Epilepsy Awareness Program - EEG (Electroencephalography)


An electroencephalogram (EEG) is a test used to detect abnormalities related to electrical activity of the brain.
Scientists first captured and recorded brain waves in dogs in 1912.
German physiologist and psychiatrist Hans Berger (1873–1941) began his studies of the human EEG in 1920. He gave the device its name and is sometimes credited with inventing the EEG, though others had performed similar experiments. His work was later expanded by Edgar Douglas Adrian. In 1934, Fisher and Lowenback first demonstrated epileptiform spikes. In 1935 Gibbs, Davis and Lennox described interictal spike waves and the 3 cycles/s pattern of clinical absence seizures, which began the field of clinical electroencephalography. Subsequently, in 1936 Gibbs and Jasper reported the interictal spike as the focal signature of epilepsy. The same year, the first EEG laboratory opened at Massachusetts General Hospital.
Franklin Offner (1911–1999), professor of biophysics at Northwestern University developed a prototype of the EEG that incorporated a piezoelectric inkwriter called a Crystograph (the whole device was typically known as the Offner Dynograph).
In 1947, The American EEG Society was founded and the first International EEG congress was held. In 1953 Aserinsky and Kleitman describe REM sleep.
In the 1950s, William Grey Walter developed an adjunct to EEG called EEG topography, which allowed for the mapping of electrical activity across the surface of the brain. This enjoyed a brief period of popularity in the 1980s and seemed especially promising for psychiatry. It was never accepted by neurologists and remains primarily a research tool.

Electroencephalography (EEG) Introduction

When the brain cells send messages to each other, they produce tiny electrical signals. Your brain cells communicate via electrical impulses and are active all the time, even when you're asleep. This activity shows up as wavy lines on an EEG recording.

An EEG is one of the main diagnostic tests for epilepsy. An EEG may also play a role in diagnosing other brain disorders.
In an EEG test, electrodes (flat metal discs) are placed onto your scalp using a sticky substance. These electrodes pick up the electrical signals from your brain and send them to an EEG machine, which will record the signals as wavy lines onto paper or on a computer. The EEG machine records your brain's electrical activity as a series of traces, Each trace corresponds to a different region of the brain.
An electroencephalogram (EEG) is a painless procedure that takes 30 to 45 minutes with rarely causes of any side effects.

What is the main diagnostic application of EEG?

The main diagnostic application of EEG is in the case of epilepsy, as epileptic activity can create clear abnormalities on a standard EEG study. A secondary clinical use of EEG is in the diagnosis of coma, encephalopathies, and brain death. EEGs can also help to identify causes of other problems such as sleep disorders and changes in behavior as well it can be used to evaluate brain activity after a severe head injury or before heart or liver transplantation.
EEG used to be a first-line method for the diagnosis of tumors, stroke and other focal brain disorders, but this use has decreased with the advent of anatomical imaging techniques such as MRI and CT.

What is shown in EEG recording?

The EEG shows patterns of normal or abnormal brain electrical activity. Some abnormal patterns may occur with a number of different conditions, not just seizures. For example, certain types of waves may be seen after head trauma, stroke, brain tumor, or seizures. A common example of this type is called "slowing," in which the rhythm of the brain waves is slower than would be expected for the patient's age and level of alertness.
Certain other patterns indicate a tendency toward seizures. Your doctor may refer to these waves as "epileptiform abnormalities" or "epilepsy waves." These include spikes, sharp waves, and spike-and-wave discharges. Spikes and sharp waves in a specific area of the brain, such as the left temporal lobe, indicate that partial seizures might possibly come from that area. Primary generalized epilepsy, on the other hand, is suggested by spike-and-wave discharges that are widely spread over both hemispheres of the brain, especially if they begin in both hemispheres at the same time.


