Focal epilepsies and focal disorders

Abstract

Electroencephalographic (EEG) investigations are crucial in the diagnosis and management of patients with focal epilepsies. EEG may reveal different interictal epileptiform discharges (IED: spikes, sharp waves). The EEG visibility of spike depends on the surface area of cortex involved (>10cm 2) and the brain localisation of cortical generators. Regions generating IED (defining the "irritative zone") are not necessarily equivalent to the seizure onset zone. Focal seizures are dynamic processes originating from one or several brain regions (generating fast oscillations and called "epileptogenic zone") before to spread to other structures (generating lower frequency oscillations and called "propagation zone"). Several factors limit the expression of seizures on scalp EEG, such as area involved, degree of synchronization and depth of the cortical generators. Different scalp EEG seizure onset patterns may be observed: fast discharge, background flattening, rhythmic spikes, sinusoidal discharge, or sharp activity but to a large extent EEG changes are linked to seizure propagation. Finally, in the context of presurgical evaluation, the combination of interictal and ictal EEG features is crucial to provide optimal hypothesis about the epileptogenic zone.

Interictal pattern on scalp electroencephalogram predicts excellent surgical outcome of epilepsy caused by focal cortical dysplasia

“…Due to low numbers, the simultaneous presence of three to five characteristic interictal scalp EEG patterns was merged. Ictal EEG onset patterns were classified into the following three categories: low‐voltage fast activity, spikes or sharp waves, and slow waves 17 …”

“…Interictal and ictal EEG patterns were only considered present when consensus was reached between the two independent reviewers. Scalp EEG data analysis was performed to detect interictal EEG patterns previously reported to be associated with FCD and the ictal EEG onset patterns 16,17 . Epitashvili et al classified the characteristic interictal scalp EEG patterns of FCD into two broad categories as follows: repetitive discharges and polyspike discharges 16 .…”

Type III Sturge Weber Syndrome, An Uncommon Cause of Status Epilepticus

“…11 Seizures are common in SWS, considering that LCM acts like a structural epileptogenic focus, however, the detection in the EEG may only be made when it affects a large brain area. 14 EEG findings correlate with the LCM size, because at the beginning there is a slight vascular dilation and later dysgenesis of the superficial venous system with no changes in the EEG, but later there will be shunts with the deep system, leading to venous dilation, flow stasis, and chronic ischemia, presented in the imaging as linear cortical calcificacitions, known as tram-track sign, with features in the EEG including from focal asymmetry to epileptiform pattern. 15 Imaging for characterization of SWS often starts with tomography, useful in this case because it helps to show calcifications.…”

Improving surgical outcome with electric source imaging and high field magnetic resonance imaging

“…IEDs have been the most studied biomarker of epilepsy with ESI. They compound the irritative zone (IZ), whose evaluation is an important part of the presurgical workup [120]. Typically, similar IEDs are averaged to improve the signal-to-noise ratio (SNR) [121].…”