Temp3

Primer of EEG

Preface

This primer is written as a brief introduction to EEG that can be read on the first day as someone is exposed to an EEG for the first time. It is not meant to be a comprehensive text. There are many textbooks available that go into much more detail. The goal is to review the principles of EEG and give examples of EEG findings encountered on a routine day of EEG reading.

It is easy to read EEG, but difficult to read EEG well.

Contents

1. [#Introduction Introduction]
2. [#Electrical_Principles Electrical Principles]
3. [#Electrode_Placement Electrode Placement]
4. [#Recording_and_Display_Conventions Recording and Display Conventions]
5. [#Montages Montages]
6. [#EEG_Analysis_of_Waveforms EEG Analysis of Waveforms]
7. [#Normal_Waking_EEG Background Organization Normal Waking EEG Background Organization]
8. [#Normal_Sleep_EEG Background Organization Normal Sleep EEG Background Organization]
9. [#Normal_Variants Normal Variants]
10. [#Artifacts Artifacts]
11. [#Encephalopathic_Patterns Encephalopathic Patterns]
12. [#Interictal_Epileptiform EEG Interictal Epileptiform EEG]
13. [#Ictal_Recordings Ictal Recordings]
14. [#Critical_Care_EEG Critical Care EEG]
15. [#Quantitative_EEG Quantitative EEG]
16. [#Appendix_1_Approach_to_a_Normal_EEG_Report Appendix 1: Approach to a Normal EEG Report]

Introduction

Electroencephalography (EEG) was first described by Hans Berger in 1929. He recorded brain waves from two electrodes placed at the back of the head and described the normal resting alpha rhythm. He also identified abnormal electrical patterns among brain injured subjects. Since then, EEG’s application in evaluating numerous clinical scenarios has been expanded and refined.

Though a relatively old technology, EEG remains the best available noninvasive method for evaluating brain function in real time. EEGs are often requested to assess paroxysmal neurologic events, define and characterize seizure disorders, and determine how overall brain function correlates with a variety of clinical states. Identification of interictal epileptiform discharges may help confirm a diagnosis of epilepsy or clarify the diagnosis of specific epilepsy syndromes.

Electrical Principles

Surface electrodes are applied to the scalp to record the underlying cortical activity. Each electrode records the synchronized electrical activity arising from a population of neurons in an area of cortex in an angle subtended by the surface electrode of approximately 6 cm², slightly more than the surface area of a quarter.

Since most of the neurons in the cerebral cortex are radially oriented, the synaptic events in the area just under the surface of the scalp tend to be negative in polarity...

Electrode Placement

The surface electrodes are placed on the scalp in a systematic manner that standardizes their position in relation to underlying brain regions and ensures a reproducible study between patients or for an individual patient across multiple recordings.

EEG electrodes are placed according to an internationally agreed-upon system called the International 10-20 System. This system is based on measurements of the head that are divided into 10% and 20% fractions of the total circumference of the head...

Recording and Display Conventions

The EEG machine is a differential amplifier. It takes the electrical information from one signal source or electrode and compares it to another source, rejecting any common features (common mode rejection). The resulting signal reflects a measurement of the net difference in voltage between the two electrodes (here designated as gridpoints G1 and G2) over time...

Montages

A montage is the arrangement of channels on the EEG display or tracing, and each channel consists of paired signal sources. When channel derivations are arranged into montages their sequence provides information that identifies the polarity and location of the cortical activity of interest. Montages can be created in any array, though the most useful ones will be those that allow for rapid visual interpretation of the EEG signals.

There are two types of montages: bipolar and referential.

Bipolar Montage

A bipolar montage is created by making chains of sequential channels where each channel consists of a pair of individual electrodes...

Referential Montage

Referential montages give the same electrophysiological information but present the information in a slightly different format...

EEG Analysis of Waveforms

When approaching the evaluation of the EEG you will see continuous lines that represent fluctuating cerebral potentials and are composed of several types of waveforms. To describe a waveform in an organized and reproducible way, we break it down into its component dimensions of voltage, frequency, and morphology.

• **Voltage**: At the scalp surface, the amplitude of the cerebral activity is measured on the order of microvolts.
• **Frequency**: Many EEG patterns are rhythmic or semirhythmic in nature.
• **Morphology**: The morphology is the shape of a waveform.