Background EEG in Adults


Alpha:8-13 Hz, posterior predominant, symmetric, Amp 30-60 mV, R>L by 20-50%, ¯ by EO, drowsiness, Age 60-80, a=9.5 Hz; a < 8 in elderly suggests A.D. Slow (sub-harmonic, 4-5 Hz) and fast alpha (16-20 Hz)

Theta:  Usually low amplitude at frontal central region (6-7Hz), Rhythmic temporal theta bursts of drowsiness, Midline theta rhythm (Cz max)

Delta: Diffuse in deep sleep, metabolic encephalopathies, Focal in structural brain lesion

Beta: 18-25 Hz, frontal-central predominant, Amp < 20 mV and lower in elderly, ­ by benzodiazepine, light sleep and skull defect

EEG Montages

Since an EEG voltage signal represents a difference between the voltages at two electrodes, the display of the EEG for the reading EEG machine may be set up in one of several ways. The representation of the EEG channels is referred to as a montage.

Bipolar montage: Each channel (waveform) represents the difference between two adjacent electrodes. The entire montage consists of a series of these channels. For example, the channel "Fp1-F3" represents the difference in voltage between the Fp1 electrode and the F3 electrode. The next channel in the montage, "F3-C3," represents the voltage difference between F3 and C3, and so on through the entire array of electrodes.

Referential montage: Each channel represents the difference between a certain electrode and a designated reference electrode. There is no standard position for this reference; it is, however, at a different position than the "recording" electrodes. Midline positions are often used because they do not amplify the signal in one hemisphere vs. the other. Another popular reference is "linked ears," which is a physical or mathematical average of electrodes attached to both earlobes or mastoids.

Average reference montage: The outputs of all of the amplifiers are summed and averaged, and this averaged signal is used as the common reference for each channel.

Laplacian montage: Each channel represents the difference between an electrode and a weighted average of the surrounding electrodes.
With digital EEG, all signals are typically digitized and stored in a particular (usually referential) montage; since any montage can be constructed mathematically from any other, the EEG can be viewed by the EEG machine in any display montage that is desired.

Who can read EEG?

The EEG is read by a neurologist, optimally one who has specific training in the interpretation of EEGs. This is done by visual inspection of the waveforms, called graphoelements.


Clinical use

A routine clinical EEG recording typically lasts 20–40 minutes (plus preparation time) and usually involves recording from scalp electrodes. Routine EEG is typically used in the following clinical circumstances:

*-* to distinguish epileptic seizures from other types of spells, such as psychogenic non-epileptic seizures, syncope (fainting), sub-cortical movement disorders and migraine variants.

*-* to differentiate "organic" encephalopathy or delirium from primary psychiatric syndromes such as catatonia
*-* to serve as an adjunct test of brain death

*-* to prognosticate, in certain instances, in patients with coma

*-* to determine whether to wean anti-epileptic medications

At times, a routine EEG is not sufficient, particularly when it is necessary to record a patient while he/she is having a seizure. In this case, the patient may be admitted to the hospital for days or even weeks, while EEG is constantly being recorded (along with time-synchronized video and audio recording). A recording of an actual seizure (i.e., an ictal recording, rather than an inter-ictal recording of a possibly epileptic patient at some period between seizures) can give significantly better information about whether or not a spell is an epileptic seizure and the focus in the brain from which the seizure activity emanates.

Epilepsy monitoring is typically done:

** to distinguish epileptic seizures from other types of spells, such as psychogenic non-epileptic seizures, syncope (fainting), sub-cortical movement disorders and migraine variants.

** to characterize seizures for the purposes of treatment

** to localize the region of brain from which a seizure originates for work-up of possible seizure surgery

Additionally, EEG may be used to monitor certain procedures:

# # to monitor the depth of anesthesia

# # as an indirect indicator of cerebral perfusion in carotid endarterectomy

# # to monitor amobarbital effect during the Wada test

EEG can also be used in intensive care units for brain function monitoring:

~ to monitor for non-convulsive seizures/non-convulsive status epilepticus

~ to monitor the effect of sedative/anesthesia in patients in medically induced coma (for treatment of refractory seizures or increased intracranial pressure)

~ to monitor for secondary brain damage in conditions such as subarachnoid hemorrhage (currently a research method)

Methods used in EEG:

Photic Flash Stimulation: The flashing light device consists of a Photic light or also can be called flash light where intensity, frequency and duration of the emitted light are operator controlled. The Flash light is positioned about a meter distance from the patient head and usually flash light is not given continuously when frequency is changed as a pause for a certain time separates different flash light frequency values.
The Photic stimulation is usually used as a part of routine EEG test and can provoke seizure in certain percentage of patients.


