Ictal and interictal EEG

Question 1

In patients with epilepsy, an initial routine EEG detects epileptiform discharges (IEDs) what percent of the time?

After four routine EEGs?

Answer 1

In patients with epilepsy, an initial routine EEG detects epileptiform discharges in about 50% of times. After four routine EEGs, this yield increases to over 90%.

Question 2

Which type of epilepsy is commonly associated with IEDs? Which is least associated?

Answer 2

Temporal lobe epilepsy tends to be commonly associated with IEDs while frontal lobe epilepsy is least associated with IEDs on scalp EEG.

Question 3

Spikes vs sharps

Answer 3

Focal spikes on scalp EEG are sharply contoured waveforms with durations between 20 and 70ms. Sharp waves, on the other hand, are similar morphologically except that their duration is longer than 70ms.

Question 4

What is the physiological basis of a discharge?

Answer 4

The physiologic basis of a focal epileptiform discharge is the paroxysmal depolarizing shift (PDS).

Question 5

Main features of focal spikes/sharps

Answer 5

(1) They are distinct from the background. That means that they are not part of a preceding rhythm, like wicket spikes. Rather, they have an amplitude large enough to stand out from the background, and appear to abruptly arise from a morphologically different background.

(2) They are often followed by a slow wave.

(3) They tend to disrupt the background. The sharply contoured component is often followed by an irregular, slow EEG that is different from the preceding EEG. Thus, even if the sharply contoured component is hidden, the reader may still be able to tell that a disruptive event has just taken place.

(4) On scalp EEG, IEDs are often surface negative. Exceptions may occur in individuals who have undergone craniotomy; in whom, spikes may occasionally be positive (Fig.5.1). Also, infants with intraventricular hemorrhage or periventricular disease may have positive spikes in the central region, the significance of which is encephalopathy rather than propensity for epilepsy

(5) The slopes of The IED are often asymmetrical. The initial, negative component is typically the steepest, followed by a slower positive component with larger amplitude.

(6) IEDs have a field that often extends over a few electrodes. If a relatively high-voltage sharply contoured waveform is seen only on one contact, but not on neighboring ones, it is often more suggestive of an artifact.

Question 6

Clinical significance of IEDs

Answer 6

The clinical significance of IEDs of different locations is not the same [4]. For instance, seizures occur in 90% of children with anterior temporal spikes, but in only 40% of those with rolandic spikes or occipital spikes. Occipital spikes can be seen in migraine [5] or in children with congenital blindness [6].

Question 7

IEDs in BECTs

Answer 7

In Benign Epilepsy with Centrotemporal Spikes (BECTS), or Benign Rolandic Epilepsy, spikes are equally negative over the central and temporal derivations with the positive end of the dipole appearing typically in the frontal regions.

Question 8

Multifocal IEDs

Answer 8

Multifocal IEDs referred to spikes or sharp waves are seen independently on both sides. These are often associated with background slowing and the vast majority of patients have seizures, with generalized seizures being very common. In addition, seizure frequency is often very high and medical intractability common. Frequent comorbidities of individuals with multifocal IEDs include cognitive and motor deficits.

Question 9

PLEDs

Answer 9

IEDs that occur on one side on the brain at regular intervals of 0.3–4s (Fig.5.2).

They are commonly seen in acute brain injury such as herpes encephalitis and stroke, among others.

They can also occur for prolonged periods of time after focal status epilepticus.

In addition, they can be seen in toxic encephalopathies, including aminophylline or alcohol intoxication.

PLEDs can occur in individuals with marked encephalopathy as well as in ones who at their baseline mental status. Half of all patients with PLEDs will have seizures.

When PLEDs are associated with low amplitude, high-frequency rhythmic discharges, often appearing superimposed superimposed on or after the sharply contoured waveform, they are termed PLEDs plus and have increased significance for predicting seizures.

Question 10

BiPLEDs

Answer 10

BiPLEDs are PLEDs that occur independently on either side of the brain. They occur in individuals with severe brain disease and are associated with a poor prognosis. Multifocal PLEDS refer to 3 or more foci of PLEDs involving both sides of the brain. They are associated with multifocal lesions or severe diffuse brain disease. The majority of patients with multifocal PLEDS have seizures.