Normal Waking EEG Background Organization

The normal waking EEG background is characterized by anterior-posterior voltage and frequency gradients. The EEG activity is generally lower in amplitude and faster in frequency anteriorly and higher in amplitude and slower in frequency posteriorly.

Another feature of a normal waking EEG background is the posterior dominant rhythm, sometimes called the alpha rhythm, which is best seen at O1 and O2 electrodes...

Normal Sleep EEG Background Organization

Sleep is divided into two states: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep is further subdivided into N1-3 based on the depth of sleep and certain EEG characteristics.

The onset of N1 sleep is characterized by an attenuation of the posterior dominant rhythm, increased theta activity, reduction in muscle movements, and slow roving eye movements...

Normal Variants

There is a set of EEG waveforms that appear either sharp in contour or rhythmic in nature and may be mistakenly described as epileptiform or abnormal features. However, based on many years of experience and large studies of normal EEG, these patterns, though not evident on every normal EEG, are accepted as benign normal EEG variants.

Alpha Squeak

This finding occurs immediately after eye closure during the waking state. The posterior dominant rhythm appears to transiently increase in frequency by approximately by 1 Hertz over the normal posterior rhythm.

Artifacts

Artifacts are waveforms on an EEG caused by electrical sources other than the cortex. It is important to recognize and identify artifacts so that they are not confused with cerebral potentials. Artifacts can be generated by numerous sources, the most common of which is the body itself: the eyes, the heart, and muscle.

Ocular Artifact

The globe of the eye has an electrical potential of about 50-100 microvolts with the cornea positive and the retina negative in polarity...

Encephalopathic Patterns

When a patient is unresponsive without lateralizing exam findings and the etiology of the coma is in question, an EEG can be helpful to further assess the degree of the coma and perhaps give insight into the etiology or localization of the underlying pathology.

In general, there is a continuum of EEG findings that correspond to the severity of a diffuse clinical encephalopathy...

Interictal Epileptiform EEG

Epilepsy syndromes are classified as either Focal or Generalized epilepsies. They are differentiated by whether the seizures arise from a focal region of the brain or from activation of a generalized network. The EEG is used to differentiate focal from generalized epilepsy syndromes by the epileptiform discharges that are localized to a focal region versus a generalized pattern.

Ictal Recordings

It is rare to capture a seizure during a routine EEG, because the study is a random sampling of someone who has paroxysmal events. For this reason, patients are often admitted to the hospital for prolonged EEG monitoring with video...

Critical Care EEG

Critical Care EEG involves the continuous monitoring of patients in critical conditions...

Quantitative EEG

Quantitative EEG (qEEG) is used for analyzing brain activity based on frequency bands...

Appendix 1: Approach to a Normal EEG Report

UNC EEG Report

• **Patient**: @NAME@
• **Date of Birth**: @DOB@
• **UNC MRN**: @MRN@
• **Ordering Provider**: @REFERPROV@

History

Per chart, @NAME@ is @AGE@ years old @SEX@ with ...

References

• Beniczky S, Schomer DL. Electroencephalography: basic biophysical and technological aspects important for clinical applications. Epileptic Disord. 2020;22(6):697-715. doi:10.1684/epd.2020.1217
• Peltola ME, Leitinger M, Halford JJ, et al. Routine and sleep EEG: Minimum recording standards of the International Federation of Clinical Neurophysiology and the International League Against Epilepsy. Clin Neurophysiol. 2023;147:108-120. doi:10.1016/j.clinph.2023.01.002
• Nascimento FA, Jing J, Strowd R, et al. Competency-based EEG education: a list of "must-know" EEG findings for adult and child neurology residents. Competency-based EEG education: a list of “must-know” EEG findings for adult and child neurology residents. Epileptic Disord. 2022;24(5):979-982. doi:10.1684/epd.2022.1476

Electroencephalography (EEG) was first described by Hans Berger in 1929. He recorded brain waves from two electrodes placed at the back of the head and described the normal resting alpha rhythm. He also identified abnormal electrical patterns among brain injured subjects. Since then, EEG’s application in evaluating numerous clinical scenarios has been expanded and refined.

Though a relatively old technology, EEG remains the best available noninvasive method for evaluating brain function in real time. EEGs are often requested to assess paroxysmal neurologic events, define and characterize seizure disorders, and determine how overall brain function correlates with a variety of clinical states.

