Interictal Epileptiform EEG

Interictal Epileptiform EEG

Interictal Epileptiform EEG – outpatient or EMU

Epilepsy syndromes are classified as either Focal or Generalized epilepsies. They are differentiated by whether the seizures arise from a focal region of the brain or from activation of a generalized network. Epilepsy syndromes are also differentiated by their clinical characteristics, EEG patterns, prognosis and response to treatment. The EEG is used to differentiate focal from generalized epilepsy syndromes by the epileptiform discharges that are localized to a focal region versus a generalized pattern.

Generalized Epilepsy Syndromes

The generalized epilepsy syndromes are further subdivided into Idiopathic/Primary and Secondary Generalized Epilepsies. Most generalized epilepsies share interictal findings of diffuse paroxysmal EEG changes and have similar appearing clinical manifestations and ictal EEG patterns. The EEG abnormalities have a generalized distribution; they are seen simultaneously at the surface electrodes of both hemispheres and have a diffuse spatial distribution.

It is thought primary/idiopathic/genetic generalized epilepsy syndromes are related to dysfunction in the circuitry between the thalamus and the cortex. Occasionally, patients show an EEG trait consistent with a primary generalized epilepsy syndrome yet may have never had a seizure in their life time. With detailed questioning, a family history of epilepsy may be uncovered. In general, these patients are of normal intellect and have normal neuroimaging. Most of these patients are easily treated when appropriate medications are used.

Secondary generalized epilepsy syndromes are associated with underlying dysfunction that is less clearly characterized since these patients tend to have diffuse or multifocal neurological dysfunction. They are typically developmentally delayed and have other neurologic disorders. They frequently have a variety of seizure types that can be very difficult to treat medically. Although they have EEG features similar to other generalized epilepsy syndromes, they also have other generalized or multifocal abnormalities indicating widespread abnormalities within the cortex.

Genetic/Idiopathic/Primary Generalized Epilepsy Syndromes

Of the primary generalized epilepsy syndromes Childhood Absence Epilepsy and Juvenile Myoclonic Epilepsy are the most commonly encountered in an EEG lab. The EEG patterns are similar but differ in subtle ways.

Classically, Childhood Absence Epilepsy is associated with blank staring and unresponsiveness lasting seconds that can be induced by hyperventilation. The seizures begin at 5 years of age and tend to remit by 12 years of age. The EEG shows characteristic bursts of spikes followed by dome-shaped slow waves that occur in a rhythmic pattern at a frequency of 3 per second. The onset is synchronous in both hemispheres and tends to be predominant in both frontocentral regions, although in young children they may have a more posterior predominance. The EEG finding is a diffuse frontocentrally predominant 3 Hz Spike-Wave Complex. During hyperventilation, an activating procedure routinely performed during an outpatient EEG, absence seizures can often be provoked.

With Juvenile Myoclonic Epilepsy, the clinical onset is later in adolescence or early adulthood. This epilepsy syndrome is associated with early morning jerking and these patients can be photosensitive. The EEG also shows characteristic bursts of spike and slow waves that occur in a faster frequency of 4-5 per second compared to classic 3 Hz spike wave complexes. The spike components can also appear as a run of polyspikes. The onset is synchronous in both hemispheres and tends to be predominant in both frontocentral regions. The EEG finding is often described as diffuse frontocentrally predominant fast spike and polyspike and wave activity.

Photic stimulation, another routine induction used in the outpatient setting to increase the yield of an interictal EEG, can often induce discharges at particular flash frequencies. This phenomenon is called a photoparoxysmal response. Typically a technologist will repeat the photic stimulation to ensure that the discharges did not occur by chance but were a reproducible finding. It can then be concluded that this patient is photosensitive to a particular flash frequency range.

