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Often the onset of a clinical seizure is characterized by a sudden change of frequency in the EEG measurement. It is normally within the alpha wave frequency band with slow decrease in frequency (but increase in amplitude) during the seizure period. It may or may not be spiky in shape. Sudden desynchronization of electrical activity is found in electrodecremental seizures. The transition from preictal to ictal state, for a focal epileptic seizure, consists of gradual change from chaotic to ordered waveforms. The amplitude of the spikes does not necessarily represent the severity of the seizure. Rolandic spikes in a child of 4–10 years old for example, are very prominent, however the seizure disorder is usually quite benign or there may not be clinical seizure [47].

In terms of spatial distribution, in childhood the occipital spikes are very common. Rolandic central–midtemporal–parietal spikes are normally benign, whereas frontal spikes or multifocal spikes are more epileptogenic. The morphology of the spikes varies significantly with age. However, the spikes may occur in any level of awareness including wakefulness and deep sleep.

Distinction of seizure from common artefacts is not difficult. Seizure artefacts within an EEG measurement have prominent spiky but repetitive (rhythmical) nature, whereas the majority of other artefacts are transients or noise‐like in shape. For the case of the ECG, the frequency of occurrence of the QRS waveforms is approximately 1 Hz. These waveforms have a certain shape which is very different from that of seizure signals.

The morphology of an epileptic seizure signal slightly changes from one type to another. The seizure may appear in different frequency ranges. For example, a petit mal discharge often has a slow spike around 3 Hz, lasting for approximately 70 ms, and normally has its maximum amplitude around the frontal midline. Conversely, higher frequency spike wave complexes occur for patients over 15 years old. Complexes at 4 and 6 Hz may appear in the frontal region of the brain of epileptic patients. As for the 6 Hz complex (also called benign EEG variants and patterns), patients with anterior 6 Hz spike waves are more likely to have epileptic seizures and those with posterior discharges tend to have neuro‐autonomic disturbances [48]. The experiments do not always result in the same conclusion [47]. It was also found that the occipital 6 Hz spikes can be seen and are often drug related (due to hypoanalgetics or barbiturates) and withdrawal [49].

Among nonepileptics, the discharges may occur in patients with cerebrovascular disorder, syncopal attacks, and psychiatric problems [47]. Fast and needle‐like spike discharges may be seen over the occipital region in most congenitally blind children. These spikes are unrelated to epilepsy and normally disappear in older age patients.

Bursts of 13–16 Hz or 5–7 Hz, as shown in Figure 2.14, (also called 14 and 6 Hz waves) with amplitudes less than 75 μV and arch shapes may be seen over the posterior temporal and the nearby regions of the head during sleep. These waves are positive with respect to the background waves. The 6 and 14 Hz waves may appear independently and be found respectively in younger and older children. These waves may be confined to the regions lying beneath a skull defect. Despite the 6 Hz wave, there are rhythmical theta bursts of wave activities relating to drowsiness around the midtemporal region with a morphology very similar to ictal patterns. In old age patients other similar patterns such as subclinical rhythmic EEG discharges of adults (SREDA) over the 4–7 Hz frequency band around the centroparietal region, and a wide frequency range (2–120 Hz) temporal minor sharp transient and wicket spikes over anterior temporal and midtemporal lobes of the brain may occur. These waves are also nonepileptic but with seizure‐type waveform [47].

The epileptic seizure patterns, called ictal wave patterns, appear during the onset of epilepsy. Although the next chapters of this book focus on analysis of these waveforms from signal processing and machine learning points of view, here a brief explanation of morphology of these waveforms is given. Researchers in signal processing may exploit these concepts in the development of their algorithms. Although these waveform patterns are often highly obscured by the muscle movements, they normally maintain certain key characteristics.

