Electroencephalogram (EEG): what is it and what is the exam for?


What is electroencephalogram?

Electroencephalogram (EEG) is a non-invasive monitoring exam that records the electrical activity of the brain. It is carried out with electrodes fixed on the scalp by means of an electrically conductive paste. It aims to record brain activity to detect possible neurological abnormalities.

What is the exam for?

The test is best used to detect a variety of psychiatric and neurological diseases, both infectious and degenerative, such as:

  • Epilepsy;
  • Problems with loss of consciousness or dementia;
  • Encephalitis (inflammation or infection of the brain);
  • Brain tumors;
  • Brain injuries;
  • Hemorrhage (excessive bleeding);
  • Headache or other types of migraine;
  • Cerebral edema (partial or whole swelling);
  • Apneas and other sleep disorders;
  • Excessive alcohol or drugs.

In addition, the EEG also detects the degree of certain comas or brain death, a condition in which the brain loses its functionality. It also helps to detect when a person has a physical disability or mental health problems , which include problems with the brain, spinal cord or nervous system.

Who is it indicated for?

The electroencephalogram can be performed on anyone, being indicated in cases of suspected abnormality in brain activity, evidence of epilepsy and evidence of impaired consciousness.

Because brain activity does not change with age or sex, this test can be performed at all stages of life. It can also be performed during pregnancy, as it is not invasive.

Types of electroencephalogram

It is important to note that there is not just one type of EEG and that each has its own specialties. Understand:

Routine EEG (normal)

Electrodes are placed on the patient’s scalp and their brain activity is recorded during some activities. The routine electroencephalogram can last from 20 to 40 minutes and the patient may be asked to close and open his eyes, perform breathing exercises such as inhaling and exhaling rapidly (hyperventilation), or a pulsating light can be placed in front of the patient in order to see the change in brain activity.

EEG in sleep or wake

Sleep EEG is a procedure in which the person spends the night in the hospital, used to detect possible sleep disorders (apneas, narcolepsy ). The waking test is used to diagnose spontaneous actions in the brain’s behavior (epilepsy, dementias, brain tumor).

The patient may be deprived of sleep one night prior to the exam, to ensure that he / she sleeps for sufficient hours to capture brain activities on the sleep electroencephalogram (also called sleep-deprived EEG).

Sleep EEG with photo

The EEG with photo or video serves to have more information about brain activity, so the doctor can know what happens before, during and after a possible seizure. It also helps to perceive the area of ​​the brain affected during an attack.

It is performed in a period that usually varies between 3 and 7 days, in which electrodes are placed on the patient during sleep and waking (awake). The video is recorded by the computer and kept under surveillance.

EEG with brain mapping

Brain mapping is a new resource used by doctors. In this examination, the electrodes placed on the scalp and the signals are transmitted to the computer, which transforms them into a map of colors and signals from brain activity.

It is used to perceive specific places in the brain when a change has been detected. There are still several studies to improve the use of brain mapping.

Ambulatory EEG

In this examination, the electrodes are connected to a small portable EEG device that can be attached to clothing. The patient’s activities can happen normally while it is being recorded, but care is needed not to get the device wet.

The outpatient examination works by monitoring brain activity throughout the day. In this modality, fewer electrodes are placed and the result may be lower than the routine or the one performed in the hospital.

Extended EEG

During the examination, there may be cases in which the patient has an epileptic seizure or other types of complications, which may be related to the alteration of the brain’s electrical waves. In these situations, the doctor responsible for the exam may prolong the exam for a few hours, depending on the severity of the complication. This case is called a prolonged EEG or hourly prolonged EEG.

How to prepare for the exam?

Patients using drugs that affect the central nervous system, such as anticonvulsants, stimulants or antidepressants, may be recommended to discontinue use for a day or two before the test is performed (seek medical advice).

It is also recommended that you do not eat foods or drinks that contain caffeine.

The patient must make sure to wash, comb and dry the hair before the exam, as it helps the electrodes to be fixed more easily.

Depending on the purpose of the examination, the patient may still be instructed not to sleep the night before in order to obtain more accurate results while sleeping. Therefore, the patient should not drive and it is recommended to ask someone to take him or her to the examination site. It is also important to go well fed.

There will be complications for the exam if the patient:

  • Have a hair prosthesis (megahair);
  • You have applied hair dye in less than 7 days;
  • Presenting post-surgery dressings in the head region;
  • You have had alcoholic beverages in the last 24 hours;
  • You have been drinking caffeine in the last 12 hours.

How is the exam done?

EEG is painless, fast and safe. The patient is asked to lie down (or sit) on a reclining stretcher. The neurophysiologist doctor first measures the patient’s head and marks with a special pencil some points where the electrodes will be inserted.

Then the electrodes are connected, with a paste, to the scalp to record brain activity. The paste, in addition to fixing the electrodes, allows a better capture of electrical signals from the brain.

The electrodes are connected to a computer that translates these brain activities into ripples that can be recorded on digital graphs or on sheets of paper.


In children who are reactive to the exam, resources such as sedation are used. In this case, the procedure is done during induced sleep, and then the child is awakened to perform the EEG while awake.


