Epilepsy EEG

Definition/Introduction

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.[1][2][3][4][3]

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.[2][5][6]

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.[2][5][6]

Generalized Seizures[2]

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.

• An absence seizure is when there is a disturbance in consciousness. It usually lasts for about <10 seconds in typical absence with an abrupt onset and offset. The patient can have automatism, such as repetitive blinking and lip-smacking. In typical absence seizure, the patients usually do not have a postictal state and resume their normal activity right away. Absence seizures are so brief that most of the time, it escapes detection, even though the patient can experience the attacks 50 to 100 times daily. As patients usually stare blankly into space or out of the window, they usually thought to be daydreaming. Electroencephalography performed during this time shows generalized 3-Hz spikes and wave activity. Absence epilepsy is most common in school-age children between ages 4 and 12 years and rarely begins after age 20.

• Atonic or astatic seizures are when the patient suddenly loss of postural tone. The patients can suddenly fall, which may be concerning as they can hit an object. This condition is commonly associated with Lennox-Gestaut syndrome. Electroencephalography performed during this time may show a slow spike-wave or an electrodecremental pattern with generalized fast activity.
• Myoclonic seizures are when there is a sudden, brief jerk or twitching of any extremity or axial musculature. If brief, patients are not losing consciousness during the episodes. Electroencephalography performed during this time may show generalized polyspikes correlating with motor activity.
• Tonic seizures are when there is a sudden generalized tonic stiffening. It can last for less than 20 seconds. Electroencephalography performed during this time may show generalized paroxysmal fast activity or an electrodecremental pattern.
• Tonic-Clonic seizure is where there is a tonic phase where the patient's body suddenly stiffens, then followed by a clonic phase. Eventually, the patient goes into a postictal state. The patient can have a loss of bladder or bowel continence. Electroencephalography performed during this time may show generalized polyspikes, approximately 10Hz frequency, called "recruitment rhythm." EEG during the tonic phase becomes obscured by a diffuse myogenic artifact that becomes intermittent during the clonic phase.

Focal Seizures[2]

In focal seizures, the onset of the seizure arises from one area of the brain. It also was previously known as a partial seizure.

• Focal seizure with intact awareness (previously called simple partial seizure): During the attack, the patient is fully alert and conscious. Electroencephalography performed during this condition rarely shows an ictal EEG discharge, but most of the time, scalp EEG cannot pick up the ictal activity.
• Focal seizure with impaired awareness (previously called complex partial seizure): During the attack, the patient develops a variable degree of impaired awareness, or consciousness gets altered. The clinical behavior of this condition will depend on region or lobe of onset.

Combined Generalized and Focal Epilepsies[5]

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. 

Issues of Concern