Tonic–clonic seizure (also called grand mal) is the most common type of epileptic seizure. It appears in all electrodes but more towards the frontal electrodes (Figure 2.15). It has a rhythmic but spiky pattern in the EEG and occurs within the frequency range of 6–12 Hz. Petit mal is another interictal paroxysmal seizure pattern which occurs at approximately 3 Hz with a generalized synchronous spike wave complex of prolonged bursts. A temporal lobe seizure (also called a psychomotor seizure or complex partial seizure) is presented by bursts of serrated slow waves with relatively high amplitude of above 60 μV and frequencies of 4–6 Hz. Cortical (focal) seizures have contralateral distribution with rising amplitude and diminishing frequency during the ictal period. The attack is usually initiated by local desynchronization, i.e. very fast and very low voltage spiky activity, which gradually rises in amplitude with diminishing frequency. Myoclonic seizures have concomitant polyspikes seen clearly in the EEG signals. They can have generalized or bilateral spatial distribution more dominant in the frontal region [50]. Tonic seizures occur in patients with Lennox–Gastaut syndrome [51] and have spikes which repeat with frequency approximately 10 Hz. Atonic seizures may appear in the form of a few seconds drop attack or be inhibitory, lasting for a few minutes. They show a few polyspike waves or spike waves with generalized spatial distribution of approximately 10 Hz followed by large slow waves of 1.5–2 Hz [52]. Akinetic seizures are rare and characterized by arrest of all motion, which, however, is not caused by sudden loss of tone as in atonic seizure and the patient is in an absent‐like state. They are rhythmic with frequency of 1–2 Hz. Jackknife seizures also called salaam attacks, are common in children with hypsarrhythmia (infantile spasms, West syndrome) are either in the form of sudden generalized flattening desynchronization or have rapid spike discharges [51].

Figure 2.14 Bursts of 3–7 Hz seizure activity in a set of adult EEG signals.

Figure 2.15 Generalized tonic–clonic (grand mal) seizure. The seizure appears in almost all the electrodes.

There are generally several varieties of recurring or quasi‐recurring discharges, which may or may not be related to epileptic seizure. These abnormalities may be due to psychogenic changes, variation in body metabolism, circulatory insufficiency (which appears often as acute cerebral ischaemia). Of these, the most important ones are: periodic or quasiperiodic discharges related to severe CNS diseases; periodic complexes in subacute sclerosing panencephalitis (SSPE); periodic complexes in herpes simplex encephalitis; syncopal attacks; breath holding attacks; hypoglycaemia and hyperventilation syndrome due to sudden changes in blood chemistry [53]; and periodic discharges in CJD (mad cow disease) [54, 55]. The waveforms for this latter abnormality consist of a sharp wave or a sharp triphasic transient signal of 100–300 ms duration, with a frequency of 0.5–2 Hz. The periodic activity usually shows a maximum over the anterior region except for the Heidenhain form, which has a posterior maximum [47]. Other epileptic waveforms include periodic literalized epileptiform discharges (PLED), periodic discharges in acute cerebral anoxia, and periodic discharges of other etiologies.

Despite the above epileptiform signals there are spikes and other paroxysmal discharges in healthy nonepileptic persons. These discharges may be found in healthy individuals without any other symptoms of diseases. However, they are often signs of certain cerebral dysfunctions that may or may not develop into an abnormality. They may appear during periods of particular mental challenge on individuals, such as soldiers in the war front line, pilots, and prisoners.

Generation of epileptiform brain discharges from deeper brain layers such as the hippocampus during pre‐ictal or interictal periods is an indication of upcoming seizure. These discharges which are spike‐type and have particular morphology which can be seen by inserting electrodes such as multichannel foramen ovale electrodes deep into the hippocampus. More than 90% of these discharges cannot be seen over the scalp due to their attenuation and smearing. A comprehensive overview of epileptic seizure disorders and nonepileptic attacks can be found in many books and publications such as [53, 56]. In this book a chapter is dedicated to the methods for analyzing intracranial and scalp EEGs.