What is electroencephalogram?

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Electroencephalogram (EEG) is a test that evaluates the electrical activity of nerve cells (neurons) in the cerebral cortex (the nerve tissue that lines the brain). It is a neurophysiological test that detects and measures the function of neurons from an electrical point of view, using electrodes that are placed on the person's scalp.

This test means the brain activity of neurons in the two cerebral hemispheres and their four lobes (frontal, temporal, parietal and occipital) can be detected. It cannot detect the activity of neurons in the cerebellum, the brainstem, or the deep areas of the brain. Brain cells communicate through electrical impulses and are always active, no matter whether the person is awake or asleep. This electrical activity manifests itself in the form of waves of varying intensity: low, moderate and high.

Why it is recommended?

Electroencephalogram is one of the main tests used to diagnose epilepsy. In patients with epilepsy, electroencephalogram often detects abnormal figures that stand out against the normal alpha and beta waves. The EEG serves to show whether the epilepsy is generalised.

Abnormal brain waves may appear in both hemispheres and all four lobes, or only in one lobe of one of the two hemispheres. In addition to being used for diagnosis, this test can be used for clinical follow-up or for the withdrawal or introduction of anti-epileptic medication in patients with epilepsy.

EEG can also be used to detect multiple brain disorders such as:

  • Non-convulsive status epilepticus
  • Convulsive status epilepticus
  • Specific epileptic syndromes, such as absence seizures
  • Dissociative seizures: epileptic seizures without dysfunction of the nervous system
  • Lennox-Gastaut syndrome: a type of childhood epilepsy
  • West syndrome: epileptic encephalopathy that begins in the first year of life
  • Rolandic epilepsy: infantile focal epilepsy
  • Encephalopathies of any origin (metabolic, infectious, toxic)
  • Herpes encephalitis: severe infection of the central nervous system
  • Creutzfeldt-Jakob disease: a degenerative disorder that progresses rapidly and causes dementia
  • Comas of uncertain origin
  • Cerebral hypoxia: a lack of oxygen to the brain  

Electroencephalogram is also performed to diagnose brain death. This is usually confirmed by a ‘flat EEG’, which does not react to painful stimuli or intermittent light stimulation.

What preparations are needed?

In the days leading up to the test, the patient should:

Correct blue and green pharmaceuticals

Take their usual medication, without changing the frequency or dosage (this is especially important in the case of anti-epileptic drugs).

Following a routine

Maintain a regular schedule

Drug crossed out

Avoid introducing new medicines

Cannabis, pills and syringe crossed out

Avoid using drugs

Two beer mugs crossed out indicating that alcohol is forbidden.

Avoid consuming excess alcohol

Strikethrough coffee

Avoid caffeine intake

A person taking a shower

Wash their hair the night before the day of the study. The patient should not use hairspray or gel. These hair products can make it difficult for the electrodes to adhere to the scalp properly, causing them to fall off during the test.

Person with insomnia

Occasionally, the doctor will ask for the EEG to be done after a day's sleep deprivation. This means that the patient should not sleep the night before the test. In these cases, the test is performed first thing in the morning.

How it is performed?

To perform an EEG, the proper equipment and a technician skilled in the technique are required.

The equipment consists of:

  • A tape measure
  • Thick-tipped coloured pencils
  • Cotton swabs
  • Surface electrodes
  • Scalp cleansing paste
  • Conductive paste
  • Gauze
  • Input box for a signal amplifier
  • Digital acquisition and record review device

The electrodes should be positioned so as to allow electrical activity to be recorded efficiently, and with minimum distortion. They are placed on the scalp so as not to cause pain or become displaced throughout the entire test. Their positioning should cover all regions of the cerebral cortex and the midline. 21 electrodes are put in place, and each one is given a name (frontopolar, frontal, central, parietal and occipital) and a location (left, right, midline).

Positioning takes 15-30 minutes to ensure that the electrodes are placed correctly and symmetrically on the anatomical areas. An electrode is placed on each earlobe and to the side of the eyes. This allows an electrooculogram to also be carried out, to assess the condition of the retina. It is very useful for the EEG recording to be synchronised with an audio-visual recording, in order to detect abnormal behaviour.

The test usually takes a minimum of 30 minutes, and the patient tends to be lying on a bed, or sitting on a comfortable sofa. They are asked to open and close their eyes and take quick, deep breaths for 3 minutes. At the beginning or at the end of the test, a high-intensity light will be placed in front of the eyes and flashes of light will be produced on the closed eyes at different frequencies.