Question 11

TIRDA

Answer 11

TIRDA refers to intermittent rhythmic activity of 1–3Hz frequency occurring over the anterior-to-mid temporal derivations on one side. The duration of the train varies, often lasting for approximately 5s. The presence of TIRDA is as significant for temporal lobe epilepsy as temporal IEDs are. Indeed, concomitant depth and scalp electrode recordings have shown that TIRDA correlates with intracranially recorded mesial temporal spikes.

Question 12

Generalized IEDs - Absence epilepsy

Answer 12

The 3-Hz spike-and-wave discharges are the EEG signature of absence epilepsy, often presenting in bursts lasting 1–3s, and typically activated by hyperventilation. They are often bilaterally synchronous and have a generalized field, typically appearing maximum over the frontal and midline derivations. However, variations of the field of generalized IEDs are not uncommon.

Occasionally, some asynchrony or asymmetry may be noted, but often such asymmetries (referred to as fragments of generalized epileptiform discharges) shift in the same record. Phase reversals of the spike components may be seen over F3 and F4 contacts.

Although brief runs of 3-Hz spike-and-wave discharges may appear asymptomatic, detail assessments revealed that even brief runs may interfere with continuous motor tasks.

Question 13

Generalized IEDs - JME

Answer 13

Spike or polyspike-and-slow wave complexes often present in runs of faster frequencies, typically 4–6Hz, and also occur singly

Question 14

Atypical generalized IEDs

Answer 14

Atypical generalized spikes may occur as part of other generalized epilepsies. These are medium to high voltage without a prominent after going slow-wave component and may occur singly. They are best seen with a referential ear montage.

Question 15

Generalized IEDs - LGS

Answer 15

Slow spike-and-wave complexes present with a frequency that is slower than the 3-Hz pattern of absence epilepsy. They are a typical electrographic feature of Lennox-Gastaut syndrome. Their typical frequency is around 1.0–2.5Hz, with wider (less spiky) sharp component than in absence epilepsy. Sleep activates trains of such slow complexes in the extent that they may appear continuous as in electrical status epilepticus during sleep (ESES).

Question 16

EEG - LGS

Answer 16

The EEG in LGS is marked generalized, frontal maximum slow spike-and-slow waves (1.5–2.5Hz). Tonic, as well as atonic, seizures are associated with a low-voltage fast pattern. As mentioned previously, the baseline EEG is slow and disorganized.

Question 17

Photoepileptiform discharges/photoparoxysmal response

Answer 17

Photoepileptiform discharges are IEDs that are elicited by photic stimulation. The elicited discharges can be generalized (most common), bilateral posterior, or unilateral predominant (least common). They may occur within the photic stimulation train or outlast it.

When they outlast photic stimulation and are self-sustaining, they may have a higher association with epilepsy, although this is debatable. Up to three out of four patients with photo epileptiform discharges have seizure disorders. Individuals with bioccipital discharges have the least association with epilepsy.

These discharges are often part of primary generalized epilepsy and rarely focal epilepsy.

Interestingly, occipital spike-and-slow wave discharges may be scotosensitive, i.e., elicited by darkness rather than light and may occur as part of benign epilepsies as well as such mitochondrial disorders as myoclonic epilepsy with ragged red fibers (MERRF)

Question 18

Scalp positive seizures in patients with seizures without alteration of consciousness

Answer 18

Only 22% of all focal seizures that are not associated with alteration of consciousness (formerly named simple partial seizures) have an EEG correlate. In the subset of such seizures where a motor component is present, the electrographic yield increases to 33% versus only 15% of those that have no motor manifestations

Vs dyscognitive seizures, which are almost always associated with EEG changes Rare exceptions may apply to seizures originating from the parietal or frontal lobe.

Question 19

Fast vs slow ictal patterns

Answer 19

In general, when the ictal discharge consists of fast frequencies, it indicates proximity of the recording electrode to the seizure focus. On the other hand, slow discharges, for example in the delta range, typically represent propagated activity from distant sites.

Question 20

Mesiotemporal vs temporal neocortex seizures on EEG

Answer 20

In temporal lobe epilepsy, simultaneous scalp and depth electrode recordings show that no scalp EEG changes are seen when seizure discharges are limited to the hippocampus. As the seizure propagates outside of the mesial temporal structures into neocortical regions, it is then detected by scalp EEG.

A 5–9-Hz temporal ictal discharge is highly associated with seizures of hippocampal onset, while neocortical seizures often are associated with polymorphic, 2–5-Hz.

