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An EEG is a test that detects abnormalities in your brain waves, or in the electrical activity of your brain.

An electroencephalogram (EEG) is a diagnostic test that detects electrical activity in our brain using small, metal discs (electrodes) attached to the scalp. Our brain cells communicate via electrical impulses and are active all the time, even when we are sleeping. The EEG machine captures the electrical activity which is amplified and recorded as a graph on a computer screen, or may be printed out on paper. The healthcare provider can then interpret this reading to conclude the diagnosis of the treatment. 

Why it is done and what it can diagnose?

The EEG shows patterns of normal or abnormal brain electrical activity. Each wave corresponds to a particular area of the brain. The healthcare professional can use this to diagnose the patient’s condition.  

Abnormal patterns may occur with a number of different conditions like head injury, stroke, brain tumour, encephalitis, sleep disorders 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. They can look like spikes, sharp waves, and spike-and-wave discharges.

An EEG might also be used to confirm brain death in someone in a persistent coma. 


The patient relaxes in a reclining chair or lies on a bed, with eyes closed. They are asked to stay still, with no movements, such as swallowing or blinking, throughout the test. At various times, the technician might ask them to open and close their eyes, perform a few simple calculations, read a paragraph of text, look at a picture, breathe deeply for a few minutes, or look at a flashing light.

The technician will attach between 16 and 25 electrodes to the scalp with a special paste. Sometimes they will put a cap containing the electrodes on the patient.

The recording may be stopped periodically to let the patient rest or reposition.

This study is generally done by an EEG technician and may take approximately 45 minutes to 2 hours.

What you can expect

A technician measures and marks the scalp with a special pencil. They might scrub it with a gritty cream to improve the quality of the recording. Discs (electrodes) are attached to scalp using a special adhesive which has to be washed off after the test is complete.

The preparation required 

Normally, healthcare provider will explain the procedure in detail. After which the patient will be asked to sign a consent form.

The common instructions given are: 

  • Wash your hair with shampoo, but do not use any hair care products.
  • Take your usual medications unless instructed otherwise.
  • Avoid consuming anything with caffeine for 8 to 12 hours before the test.
  • Avoid fasting the night before or the day of the procedure. Low blood sugar may influence the results.
  • Based on your medical condition, your healthcare provider may request other specific preparations.

What happens after and how long to get the results

EEG is a safe procedure. Once the test is complete, the technician removes the electrodes and washes off the sticky paste with warm water. 

Generally, there are no special instructions for after-care. However, you will need to wash your hair thoroughly to remove all traces of gel and other fluids.

Are there any associated risks?

EEGs are safe and painless. The test causes no discomfort. In rare instances, an EEG can cause seizures in a person with a seizure disorder. This is due to the flashing lights or the deep breathing that may be involved during the test. If the patient does have a seizure, the healthcare provider will treat it immediately.

Other risks may be present, depending on your specific medical condition. Certain conditions may interfere with the reading of an EEG test. These are low blood sugar (hypoglycemia) or certain medicines, such as sedatives.

Skin irritation or redness may be present at the locations where the electrodes were placed, but this will wear off in a few hours.

Frequently asked questions

Can an abnormal EEG be wrong?

Misdiagnosis of epilepsy is the tendency to over-read normal tracings as abnormal. Reasons for the common over-interpretation of normal EEGs are mostly related to the lack of standards or mandatory training in EEG. It is also true that a normal EEG result does not always rule out epilepsy. This is because some people with seizure disorders have normal EEG readings between seizures.

Can anxiety cause abnormal EEG?

Long-term anxiety and panic attacks can cause your brain to release stress hormones on a regular basis. This can increase the frequency of symptoms such as headaches, dizziness, and depression. All of which can alter the EEG.

How long does an EEG test take?

A routine EEG recording usually takes 20 to 40 minutes. Although a typical appointment will last about an hour, including some preparation time at the beginning and some time at the end.

What does an abnormal EEG indicate?

An abnormal EEG means that there is a problem in an area of brain activity. This can offer a clue in diagnosing various neurological conditions.

Why no  caffeine before EEG?

Drinks containing caffeine, such as coffee, cola, and tea can occasionally alter the EEG results. However, this almost never interferes significantly with the interpretation of the test. Studies shows that caffeine can prevent a decline in arousal level and changes in EEG spectral parameters that occur spontaneously during rest.

How long do EEG results take?

This study is generally done by an EEG technician and may take approximately 45 minutes to 2 hours. 

Is EEG test painful?

You’ll feel little or no discomfort during an EEG . The electrodes don’t transmit any sensations.

How can some drugs affect the electroencephalogram eeg?

The effects of drugs on the EEG vary and are often dose dependent. In general, the way drugs affect the EEG are fairly predictable. For example the effect could be accentuation of electrical activity, decreased seizure activity, and lowering of the seizure threshold. Sometimes there is no effect at all. In severe overdose cases, the EEG may demonstrate burst, suppression, or even a “flat” EEG pattern.