Epilepsy is a condition where there are at least two seizures (unprovoked) that occur in more than 24 hours apart. The term epilepsy syndrome is to describe a condition that incorporates clinical features, EEG/seizure type, and imaging as a prognosticate treatment response and clinical course. Features that are important in epilepsy syndromes are age, seizure triggers, comorbidities such as psychiatric and intellectual dysfunction, combined with specific findings on electroencephalography (EEG) and imaging studies. Examples of epilepsy syndromes are West syndrome, childhood absence epilepsy, and Dravet syndrome.
EEG is the method that specifically defines the epileptogenic cortex. The characteristic finding of the epileptiform discharge is rarely recorded in healthy, young individuals. To localize the epileptic zone, it is important to have an EEG recording. EEG used in a patient with epilepsy may demonstrate generalized or focal slow activity of the background. The most indicative diagnostic finding to support the diagnosis of epilepsy is the presence of interictal epileptiform discharges (IEDs). Epileptiform discharge (interictal discharges; IEDs) is a term to describe EEG patterns that are associated with a high risk of having seizures. The morphologic characteristic of an IED is a very sharp rise time, a complex waveform with several phases or baseline crossings, and an after-going slow-wave discharge that disrupts the continuity of the background rhythm. They morphologically classify into sharp waves or spikes, but that has no clinical implication. Spikes is an epileptiform discharge that infers to a 'pointy' shape with a duration of 70 ms or less. Sharp waves are blunter with a duration of 70 to 200 ms. IEDs can be focal or generalized. Focal IEDs are associated with focal (one brain region) seizures, while generalized IEDs are associated with bihemispheric (more than one brain region) seizures. IEDs are called multi-focal when three or more independent foci distributed in both hemispheres. It is important to distinguish between benign variants or normal brain waves and IEDs to avoid the possibility of overinterpretation of benign variants that may be mistaken for epileptiform activity.
Generalized epileptic seizures are originated at some point within the brain that rapidly spreads bilaterally as recorded on surface EEG. Bilateral networks can be subcortical and cortical structures. It does not have to include the entire cortex, as the name may falsely imply. EEG findings of the patient usually show generalized spike-wave activity. The diagnosis of this condition is made by clinical history and supported by EEG. The patient can have generalized tonic-clonic seizures but a normal EEG. About 50% of patients with generalized tonic-clonic seizures have shown IED, but between 1 to 13% of normal individuals can show IED and also in those with the first-degree relative with generalized epilepsy. Examples of generalized epilepsies are myoclonic, absence, tonic, atonic, and tonic-clonic seizures.
Focal epilepsies refer to a seizure that involving one region in one hemisphere. It can originate in subcortical structures. EEG may show focal epileptiform discharges. Focal IEDs most commonly present in temporal lobe epilepsy, followed by frontal lobe epilepsy. Parietal and occipital epilepsies are less common. Examples of focal epilepsies are focal aware seizures, a focal non-motor seizure, focal motor seizures, focal to a bilateral tonic-clonic seizure.
The classification of primary generalized seizures is the result of seizures affecting both the cerebral hemispheres from the beginning of the seizure. Electroencephalography of this condition may indicate that seizures start 'all over' the brain. Those could be genetic (previously called idiopathic) or secondary/symptomatic due to structural/metabolic etiologies.
In focal seizures, the onset of the seizure arises from one area of the brain. It also was previously known as a partial seizure.
Combined Generalized and Focal Epilepsies
In 2017 International League Against Epilepsy (ILAE) updated the epilepsies classification. The new classification group is Combined Generalized and Focal Epilepsies. This group is for patients who experience both focal and generalized seizures. Clinical history is the most important part of the diagnosis, with support from EEG findings. The EEG characteristic of this condition is showing both focal epileptiform discharge and generalized spike-wave discharges.
When the patient is diagnosed with epilepsy, but due to insufficient data, the clinician is unable to categorize the type of epilepsy, it is considered 'unknown' type.
EEG sensitivity and specificity depend on several factors such as age, states recorded, frequency of seizures, and use of activation procedures. Activation procedures include sleep deprivation, hyperventilation, and photic stimulation).
EEG is the key tool for the diagnosis of epilepsy. Surface EEG provides the best overview of the approximate location of the epileptogenic zone. The interictal and ictal EEG findings are essential for the confirmation and classification of the disorders. Invasive EEG is used only used if the zone cannot be located with non-invasive diagnostic methods, and there is a good chance of finding essential localizing information, and the area is potentially resectable. Intracranial EEG with subdural or intraparenchymal electrodes can localize the seizure focus and map the areas where it is required to plan the surgery.
Epilepsy diagnosis requires an interprofessional team of healthcare professionals that includes a nurse and clinicians. EEG is an important tool to support the patient’s diagnosis of epilepsy. Normal EEG does not exclude epilepsy diagnosis necessarily. The need for meticulous history taking and examination can lower the morbidity and improve the outcomes. [Level 1]
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