Response to Photic stimulation:

*- Asymmetrical: unilateral destructive occipital lesion

*- Photomyoclonic response at f=12-18 Hz; associated with brainstem lesion or psychiatric disorders but not epilepsy

*- Photoparoxysmal response most easily elicited at f= 15-20 Hz; not time locked to flash stimulation

Photic driving response

Rhythmic occipital dominant waveform
Occurs at stimulus frequency 5-30 Hz, especially at 8-13 Hz
Associated with lamda and POST

Hyperventilation: In medicine, hyperventilation (or overbreathing) is the state of breathing faster and/or deeper than necessary. Hyperventilate - To breathe fast and deeply.

During the EEG test, Hyperventilation is a method used to cause rapid, deep breathing; this technique may provoke epileptiform waves or seizures (especially absences seizures) during an EEG recording.

Response to Hyperventilation

Asymmetrical slow waves imply focal pathology
Accentuation of epileptiform discharges
3 Hz spike wave discharge in absence epilepsy
Trigger seizures, generalized or focal
Contraindicated in patients with recent stroke, SAH, or cardiac disease


Limitations of EEG

EEG has several limitations. Most important is its poor spatial resolution. EEG is most sensitive to a particular set of post-synaptic potentials: those generated in superficial layers of the cortex, on the crests of gyri directly abutting the skull and radial to the skull. Dendrites, which are deeper in the cortex, inside sulci, in midline or deep structures (such as the cingulate gyrus or hippocampus), or producing currents that are tangential to the skull, have far less contribution to the EEG signal.
The meninges, cerebrospinal fluid and skull "smear" the EEG signal, obscuring its intracranial source.
It is mathematically impossible to reconstruct a unique intracranial current source for a given EEG signal, as some currents produce potentials that cancel each other out. This is referred to as the inverse problem. However, much work has been done to produce remarkably good estimates of, at least, a localized electric dipole that represents the recorded currents.


The procedure is very safe. However, the flashing lights or fast breathing (hyperventilation) required during the test may trigger seizures in those with seizure disorders.

How the Test is Performed

Brain cells communicate with each other by producing tiny electrical impulses. In an EEG, this faint electrical activity is measured by putting electrodes on the scalp.
The test is performed by an EEG technician in your health care provider's office, at a hospital, or at an independent laboratory. You will be asked to lie on your back on a bed or in a reclining chair.
The technician will apply between 16 and 25 flat metal disks (electrodes) in different positions on your scalp. The disks are held in place with a sticky paste. The electrodes are connected by wires to an amplifier and a recording machine.
The recording machine converts the electrical impulses into patterns that can be seen on a computer screen, as well as stored on a computer disk. Before computers, the activity was printed on paper. In either case, the electical activity looks like a series of wavy lines. You will need to lie still with your eyes closed because any movement can alter the results.
You may be asked to do certain things during the recording, such as breathe deeply and rapidly for several minutes or look at a Photic flash light.


How the electrodes are placed on patients head?

Electrodes are placed on the scalp in special positions. These positions are identified by using the International 10/20 System. This relies on taking measurements between certain fixed points on the head. The electrodes are then placed at points that are 10% and 20% of these distances.
Each electrode site is labelled with a letter and a number. The letter refers to the area of brain underlying the electrode e.g. F - Frontal lobe and T - Temporal lobe. Even numbers denote the right side of the head and odd numbers the left side of the head.



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What do the letters and numbers mean?

