclonic Epilepsy
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. The imaging findings are normal. This condition may account for as many as 10% of all patients with epilepsy. 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, as a correct diagnosis helps guide management. This, in turn, affects prognosis, because the drugs used in this entity differ from those used in most other seizure types. 
The interictal electroencephalogram (EEG) shows a normal background with frequent generalized polyspike and wave discharges that may be anteriorly dominant or diffuse (see the following images. By definition, polyspike and wave discharges have at least 3 spikelike components in them. 
For more information, see Juvenile Myoclonic Epilepsy.