Localization-Related Epilepsies on EEG 

The International League Against Epilepsy (ILAE) classifies the epilepsies along 2 dichotomies^[1] : partial (ie, localization-related) versus generalized and idiopathic versus cryptogenic or symptomatic. This double dichotomy conveniently allows presentation of the epilepsy classification in a simple table, as shown in Table 1, below.

Table. Classification of the Epilepsies (Open Table in a new window)

The term idiopathic is often misunderstood in this setting and requires clarification. Although “idiopathic” usually means "of unknown cause," idiopathic epilepsies are not truly of unknown cause (This confusing terminology was raised in the 1998 ILAE classification and is discussed in the 2010 ILAE report.^[3] ). Idiopathic epilepsies are determined genetically and have no apparent structural cause, with seizures as the only manifestation of the condition. Findings of the neurologic examination and imaging studies are normal, and electroencephalography (EEG) is normal other than the epileptiform abnormalities. In some syndromes, the genetic substrate has even been identified.

Most idiopathic epilepsies are generalized, but a few are focal. Nonidiopathic epilepsies are by definition not genetic, although some may be associated with a minor genetic predisposition; they are the result of a brain insult or lesion. If the damage is focal, it results in a localization-related epilepsy; if it is diffuse, it results in a generalized epilepsy. The difference between symptomatic and cryptogenic is subtle: symptomatic means that the etiology is known, whereas cryptogenic means that an underlying etiology is apparent but cannot be documented objectively. Thus, the boundary between the 2 conditions is largely dependent on the capabilities of our diagnostic and imaging techniques.

This article briefly discusses EEG findings in the localization-related (also known as focal or partial) epilepsies.

For patient education information, see the Procedures Center and Brain and Nervous System Center, as well as Electroencephalography (EEG) and Epilepsy.

For more information, see the following:

• Epilepsy and Seizures
• EEG in Status Epilepticus
• Epileptiform Normal Variants on EEG
• EEG in Common Epilepsy Syndromes
• Generalized Epilepsies on EEG
• EEG Video Monitoring

Spikes and sharp waves as well as electroencephalographic seizures are briefly reviewed in this section.

Spikes and sharp waves

Spikes and sharp waves are sharp transients that have a strong association with epilepsy. The 2 are distinguished only by their duration (spikes, < 70 ms; sharp waves, 70-200 ms), but they have no differences in terms of clinical significance. Several characteristics distinguish these from benign epileptiform variants (see Focal (Nonepileptic) Abnormalities on EEG), including high amplitudes, which make them "stand out" from ongoing background activity, and aftergoing slow waves, which give the appearance of their "disrupting" background activity. See the following images.

Polyspikes are rarely focal, although focal spikes can at times have a multiphasic polyspikelike morphology.

EEG seizures

Electroencephalographic seizures are discharges characterized by rhythmicity and evolution ("build-up") in frequency and amplitude (see the image below). The discharge can consist of rhythmic theta or delta activity, or repetitive spikes or sharp waves, but the most characteristic features of electrographic focal seizures are rhythmicity and evolution.

Benign focal epilepsy of childhood is the main localization-related epilepsy that is idiopathic. Two varieties have been well described and are in the 1989 International League Against Epilepsy (ILAE) classification: centrotemporal and occipital. A third type has been described: autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE).^{[4, 5, 6]}

BECTS

Benign childhood epilepsy with centrotemporal spikes (BECTS) is by far the more common. The age of onset is between 4 and 12 years (peak age, 8-9 y). The seizures are simple partial with motor symptoms involving the face, and they tend to occur during sleep or on awakening. Although these focal seizures are the most characteristic seizure types in BECTS, they can be quite subtle and are missed easily, so that the most common mode of presentation is a (secondary) generalized tonic-clonic seizure. As with all idiopathic epilepsy syndromes, neurologic examination findings are normal.

The electroencephalographic (EEG) findings are characteristic, with stereotyped centrotemporal sharp waves that have a characteristic morphology. They are activated markedly by non–rapid eye movement (NREM) sleep, often occur in repetitive bursts, and can be bilateral and independent. Notably, the interictal sharp waves of BECTS often occur in asymptomatic children. In fact, only a minority of children with these discharges may have seizures.

Childhood epilepsy with occipital paroxysms

Childhood epilepsy with occipital paroxysms is less common than BECTS and less consistently benign. It shares all the characteristics of an idiopathic syndrome (ie, normal findings on examination, intelligence quotient [IQ] testing, and neuroimaging studies). The age of onset is 4-8 years. These seizures are rare and primarily nocturnal and often involve visual symptoms. The sharp waves have a maximum occipital negativity, often occur in long bursts of spike-wave complexes, and are activated markedly by eye closure.

ADNFLE

ADNFLE is a genetic localization-related epilepsy. Several mutations of the neuronal nicotinic acetylcholine receptor alpha4 subunit have been identified in association with this epilepsy. It has the expected features of idiopathic (ie, genetic) epilepsies, including onset early in life and normal imaging findings. The seizures are nocturnal and occur in clusters, mimicking parasomnias. They are mostly brief tonic seizures and rare (secondarily) generalized tonic-clonic convulsions, often preceded by a nonspecific aura.

Interictal EEGs may show epileptiform discharges with a frontal predominance, often seen only in sleep. Ictal EEG does not always show definite ictal discharges. Thus, the electroclinical features of ADNFLE are not different from those of symptomatic or cryptogenic frontal lobe epilepsy. Because the genetic findings are variable (ie, locus heterogeneity), its definite diagnosis is largely one of exclusion.

Cryptogenic focal epilepsy is by far the most common type of adult-onset epilepsy. By definition, the seizures arise from a localized region of the brain. If the cause is found, they are said to be symptomatic. If imaging study findings are normal, the cause remains presumptive, and they are said to be cryptogenic. As stated previously, the boundary between symptomatic and cryptogenic is largely dependent on our diagnostic and imaging techniques, and etiologies such as low-grade tumors, hippocampal sclerosis, and subtle cortical dysplasias are identified more and more often, owing to advances in neuroimaging.

Clinically, the seizures may be simple partial or complex partial, with or without secondary generalization. Interictal electroencephalography (EEG) shows focal spikes or sharp waves, and ictal EEG shows a focal or regional discharge at onset. The main clinical entities are mesial temporal lobe epilepsy, neocortical focal epilepsies, and hemispheric syndromes.