Introduction

EEG is an important tool in pediatric neurology. EEG abnormalities occur in all categories of neurological disorders. These abnormalities are useful for lateralizing and localizing a neurological process, but are nonspecific and do not make a specific diagnosis. Epileptiform activity, described as an EEG waveform recorded in a proportion of those suffering from an epileptic disorder, may occur in conditions other than epilepsy. Therefore, the presence of overt epileptiform features suggests, but is not absolutely diagnostic of epilepsy. For example, in a sample of children referred for EEG who had spikes, Kellaway reported a location-dependent incidence of epilepsy, ranging from 38 to 91% (central the lowest, temporal the highest).

EEG interpretation starts with an assessment of the EEG background, which refers to the entire EEG. This starts with whether the background is continuous or symmetric. It is important to know the child's age, and, if a newborn, the conceptual age rather than the legal age, because background continuity decreases with a decreasing conceptual age. The most premature infants have a very discontinuous background, and may even have prolonged periods of electrocerebral inactivity, called trace discontinu. Even in infants and older children, specific waveforms, such as the posterior rhythm, have a developmental appearance. Next, evaluate the overall symmetry from side to side, and then look for the presence of generalized or focal abnormalities, such as slowing or epileptiform activity. If an actual seizure occurs during the EEG, it is called an ictal EEG. The majority of EEGs do not record actual seizures; these are called interictal EEGs.

The EEG is especially useful in the evaluation and management of seizures, epilepsy, and the epilepsy syndromes. A recent practice parameter on evaluating a first nonfebrile seizure in children recommended routine EEG. EEG findings may help to differentiate an epileptic

From: The Clinical Neurophysiology Primer Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ

from a nonepileptic event. Many paroxysmal disorders in childhood mimic epilepsy, but do not have an epileptic mechanism. These are called nonepileptic paroxysmal events. Although nonepileptic paroxysmal events typically have a normal interictal EEG, the EEG may change at the time of the event. With syncope, slowing and even electrocerebral inactivity may occur at the time of the event. It is especially important to include cardiac disorders in the differential diagnosis, because arrhythmias, such as the prolonged QT syndrome, may present with a "seizure." However, a normal EEG does not exclude the diagnosis of epilepsy, and an abnormal EEG does not, in itself, establish a diagnosis of epilepsy. In a longitudinal study of 3726 normal children, aged 6 to 13 yr, Cavazutti et al. reported that 3.5% had epileptiform patterns. The EEG is "diagnostic" of a seizure only when actual clinical manifestations occur at the time of the electrographic discharge; if not, this finding is only very suggestive of epilepsy.

With a definite diagnosis of epilepsy, EEG helps to classify seizure type or the associated epileptic syndrome. For example, with a generalized tonic-clonic seizure (GTCS), a focal interictal EEG suggests a focal onset. EEG monitoring is also useful in the presurgical evaluation of refractory epilepsy, because identifying the location of the seizure focus is a very important part of the presurgical evaluation.

There are epileptic disorders associated with EEG epileptiform activity, such as Landau-Kleffner syndrome (LKS) or continuous spike waves of sleep (CSWS), that may not always be associated with overt clinical seizures but have cognitive dysfunction with cognitive or language regression. These specific disorders typically have sleep-activated epileptiform activity, with regression related to the duration of this EEG activity. The entity transient cognitive impairment refers to altered cognitive function associated with epileptiform activity, although it may take specific computerized testing to detect. EEG is used in both the diagnosis and treatment of these disorders, because the treatment goal is both clinical and electrographic improvement.

EEG is also used to exclude epilepsy in disorders that mimic epilepsy, such as the various childhood migraine syndromes, attention deficit disorders, or other psychiatric disorders. The incidence of paroxysmal abnormalities in headaches and migraines varies from 9 to 30%. In a recent study by Richer et al. in children with ADHD, epileptiform activity occurred in 6.1% of 347 EEGs. However, only 3 of 21 children with epileptiform activity developed a seizure disorder. In the era before modern neuroimaging, EEG was used in the diagnostic evaluation for brain tumors. However, practice parameters for headache no longer recommend routine EEG. Certain EEG findings may also be associated with specific metabolic or genetic disorders. For example, a "comb-like" central rhythm has been observed in neonates with maple-syrup-urine disease, and bifrontal slow spike-and-wave discharges have been reported in the ring 20 chromosome syndrome.

The International League Against Epilepsy system for seizure classification, the International Classification of Epileptic Seizures (ICES), starts with seizure type, whether focal or generalized, and then classifies the epileptic syndrome. There is also an unclassified category that includes neonatal seizures. An epileptic syndrome refers to a complex of signs and symptoms that define a unique epilepsy (Table 1), and is useful for choosing antiepileptic drug (AED) treatment and for predicting prognosis. Etiology is also included in epilepsy classification, whether symptomatic, cryptogenic, or idiopathic, and either generalized or focal.

Epileptic syndromes are divided into benign or malignant, depending on the expected outcome. In general, the "idiopathic" epilepsies are analogous to the benign epileptic syndromes, and the "symptomatic" epilepsies are analogous to the malignant epileptic syndromes.

Table 1

Features Used to Classify Epileptic Syndromes

Seizure Type(s) Partial onset

Simple, complex, secondarily generalized Generalized onset

Absence, tonic, tonic-clonic, atonic, myoclonic Specific

Spasms, gelastic, others Cluster of Signs and Symptoms Customarily Occurring Together Age of onset Etiology Anatomy

Precipitating factors

Severity: prognosis, benign or malignant EEG, both ictal and interictal Duration of epilepsy Associated clinical features Chronicity

Diurnal and circadian cycling

Benign syndromes are those in which seizures are successfully treated with AEDs, require no other specific treatment, or may even remit without sequelae. Malignant syndromes generally are resistant to treatment and have a poor prognosis. However, not every "benign" epileptic syndrome is associated with a normal outcome, and not every "malignant" epileptic syndrome has a poor outcome. For example, a small number of children with benign familial neonatal seizures will have developmental problems and persistent seizures, some children with absence epilepsy can have learning difficulties, and photosensitive epilepsy can occasionally be severe. The term catastrophic epilepsy is used for the malignant epileptic syndromes.

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