Developmental Epileptic Enephalopathy are typically secondary to some other neurological dysfunction of either known or unknown etiology. The patient is commonly clinically developmentally delayed and may have numerous seizure types. Often they are classified as West Syndrome or Lenox Gastaut Syndrome depending on the age of onset. The characteristic EEG pattern is called hypsarrhythmia This pattern is a chaotic one without any normal organization and is of high voltage with intermixed multifocal sharp waves and slow spike and wave activity. The Slow Spike and Wave is on the order of 1-2 discharges per second and can occur in prolonged runs lasting more than 20 seconds during which the clinical correlation is a protracted atypical absence seizure.

Focal Epilepsy Syndromes

Localization-related epilepsies are a mixed bag of epilepsy syndromes that can be idiopathic or symptomatic in etiology. The clinical characteristics or semiology of the seizures is dependent on their region of onset or the symptomatogenic zone. The interictal EEG may provide evidence about the location of the epileptogenic zone responsible for the epilepsy syndrome. The epileptogenic zone is the region that demonstrates cortical irritability. This irritability is represented by spikes or sharp waves. Formally the definition of a spike is a paroxysmal discharge that stands out from the background and in amplitude and frequency. They are less than 70 msec and typically followed by a slow wave. Neurophysiologically, this discharge represents a brief synchronous depolarization in a population of cortical neurons followed by a zone of inhibition with hyperpolarization. Otherwise, the informal definition is that a spike looks like something that would hurt to sit on! A sharp wave is equally uncomfortable to sit on and is 70-120 msec in duration. Localization of the discharge determines the region of epileptogenic potential.

Self-Limited Idiopathic Focal Epilepsy Syndromes

RadialDipoleHorizontal (tangential)DipoleIn the outpatient setting, the most common discharges seen are Benign Focal Epileptiform Discharges of Childhood (BFEDC). This type of discharge is often seen in the centrotemporal region near the Rolandic sulcus where the superior temporal bank of the temporal lobe is relatively perpendicular to the scalp surface. Classically there will be a horizontal dipole because of this localization. A negative charge is recorded from the central or temporal region with a simultaneous positive charge in the frontal region. Due to the orientation of the discharge, the EEG will be able to pick up both ends of the dipole. This is in contrast to typical discharges arising from the convexity when only the negativity is recorded and the positivity is buried deep within the brain (radially oriented dipole). At times, discharges of similar morphology can also be recorded from the occipital region. Only 40-60% of patients with BFEDC have or will have seizures. The the remainder have an asymptomatic EEG trait. When clinically manifested, BFEDCs are EEG finding of Benign Rolandic Epilepsy (BRE). The classic history is a child between the ages of 3-13 years who has a somatosensory aura around the face or hand with or without focal or generalized motor manifestations afterward. The seizures of BRE resolve spontaneously by adolescence, thus its “benign” designation. If the discharges are located in the occipital region, the clinical manifestations include visual alteration associated with headache and/or nausea or vomiting.

Benign Focal Epileptiform Discharges of Childhood (BFEDC) are most commonly seen in the centrotemporal region either unilaterally or bilaterally. In this average referential montage, a horizontal discharge dipole can be appreciated, with maximal surface positivity on the right at C4 and surface positive components at FP2 and F4. There is an independent discharging focus on the left at C3. BFEDCs are provoked by sleep, and may dissipate completely during wakefulness.

Focal Symptomatic Epilepsy Syndromes

The interictal findings can manifest as slowing and sharp waves arising from a focal region of the brain. The most common symptomatic focal epilepsy syndrome involves the temporal lobe, specifically the mesial part of the temporal lobe, the hippocampus. Despite a clear clinical history the interictal EEG may remain normal. The clinical manifestations of temporal lobe epilepsy typically include an aura that may include a sensation in the abdomen, alteration in taste or smell, or a sense of fear. Loss of awareness typically follows with ipsilateral automatisms and contralateral dystonia. Should these findings or other focal findings be seen on an interictal EEG, further workup including imaging and treatment with medications most likely should be pursued.

The same focal epileptiform discharges in the right temporal lobe are shown in both of these examples, in a bipolar montage in the upper panel and an average reference montage in the lower panel. The discharges are maximal at F8 and T2, clearly set apart from the background, and are associated with after-going slow waves.