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  • Updated 04.09.2021
  • Released 11.10.2008
  • Expires For CME 04.09.2024

EEG in epilepsy

Introduction

Overview

The electroencephalogram (EEG) provides a window to the brain regarding its functional properties. It is a widely available, cost-effective, portable neurophysiological study with worldwide applications and remains the foundational diagnostic tool to evaluate people suspected of seizures. It is a safe, noninvasive, diagnostic, 2-dimensional clinical study used to assess the brain’s 3-dimensional electrophysiological activity. EEG is the most useful test for evaluating people with possible epilepsy when epileptiform activity is demonstrated. It may provide specific neurophysiological information to support clinical diagnoses in the EEG laboratory, epilepsy monitoring unit, intensive care unit, or operating room and can assist in the evaluation of people in an ambulatory setting. When interictal epileptiform discharges are recorded on EEG, it can help to classify the seizure type or epilepsy syndrome suspected clinically. Furthermore, EEG results can guide management, from directing antiseizure drug initiation and maintenance to localizing the irritative and seizure-onset zones for neurosurgical treatment, in people with focal epilepsies. EEG has become an important adjunct to the clinical examination for diagnosing and treating unrecognized seizures and nonconvulsive status epilepticus in critically ill patients when the clinical examination is unrevealing. In the operating room, electrocorticography may help surgeons define surgical borders of functional and epileptogenic tissues. With the increasing emphasis on outpatient management, ambulatory EEG has become an important aspect of recording EEG in the home environment. The overall utility of EEG has evolved to a sophisticated computer-based clinical and research tool that is fundamental for exploring essential brain functions. Despite the innovation of neuroimaging, EEG continues to have a vital role in the dynamic evaluation of neurologic disease, extending from primary use in seizures and epilepsy to include the diagnosis and monitoring of neurologic disorders, such as encephalopathy, traumatic brain injury, sleep disorders, coma, and brain death.

Key points

• EEG is the diagnostic test of choice when evaluating a patient with seizures and epilepsy, though pitfalls exist in interpretation because of benign variants.

• Epilepsy is a clinical diagnosis that is supported by epileptiform activity on EEG and is confirmed when EEG records an electrographic seizure.

• EEG may help classify seizures and epilepsy syndromes for treatment by defining the distribution of epileptiform abnormalities and quantifying the frequency of seizure occurrence.

• Misdiagnosis of nonepileptic events as seizures is probably not rare due to EEG misinterpretation, and long-term EEG monitoring in the epilepsy monitoring unit and the intensive care unit may yield diagnostic clarity of the seizure burden.

• The surgical treatment of epilepsy relies on ictal EEG to characterize the electroclinical localization of the epileptogenic zone.

• Special waveforms, recording conditions, and new techniques are expanding EEG usefulness beyond the application in epilepsy to other areas involved in brain disease.

Historical note and terminology

In his seminal work “Uber das Elektroenkephalogram des Menschen” (“On the EEG of Man”), Hans Berger pioneered the discovery of the human EEG, first recorded in 1929 (27). The practical usefulness of EEG became apparent in the 1930s after interictal discharges were demonstrated first by Fisher and Lowenback and later by Gibbs, David, and Lennox in the United States.

In 1936, W Gray Walter demonstrated that this technology could aid in the diagnosis of tumors, stroke, and other focal brain disorders. For 40 years, EEG was the cornerstone to the diagnosis and treatment of seizures and epilepsy. Until the advent of CT and MRI, it was the first-line neurodiagnostic test for diagnosing tumors, stroke, and other focal brain disorders.

EEG data were analyzed by visual inspection until the 1960s, when digital equipment was introduced to begin the age of computerizing the EEG. In the1970s, the Fourier transform, computer-based algorithms and analytics, quantitative EEG, and trend analysis became a reality.

Terminology. In 2017, after 35 years, the International League Against Epilepsy (ILAE) released a new classification of seizure types, largely based on the existing classification originally formulated in 1981 (22) and further standardized terminology for seizure classification. In broad terms, epilepsy syndromes are classified as generalized, focal, or unknown with an etiology of genetic, structural-metabolic, or unknown (22). Seizures are classified according to where they start in the brain. Generalized seizures involve both sides of the brain or large hemispheric networks of neurons at onset. Focal seizures (ie, partial seizures) refer to those seizures that start in one side of the brain and involve regional networks. Focal seizures may start on the surface of the brain or in deeper structures and remain restricted to an area or spread to involve larger network. Unknown seizures reflect those without a clear onset at the beginning or when atypical and mixed forms of focal and generalized seizures co-exist. The new terminology correlates to older language (Table 1), which is still often used clinically.