The names of the electrode sites use alphabetical abbreviations that identify the lobe or area of the brain to which each electrode refers:
F = frontal
Fp = frontopolar
T = temporal
C = central
P = parietal
O = occipital
A = auricular (ear electrode).

The localization of the brain waves within the brain regions or lobes is further narrowed by adding electrodes, which are given numbers such as T3, T4, P3, P4. Even numbers identify electrode positions on the right side of the head, and odd numbers refer to the left side. The label "z" points to electrode sites in the midline of the head. For example, Cz refers to the midline central region of the head.


Types of EEG test:

There are several different types of EEG tests and can be listed as the following:

Routine EEG tests

Usually, you have a routine EEG test at an outpatient’s appointment at the hospital. The appointment normally lasts about one hour. You can go home as soon as the test has been done.

During the test, you sit or lie down. The person who does the test may be a nurse or a technician. They attach the electrodes to your head with a sticky gel. They may ask you to open and close your eyes, breathe deeply for some minutes and look at a flashing light. All of these activities can change the electrical activity in your brain. This can help the doctor to make a diagnosis.
It’s helpful to keep as still as possible during the test, because any movement can also change the electrical activity in your brain. This can affect the results.

Ambulatory EEG tests

Ambulatory means designed for walking. Your doctor may ask you to have an ambulatory EEG test if they want to record the activity in your brain over a few hours, days or weeks. This allows more time for the test to pick up any unusual electrical activity in your brain, than during the routine EEG test.
An ambulatory EEG uses electrodes similar to those used on a routine EEG test. However, the electrodes plug in to a small monitor that records the results. You can wear the machine on a belt, so you are able to carry on with your life as normal. You don’t usually stay in hospital while the test is being done.
Your doctor may ask you to keep a brief diary while you’re wearing the ambulatory EEG. This can show if there is anything you do or any particular situations that cause your brainwave patterns to change.

Sleep EEG tests

Your doctor may ask you to have an EEG test while you’re asleep. This could be because your seizures happen when you’re asleep or when you are tired. Or, you may have had a routine EEG test when you were awake, but it didn’t show any unusual electrical activity. When you’re asleep, your brainwave patterns change and may show more unusual electrical activity.
A sleep EEG test is usually done in hospital, using a routine EEG machine. Before the test, you may be given some medicine to make you go to sleep. The test lasts for one to two hours or up to 8 hours and you usually go home once you’ve woken up.

Sleep-deprived EEG tests


A sleep-deprived EEG test is done when you’ve had less sleep than usual. When you’re tired, there’s more chance that there will be unusual electrical activity in your brain. Your doctor may ask you to have this test if you’ve had a routine EEG test, but it didn’t show any unusual electrical activity.
Before a sleep-deprived EEG test, your doctor may ask you not to go to sleep at all the night before, or just to wake up much earlier than you usually do.
The beginning of the sleep-deprived EEG test is the same as the routine EEG test. You then try to fall asleep or doze while the EEG is still recording the activity in your brain. The test lasts for a few hours and you usually go home once you’ve woken up.

Video-telemetry tests


You would usually only have a video-telemetry test if you have already been diagnosed with epilepsy. Here are some examples of why your doctor might ask you to have a video-telemetry test.

*- It’s not clear what type of seizures you have.
*- Your anti-epileptic drugs are not working well.
*- There’s a possibility that your seizures are not caused by epilepsy, but something else.
*- You are considering having epilepsy surgery.

During a video-telemetry test, you wear an ambulatory EEG. At the same time, all your movements are recorded by a video camera. After the test, doctors can watch any seizures that you had and see if there were any changes to your brainwave patterns at the time of the seizure(s).

If you have a video-telemetry test you need to stay in hospital. The test is usually carried out over a few days. Sometimes your anti-epileptic drugs may be reduced or withdrawn. This is to increase the chances of recording your seizures.


Can an EEG test show what type of seizures?