Identification of interictal epileptiform discharges may help confirm a diagnosis of epilepsy or clarify the diagnosis of specific epilepsy syndromes. Differentiating between a generalized epilepsy syndrome and a localization-related epilepsy syndrome can be crucial in prognosis and decision making for medical and/or surgical management. EEGs are also used to help establish diagnosis and prognosis in certain encephalopathic or coma states. Commonly the EEG will reveal slowing to some degree paralleling the clinical examination. If a particular coma pattern is seen on the EEG, the pattern and its evolution over time may provide prognostic information to assist in clinical decision making for further treatment. One condition where the information provided by EEG is indispensable is nonconvulsive status epilepticus. This condition is difficult to diagnose unless the index of suspicion is high at the onset because the clinical presentation can be subtle. The morbidity and mortality of status epilepticus can be high if proper diagnosis and treatment are delayed.

Electrical Principles

Surface electrodes are applied to the scalp to record the underlying cortical activity. Each electrode records the synchronized electrical activity arising from a population of neurons in an area of cortex in an angle subtended by the surface electrode of approximately 6 cm2, slightly more than the surface area of a quarter.

Since most of the neurons in the cerebral cortex are radially oriented, the synaptic events in the area just under the surface of the scalp tend to be negative in polarity, reflecting the electrical activity in the extracellular space around apical dendrites as positive ions rush into the cell during excitation. Electrical activity surrounding the soma is generally positive in polarity, but due to its depth, is not typically recorded by surface electrodes.

Electrode Placement - International 10-20 Electrode Placement System

The surface electrodes are placed on the scalp in a systematic manner that standardizes their position in relation to underlying brain regions and ensures a reproducible study between patients or for an individual patient across multiple recordings.

EEG electrodes are placed according to an internationally agreed-upon system called the International 10-20 System. This system is based on measurements of the head that are divided into 10% and 20% fractions of the total circumference of the head, the ear to ear measurement and the anterior-posterior measurement from the nasion to the inion.

The electrodes are named based on convention, with letters that represent particular cerebral regions (Frontal, Temporal, Central, Parietal, Occipital), and numbers that represent sidedness. Odd numbered electrodes are on the left, even numbered on the right, and electrodes with a “z” designation at the midline.

Additional electrodes placed in between at the 10% locations to improve the ability to localize. This system is called the 10-10 electrode placement system. Electrodes F9/10, T9/10 and P9/10 are examples on this head diagram.

Recording and display conventions

The EEG machine is a differential amplifier. It takes the electrical information from one signal source or electrode and compares it to another source, rejecting any common features (common mode rejection). The resulting signal reflects a measurement of the net difference in voltage between the two electrodes (here designated as gridpoints G1 and G2) over time. In this way the EEG displays a continuous tracing of the electrical activity from a particular region of the cortex in relationship to another cortical region or in relationship to a non-brain reference.

By convention, the tracing for each channel of the EEG reflects the electrical activity of the first electrode or grid point (G1) in reference to the second (G2). Again by convention, each derivation or channel displays a waveform with an upward deflection when the net difference of the voltage between the two electrodes is negative (G1-G2<0). If G1 is positive or less negative than G2 (G1-G2>0), then the waveform has a downward deflection. Thus, there are two possible interpretations for the waveform produced by each derivation pair. For example, if G1 is more negative than G2 or G2 is more positive than G1, then the waveform points upwards. It depends on which electrode is the active electrode and in what direction.

There is a rational basis for the convention of displaying electrically negative events as an upward deflection on the EEG. Epileptic discharges result in a paroxysmal negative charge at the surface of the cortex, and conventional EEG tracings produce a rapid upward deflection often described as a sharp wave or spike. This is why a transient negative potential at any given electrode is described as a surface negative event. Vice versa, a transient positive potential is described as a surface positive event.

Montages:

A montage is the arrangement of channels on the EEG display or tracing, and each channel consists of paired signal sources. When channel derivations are arranged into montages their sequence provides information that identifies of polarity and location of the cortical activity of interest. Montages can be created in any array, though the most useful ones will be those that allow for rapid visual interpretation of the EEG signals. There are two types of montages: bipolar and referential. The rules for interpretation for each type of montage are different based on the way they display the information.