Table 1. Comparison of Old and New Terminology Set Forth by the ILAE in 2017

Characteristic

New terminology

Old terminology

Type of seizure and origin of onset

Originating within, and rapidly engaging, bilaterally distributed networks

Generalized

Generalized

Originating within networks limited to one hemisphere

Focal

Partial

Descriptors of focal seizures

Wording changes

Focal aware*

With observable motor or autonomic components

Simple partial

Involving subjective sensory or psych phenomena only

Aura

Focal impaired awareness

Dyscognitive

Complex partial dyscognitive

Focal to bilateral tonic-clonic seizure**

---

Secondarily generalized seizure

Seizure etiology

Epilepsy as a direct result of a known or presumed genetic defect in which seizures are the core symptom of the disorder

Genetic

Idiopathic

Distinct structural or metabolic condition or disease that substantially increases epilepsy risk

Structural/metabolic

Symptomatic

Nature of underlying cause is unknown

Unknown onset, seizure type

Cryptogenic


*Focal seizures are classified as motor and nonmotor. Adding descriptions of other signs and symptoms is suggested (ie, focal aware motor seizure).

**The term “bilateral” is used for tonic-clonic seizures that propagate to both hemispheres, and “generalized” is a term used for seizures that appear to begin simultaneously in both hemispheres.

The application of the standard scalp EEG in epilepsy has relied on the electrocerebral activity largely between the 1 to 30 Hz bandwidth (ie, Berger’s bandwidth). Filter settings are increasingly being “opened” during video epilepsy monitoring to obtain full-band EEG, providing a more comprehensive approach (67). Newer applications of frequencies (ie, gamma and high-frequency oscillations) are now being explored to enhance disclosure of brain regions and the networks that are involved in seizure genesis (Table 2).

Table 2. Bandwidth and Interpretation of EEG Waveform Frequencies

Frequency (Hz)

Bandwidth

Normal

Pathological

0.0 - 0.5

Infraslow activity*

Artifacts

Onset of focal seizures

0.5 - 3.5

Delta

Sleep, HV, PSWY, elderly

Encephalopathy, white matter lesion

>3.5 - <8.0

Theta

Drowsiness, children, elderly

Encephalopathy, white matter lesion

8 - 13

Alpha

PDR, mu rhythm, “third” rhythm

Ictal rhythm in seizure, alpha coma

13 - 30

Beta

Medication, drowsiness

Breach rhythm, drug overdose, ictal rhythm

30 - 80

Gamma*

Voluntary motor movement, learning/memory

Seizures

80 - 250

Ripples*

Cognitive processing/memory

Interictal and ictal seizure frequency, possible epileptogenesis

250 - 500

Fast ripples*

?

Focal seizures

500 - 1000

Very fast ripples*

Acquisition of sensory information

Seizures


* = Expanded frequencies currently under investigation

The characteristic interictal EEG features of epilepsy are spikes (20 to 70 msec) and sharp waves (70 to 200 msec) when displayed on a review monitor at a display speed of 30 mm/second. Pathological discharges are distinguished from the background and contain a sharp contour, physiological field, rapid rise, and brief duration, occasionally with an after-going slow wave. However, criteria for identifying them require several of the features to be convincing (35). Spikes and sharp waves can present in isolation or as polyspikes or polysharp waves. Spikes and sharp waves possess the same potential for seizure genesis independent of morphology.

Each type of EEG recording in epilepsy has its own advantages and disadvantages. Standard scalp EEG recordings, short-term EEG, computer-assisted ambulatory EEG (CAA-EEG), and in-patient continuous video EEG monitoring (VEM) are different methods of EEG recording (38), each with different clinical benefits and limitations (Table 3).

Scalp EEG is the simplest, least expensive, most practical, and, therefore, most common method to acquire EEG during routine clinical use. A standard scalp EEG is usually a brief 20- to 30-minute recording (but may extend up to 60 minutes duration). It is, therefore, typically inadequate for capturing infrequent paroxysmal neurologic events and abnormalities (06). A prospective study of EEGs from 1803 patients revealed that only 19% of the patients had interictal epileptiform discharges in the first 30 minutes of recording, but the event capture rate increased by 30% during longer recordings (12). However, increase in diagnostic yield with longer duration is not supported by other studies (41). Prolonged EEG is better able to capture seizures and neurologic events. One 5-year study of 175 outpatient short-term EEGs (shorter than 24 hours) found that 7% yielded seizures during the recording (54).

A higher diagnostic yield (40%) was found in children when EEG monitoring was longer than 6 hours (58). Prolonged the EEG recording either using CAA-EEG or inpatient video EEG monitoring offers distinct advantages. The yield of identifying epileptiform discharges to support a clinical diagnosis of epilepsy has been approximately 2.0 to 2.5 times that of a standard EEG and was cost-effective when compared to the gold standard of video EEG monitoring with a yield of ambulatory-EEG greater than 70% (17).

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