When an EEG test picks up unusual electrical activity, it shows the areas of your brain where it’s coming from. Each electrode picks up the activity in the part of the brain directly underneath it. If there is a generalised seizures, unusual electrical activity would be recorded from the electrodes on both sides of the brain. If there is a partial seizures, unusual electrical activity would be recorded from electrodes on certain areas of the brain.

                     More information about different types of seizure.

Will an EEG test causes a seizure?

There’s a very small risk that an EEG test would cause you to have a seizure. This could happen when you’re looking at a flashing light or breathing deeply also called Hyper ventilation (HV). The risk of having a seizure would also increase if your doctor asks you to reduce your anti-epileptic drugs or have less sleep than usual before you have an EEG test.


How many EEG Tests I have to have?

If you have an EEG test that doesn’t show any unusual electrical activity in your brain, your doctor may ask you to have another. It can be helpful, if possible, to have an EEG test at times when you’re more likely to have a seizure, for example early in the morning or, for some women, around the time of your period.

Normal Results

Brain electrical activity has certain frequencies (the number of waves per second) that are normal for different levels of consciousness. For example, brain waves are faster when you are awake, and slower when you're sleeping. There are also normal patterns to these waves. These frequencies and patterns are what the EEG reader looks for.

Note: A normal EEG does not mean that a seizure did not occur

Abnormal Results

Abnormal results on an EEG test may be due to:

An abnormal structure in the brain (such as a brain tumor)
Attention problems
Tissue death due to a blockage in blood flow (cerebral infarction)
Drug or alcohol abuse
Head injury
Inflammation of the brain (encephalitis)
Hemorrhage (abnormal bleeding caused by a ruptured blood vessel)
Migraines (in some cases)
Seizure disorder (such as epilepsy or convulsions)
Sleep disorder (such as narcolepsy)


An abnormal EEG may consist of:

Abnormal changes in normal rhythm: If asymmetrical, the side with lower amplitude is usually pathological
Abnormal slow activity: A sensitive indicator of encephalopathy if diffuse; correlate with regional cerebral dysfunction if localized; may appear as intermittent rhythmic delta (FIRDA in adult or OIRDA in children)
Distinctive abnormal pattern: Regular repetition of spikes, sharp waves, slow waves or any of the combination
e.g. PLED, burst suppression, triphasic waves, pseudoperiodic generalized sharp waves in CJD, pseudoperiodic slow complexes in HSV encephalitis

Epileptiform discharges: Spikes, polyspikes, sharp and slow waves


Factors affecting EEG interpretation

Age: maturation of EEG
Arousal: refers to different sleep stages
Medications e.g. benzodiazepines
Pathological brain condition (e.g.craniotomy)
Environment e.g. a.c. interference, ICU setting
Quality of recording – aware of artifacts

Common factors affecting EEG recording ( artifacts)


The biggest challenge with monitoring EEG is artifact recognition and elimination. There are patient related artifacts (e.g. movement, sweating, ECG, eye movements) and technical artifacts (50/60 Hz artifact, cable movements, electrode paste-related), which have to be handled differently. Common factors are:

• Electrical mains
• 60 Hz
• Radio frequency (RF) waves
• Electromagnetic interference
• Eye movement
• Head movement
• Muscle
• Sweating
• Electrode


*- Trescher WH, Lesser RP. The Epilepsies. In: Bradley WG, Daroff RB, Fenichel GM, Jankovic J, eds. Neurology in Clinical Practice. 5th ed. Philadelphia, Pa: Butterworth-Heinemann; 2008:chap 71.

*- Krumholz A, Wiebe S, Gronseth G, et al. Practice parameter: evaluating an apparent unprovoked first seizure in adults (an evidence-based review): report of the Quality Standards Subcommittee of the American Academy of Neurology and the American Epilepsy Society. Neurology. 2007;69(21):1991-2007.

*- Epilepsy Action |Medline Plus |NHS |Wikipedia |Mayo Clinic | Epilepsy.com


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See Also: Epilepsy Health Corner

See Also: Neurophysiology Health Corner

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