As the name implies, PLEDs (also known as Lateralized Periodic Discharges or LPDs) are IEDs that occur on one side on the brain at regular intervals of 0.3–4 s (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 on or after the sharply contoured waveform, they are termed PLEDs plus and have increased significance for predicting seizures.

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.

TIRDA refers to intermittent rhythmic activity of 1–3 Hz frequency occurring over the anterior-to-mid temporal derivations on one side. The duration of the train varies, often lasting for approximately 5 s. 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.

The 3-Hz spike-and-wave discharges are the EEG signature of absence epilepsy, often presenting in bursts lasting 1–3 s, 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 [7].

In other idiopathic generalized epilepsy syndromes, such as JME, the spike or polyspike-and-slow wave complexes often present in runs of faster frequencies, typically 4–6 Hz, and also occur singly (Fig. 5.3). 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.

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 1.0–2.5 Hz, 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).

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 drain 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) [8].

Recording the ictal EEG is an essential part of the surgical evaluation of patients with intractable epilepsy [9]. In such patients, it is important that the recorded seizures are semiologically typical of the patient’s habitual episodes before surgical decisions are made. In addition, recording the patient’s habitual episodes is essential for characterization of paroxysmal events in individuals with questionable nonepileptic episodes.

Ictal EEG represents a clear deviation from the baseline of a pattern that shows some evolution. By evolution, it is meant that the pattern changes in terms of its frequency, amplitude, field, or morphology as the seizure occurs. This applies most typically to focal seizures, especially temporal lobe seizures, but may start with a semi-rhythmic delta activity over one temporal region and soon evolves into a theta range spike discharge over the same distribution that is typical of mesial temporal generators. However, even in generalized epilepsies, such as absence epilepsy, an evolution pattern can be noted whereby the initial frequency of the spike-and-slow-wave discharge is higher than 3, often 3.5 Hz, whereas toward the end of the burst, the frequency slows down to 2.5 Hz.

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 [10]. On the other hand, seizures that are associated with alteration of awareness (dyscognitive seizures, formerly termed complex partial seizures) are almost always associated with EEG changes. Rare exceptions may apply to seizures originating from the parietal or frontal lobe [11]. When ictal discharges are present in seizures that do not cause alteration of awareness, they are morphologically indistinguishable from focal ictal discharge in dyscognitive seizures, manifesting as focal repetitive spike discharge, low-voltage fast activity, or focal rhythmic slowing, among others. 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.