Since it is not an invasive exam, the electroencephalogram can be performed by pregnant or lactating women.


In patients with epilepsy, the seizure tends to be induced in order to be able to check its degree. For this, breathing exercises (hyperventilation) and flashing lights are used.

Does electroencephalogram hurt?

The electroencephalogram is a painless exam, but people with long hair may feel uncomfortable with the paste used.


In most cases, the examination is not risky. After the exam, the patient may feel drowsy due to lack of sleep, which can lead to traffic or work accidents. He may prefer someone to pick him up at the hospital. However, you will be able to carry out your daily activities normally.

Exam duration

The length of the exam depends on your purpose. While the routine type can be fast, depending on the patient’s needs, sleep or wake EEG requires the patient to spend more time in the hospital. Understand:

  • Routine EEG: 20 to 40 minutes;
  • EEG in sleep or wakefulness: 8 to 12 hours;
  • Ambulatory EEG: The duration may depend on the doctor’s recommendation.

In most cases, the exam is short-lived.

Post-examination recommendations

After the data is captured, the electrodes are removed. This procedure is painless, but the scalp may have some remnants of the paste used. It is recommended that the hair is washed.

After the examination, it is recommended that the patient go home and rest, as sleep deprivation makes people very irritable and unable to perform daily actions.

The electroencephalogram does not have any side effects, but care is redoubled in patients who have been sedated. In these cases, it is recommended to go to the place accompanied, as the patient will not be able to drive or return home alone.

Patients who have been deprived of regular medications should wait for the authorization of the doctor or nurse responsible to continue the medications.


There are few contraindications, which can be classified between:


Because the test is non-invasive, there are no absolute contraindications. Brain activity does not change throughout the patient’s life, so the test can be performed by all ages, from newborns to the elderly.


The test may be contraindicated for people who have excessive seborrhea, skin infections on the scalp or pediculosis ( lice ).


Results tend to come when the exam is over or the next day. These must be taken to the doctor quickly, so that he can make the correct evaluation.

There are several types of brain waves:

  • Alpha: When the patient is awake and with eyes closed, but mentally alert, the brain produces these waves. They go away when the patient opens his eyes or is concentrated;
  • Beta: They are present when the patient is alert or taking certain medications, such as benzodiazepines;
  • Delta and Theta: Appear when the person is sleeping and, sometimes, in awake children.


During wakefulness, normal results usually show high frequency, low amplitude brain waves. In sleep, they can vary considerably in frequency and amplitude, depending on the stage.

In waking adults, alpha and beta waves are produced. Similar patterns of electrical activity are recorded on both sides of the brain. In addition, there are no abnormal changes in the frequency and amplitude of the waves.

When pulsating lights are used, a response from the occipital region (visual cortex) is expected, but there are no changes in brain waves.

Normal results do not prevent the diagnosis of epilepsy ( seizures ) in the future.


In abnormal results, the person presents waves with altered frequency and amplitude during waking. In addition, when sleeping, it may present waves that are not consistent with your sleep stage.

Changes in the amplitude and frequency of the waves, called peaks, can indicate lesions, tumors, infections, strokes , and epilepsies.

If the person has differences in the patterns of electrical activity on both sides, it can mean problems in one of the wolves.

Depending on the affected area, you can track seizure types and where it starts. In addition, when the changes affect the entire brain, they can mean drug intoxication, encephalitis , metabolic disorders.

By identifying delta and theta waves in awake adults, you can point out a brain injury or disease. However, medications can cause this change.

When there is no electrical activity, loss of brain function is configured. This can occur due to lack of oxygen and blood flow, comas or drug sedation. In such cases, the graphical representation is a straight line.

What can change the result?

Some situations may cause the results of the exam to be not entirely accurate. Are they:

  • Excessive physical activity;
  • Fast;
  • Scalp infections;
  • Certain types of medication such as anticonvulsants, stimulants or antidepressants;
  • Being unconscious due to the abuse of drugs or alcoholic beverages;
  • Hypothermia (low body temperature);
  • Dirty, oily hair or the use of sprays and creams (which can impair the adhesion of the paste used);
  • Ingestion of drinks or foods that contain caffeine (such as coffee, cola soda, tea).

The duration of the exam can also influence the result of the exam, since in 20 minutes it cannot present any significant alteration.


Normally, there are no complications in the application and performance of the electroencephalogram.

In patients with a tendency to epilepsy, a crisis may occur during the examination, especially when photo-stimulation is performed. However, if this occurs, the patient will be seen immediately and will be in a location with medical assistance. In general, in non-epileptic patients, nothing happens.

What is the price of the electroencephalogram?

The amount to perform the exam depends a lot on the health plan or the type of EEG. The price can vary from 70.00 to 500.00 reais, with the EEG with brain mapping the exam with the highest value.

To take the exam through the Unified Health System (SUS), authorization and clearance from the Municipal and State Health Secretariat is required.

Electroencephalogram is a very common and painless exam. Share it with your friends to make them aware of everything that happens when taking this exam.

Any questions, contact us and we will respond!