Bipolar montages are created by making chains of sequential channels where each channel consists of a pair of individual electrodes. In order to convey spatial resolution, the chains of sequential channels can be arranged in anatomically relevant directions. A new chain starts when the sequence begins in a new location. The chains can run in an anterior to posterior direction (AP Bipolar Montage also called the double banana), or in a transverse direction across the head; whatever manner that gives useful localizing information.

Rule for interpretation of a bipolar montage: A phase reversal in a chain identifies an area where the changes in electrical potential are maximal relative to the surrounding signals. Phase reversals can either be positive or negative in polarity. When there is a negative phase reversal, the waveforms appear to point toward each other; when there is a positive phase reversal the waveforms appear to point away from each other. Phase reversals often indicate a region of interest electrophysiologically, showing a particular spatial point on the EEG where voltage changes are maximal and/or the spatial extent of voltage changes that surround it, its field.

In this hypothetical bipolar example, systematically evaluating each channel will allow us to make an interpretation of the entire chain and by using the polarity conventions of EEG display, localize electrical events in space.

• In the first channel where a is compared to b, there is no large net difference in their electrical potentials so the display shows a relatively flat line.
• When b is compared to c, there is a small downward deflection. This finding suggests that either b is slightly more positive than c or c is more negative than b.
• When c is compared to d, there is another small deflection downwards also suggesting that c is slightly more positive than d or that d is more negative than c.
• There is a large deflection upwards when d is compared to e. In this case, d is either much more negative than e or e is much more positive than d.
• When e is compared to f, there is a small upward deflection suggesting e is either slightly more negative than f or f is slightly more positive than e.
• Finally, in the last channel f and g are equipotential and the tracing appears relatively flat.

The conclusion from this analysis suggests that either there are two positive generators on either side of the chain or electrode d is the region of maximum negativity and the potential of interest. Considering that most cortical events of interest are negative in potential, this finding would be the most logical conclusion. The deflections between channels c-d and d-e represent a negative phase reversal.

Referential montages give the same electrophysiological information but present the information in a slightly different format. The voltage at each electrode is compared to a common or neutral electrode or to mathematical average of a group of electrodes. In a referential montage, the area of highest amplitude is of the most interest because it represents the region most negative or positive in polarity. If there is an apparent “phase reversal” there are two potential interpretations: 1) The reference electrode is active or is involved in the area of interest, or 2) there is a horizontal dipole and that both ends of the dipole are apparent on the montage.

Common referential montages:

Ipsilateral ear reference – Each electrode over the left hemisphere is compared to the left ear electrode; each electrode over the right hemisphere is compared to the right ear electrode.

Contralateral ear reference – Each electrode over one hemisphere is compared to the contralateral ear electrode.

A1-A2 reference – Each electrode is compared to an average of the A1 and A2 electrodes

Cz Reference – each electrode is compared to Cz

Average reference – Each electrode is compared to the average signal of a group of electrodes (usually those least affected by eye movement or muscle artifact

Laplacian anatomic average reference – Each electrode is compared to a weighted average of an immediately surrounding set of electrodes.

In this hypothetical referential example (which shows the same electrical event as the bipolar example), systematically evaluating each channel will allow us to make an interpretation of the entire EEG.

• In the first channel when a is compared with the reference electrode, there is no net difference between the electrical potentials so the display shows a relatively flat line.
• When b is compared to the reference electrode, there is also little net difference between the electrical potentials so the display also appears as a flat line. These findings suggest that neither a nor b is involved in the discharge of interest.
• When c is compared to the reference, there is a small deflection upwards, thus suggesting that c is slightly more negative than the neutral reference.
• There is a large deflection upwards when d is compared to the reference. This finding suggests that d is much more negative than the reference.
• When e is compared to the reference, there is a small upward deflection suggesting that e is slightly more negative that the reference.
• Finally, the last channel f is again equipotential to the reference electrode.

The conclusion from this analysis suggests that electrode d is the maximally negative electrode with a field that extends to the surrounding electrodes c and to a lesser extent e. This analysis of the same set of electrodes draws us to the same conclusion as when we the bipolar montage was analyzed systematically. A referential display also allows quantification of the events recorded since the voltage of each channel is being compared to a common reference.