On the other hand, seizures originating at the temporal neocortex are often associated with irregular, polymorphic, 2–5-Hz ictal discharge

Question 21

Sphenoid electrodes

Answer 21

The main use of sphenoidal electrodes is not only to increase the overall yield of detecting epileptiform EEG abnormalities, but also to further localize interictal and ictal discharges. For example if a discharge is of higher voltage over the sphenoidal electrodes than mid-temporal electrodes (T3 or T4), it signifies more inferior and mesial origin, whereas the opposite scenario suggests a lateral neocortical origin.

Question 22

Extratemporal seizures

Answer 22

In extratemporal lobe seizures, a fast-frequency ictal discharge may be more common than in temporal lobe epilepsy, but, in general, extratemporal seizures are not associated with a clear ictal discharge as often as seizures of temporal origin

For example, only half of frontal lobe seizures have a localizing EEG pattern. Similarly, parietal lobe seizures often have no clearly localizing EEG. Occipital lobe seizures commonly show an ictal discharge over the occipital region. Both occipital and parietal lobe seizures tend to propagate to the temporal lobes, where seizures become semiologically and electrographically indistinguishable from temporal lobe seizures. Occipital lobe seizures can also propagate to frontal and insular regions.

Question 23

A new definition of generalized epilepsies

Answer 23

Seizures originating originating at some point within, and rapidly engaging, bilaterally distributed networks. These networks can include cortical and subcortical structures, but do not necessarily involve the entire cortex,” and “they can be asymmetric”

Question 24

Idiopathic vs symptomatic generalized epilepsies background

Answer 24

In idiopathic generalized epilepsies, the baseline EEG is within normal limits, and the interictal epileptiform and ictal discharges are typically bilateral and maximal over the frontal head regions. In contrast, the EEG background is slow in symptomatic generalized epilepsy.

Question 25

Ictal - Absence seizures

Answer 25

In absence seizures, rhythmic spike-and-slow-wave runs that last longer than 3s often have
clinical correlates and may be termed “ictal” as opposed to shorter runs, often termed: “interictal”. However, due to the difficulty of assessing subtle, brief alteration of awareness, and the distinction between interictal and ictal is not straightforward. [Otherwise electro graphic seizure = A) epileptiform discharges averaging >2.5 Hz for ≥10s (>25 discharges in 10s), OR B) Any pattern with definite evolution and lasting ≥ 10s]

If absence seizures are associated with automatisms, they are called complex absence seizures.

The classic ictal pattern of childhood absence epilepsy is that of 3/s spike-and-slow-wave complexes, usually starting at 3.5Hz and ending at 2.5Hz. Occasionally polyspikes may be seen.

Question 26

Ictal - JME

Answer 26

In juvenile myoclonic epilepsy (JME), bursts of diffuse, bi-frontal maximum polyspike-and-slow-wave discharges are seen interictally or with myoclonic jerks. These are typically faster than in absence epilepsies, commonly around 5–6Hz. One third of individuals with JME show a photoparoxysmal response. An ictal discharge of 10–16Hz frequency can be seen in association with some myoclonic seizures. When absence seizures occur in individuals with JME, they manifest electrographically as 3-Hz spike-and-slow waves, like in other absence epilepsies.

Question 27

Ictal - Tonic (idiopathic)

Answer 27

Tonic seizures are often associated with voltage attenuation with superimposed high-frequency activity of 20–40Hz.

Question 28

Ictal - Tonic-clonic (idiopathic)

Answer 28

Tonic seizures are often associated with voltage attenuation with superimposed high-frequency activity of 20–40Hz. In tonic–clonic seizures, this pattern evolves to patterns with higher amplitude with slower frequencies, followed by yet a slower pattern with intermittent slow waves.

What marks the switch from the tonic to the clonic phase of the seizure is when the slow-rhythm frequency reaches 4Hz. Clonic jerks correspond to bursts of multiple spikes separated by a slow wave that corresponds to the brief muscle relaxation. Longer postictal phases can be expected after longer seizures and in younger individuals.

Question 29

Ictal - Tonic (symptomatic)

Answer 29

Tonic seizures are typically associated with an electrodecremental pattern or paroxysmal fast activity of 10–25Hz frequency. This is often followed within 5s by sharp-and-slow wave complexes.

This is often followed within 5s by sharp-and-slow wave complexes. The vast majority of such discharges are bilateral and frontal maximum.