Will I be able to take my medication?

Yes. The test should be performed under the patient's typical conditions, so that it is representative of what has been happening to the patient recently. This is why the patient should continue taking their usual medicines (the same number of pills at the same times), especially those used for epilepsy.

What happens during the test?

Before the test, the technician will measure the patient's head and mark specific points on the surface of the scalp with a special thick-tipped pencil to indicate where the electrodes should be placed. They will also apply gel to the scalp to place the electrodes, as well as a conductive paste, to ensure that they adhere correctly, and to improve the quality of the recording.

During the test, the electrodes are connected to the input box that amplifies the brainwave signal using conductor cables, and the recording is made on a computer.

The patient is invited to relax and close their eyes in a comfortable position, on a bed or armchair. The technician will ask them to open and close their eyes at different points. They will be asked to breathe quickly and deeply for 3 minutes.

A light will be placed in front of the patient's eyes. This light emits intermittent bursts at different frequencies for 2 minutes, and they will be asked to open their eyes between bursts. If the patient was deprived of sleep the night before, the intensity of the light in the room will be reduced and the patient will be invited to relax and fall asleep in bed.

In special situations, depending on the patient’s characteristics and the recording, they might be asked to perform some simple calculations, read a paragraph, look at a fixed image or open and close their hand.  

The whole test is supervised by a technician who will be responsible for the quality of the recording and will take note of any incident. The patient is asked to notify the technician if they experience any abnormality during the recording.

How long does it take?

Conventional EEG recording usually takes 30 minutes. Preparing the patient for the test also takes about half an hour.

Sleep-deprived EEG recording has a duration of 60 minutes. It consists of two parts. The first part is the same as the conventional EEG. In the second part, the light in the room is dimmed and the patient is invited to sleep for 30 minutes. In these cases, intermittent light stimulation can be performed at the end of the recording.

The EEG recording for the diagnosis of brain death should last 30 minutes. It involves intermittent light stimulation on two occasions and the application of painful stimuli to the patient, to confirm that the recording does not change. In special situations, the doctor may request an EEG recording for several hours or for several days (EEG monitoring). In these cases, a solution should be applied to the scalp to ensure that the electrodes do not fall off.

What happens after the electroencephalogram?

At the end of the test, the technician carefully removes the electrodes with gauze and alcohol, in order to not damage the patient's scalp. During this period of time, which usually lasts 5-10 minutes, the technician will ask the patient if they noticed anything abnormal during the test. The patient can then leave the hospital.

The recording is stored digitally, and will be reviewed by a doctor who will produce a report. The report should include the patient's name, age and sex, the medication they take, the reason for the consultation, the date of the test, and the test report. The technical conditions, the results, their interpretation, and the conclusions should all appear in this report. The EEG report is sent to the doctor who requested the test, and the most appropriate action will be taken in the context of the individual patient, in accordance with the results.

What are the possible complications?

Electroencephalogram is a safe test that has been used for more than 100 years. During an EEG, the electrodes do not transmit any sensation, nor do they cause pain or emit discharges. They only record the brain waves. The adhesive gel and the conductive paste do not cause any alterations or allergies. If the electrodes have not been placed delicately enough, it is possible that they may cause slight irritation on the scalp, which will disappear after a few hours.

Special situations

EEGs allow epileptic activity to be detected. Therefore, provocation techniques are performed during the test, such as hyperventilation for three minutes, intermittent light stimulation and sleep onset after sleep deprivation the previous night. These techniques can provoke epileptic seizures, which means it is possible that the patient may experience seizures or loss of consciousness during the test. If this happens, the technician must press an alarm button to alert a doctor.

The first thing that should be done in the event of an epileptic seizure, is to act professionally and remain calm, as the patient usually fully recovers within minutes after the seizure. Objects that could hit the patient should be removed, and tight clothing (belt, shirt collar, tie) should be loosened. No objects should be put between the teeth. Once the convulsions have ceased, it is advisable to lay the patient on their side. This means that if vomiting occurs, it will not remain in the mouth or pass into the respiratory system. It is also important to ensure that the airways are clear. It is necessary to observe what happens during the seizure (head deviation, sequence of movements, etc.). During an EEG, the patient is routinely videotaped to allow subsequent review of the characteristics of the seizure and correlation with the simultaneous EEG recording.

Substantiated information by:

Alejandro Iranzo de Riquer

Published: 27 April 2022
Updated: 27 April 2022


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