What is an Electroencephalogram?

An electroencephalogram or EEG is a reading of a test that is done to record the electrical activity of the brain. Electrical signals are picked up and amplified by special electrodes attached to the scalp. These signals appear as a graph on a computer, which is then printed on a paper. The graph is then interpreted by a healthcare provider or a specialist to identify any unusual activity. This procedure is conducted by highly trained clinical neurophysiologists at a hospital and typically lasts for 45-120 minutes. Results are usually deciphered on the basis of waveforms, energy and responses to stimuli such as flashing lights. 

  1. Why is an EEG performed?
  2. How do you prepare for an EEG?
  3. How is EEG performed?
  4. What do EEG results indicate?

An EEG is used to interpret several brain disorders, such as epilepsy, tumours and stroke. It can also help in diagnosing Alzheimer’s disease, psychosis or sleep disorders. The primary use of an EEG is to detect and evaluate epilepsy that causes repeated seizures. An EEG will help the doctor to identify the type of triggers for better treatment. In general, it is used to determine the overall activity of brain. In certain surgical procedures, an EEG may be used to monitor blood flow. EEGs are also less commonly used to investigate dementia, head injuries, brain tumours and obstructive sleep apnoea.

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The common steps to be followed before an EEG are mentioned below:

  • Wash hair with shampoo but avoid conditioners. Do not use any hair gel or hair spray. Keep a hairbrush handy.
  • Discontinue all medications that may interfere with the procedure after permission from your healthcare provider. Do not discontinue medications on your own.
  • Avoid caffeine and caffeine products for 8 to 12 hours before the test.
  • If the procedure is to be performed during sleep, adults should not sleep more than 4 to 5 hours the previous night. Children should not sleep for more than 7 to 8 hours.
  • Avoid dieting or fasting before the test. Low sugar can influence results.

Based on the medical condition, a doctor may ask for other special preparations.

The procedure is typically done on an outpatient basis, whereas a hospital stay may sometimes be needed. Before the procedure, a technician will ask to sign a consent form to obtain permission to perform the procedure. Read the form carefully and ask questions if required.

  1. The person is asked to lie on a reclining chair or bed.
  2. Around 15 to 25 electrodes are attached to various areas of their scalp with a paste. The person must close his/her eyes, relax and be completely still during the entire procedure.
  3. After an initial recording, the individual is exposed to several stimuli, such as deep breathing or light flashing, to produce certain brain activities. If the person is being evaluated for a sleep disorder, the EEG may be done during sleep.
  4. After the test,  electrodes are removed and the paste is washed with warm water or acetone. It is recommended to wash hair after this procedure. If sedatives have been administered, the person should wait until the effect has worn off. After the EEG, the person is discharged and normal activities can be resumed.
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An EEG only records the electrical activity of brain. It does not help diagnose physical disorders of brain. EEG results need to be further interpreted by specialists to identify a condition. EEG picks up unusual electrical activity from specific areas of brain and it also shows if you are having a seizure during the test. However, it may not show focal seizures. Several types of epilepsies can be identified by the unusual activity in brain in an EEG during a seizure. However, if the EEG does not show any unusual activity during the test, it simply means there was no epileptic activity during that time. It does not prove that the person does not have epilepsy.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is purely from an educational perspective and is in no way a substitute for medical advice from a qualified doctor. 


  1. Epilepsy Society [internet]. Buckinghamshire, UK; EEG (Electroencephalogram)
  2. National Health Service [internet]. UK; Electroencephalogram (EEG)
  3. Johns Hopkins Medicine [Internet]. The Johns Hopkins University, The Johns Hopkins Hospital, and Johns Hopkins Health System; Electroencephalogram (EEG)
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