Page created on April 4, 2022. Not updated since.


Electroencephalography (EEG) assesses the electrical activity of the brain via electrodes attached to the scalp. Electrical activity is different across different parts of the brain, and so many electrodes are needed to assess the different areas of the brain, usually 10 – 20 electrodes. A routine EEG is 30 – 60 minutes long.

EEG is an important tool to evaluate a patient with possible epilepsy. Sometimes repeated recordings are necessary to make the diagnosis. It’s also used to determine the different phases of sleep during polysomnography, and in the evaluation of brain death. A form of EEG is used to estimate the depth of anaesthesia during anaesthesia.

However, EEG has many drawbacks:

  • Many EEG findings are neither sensitive nor specific
  • Many EEG findings only occur intermittently, and may not appear during the period of the recording
  • Some people have abnormal EEG but no disease


The background electrical activity consits of alpha, beta, gamma, delta, and/or theta waveforms, depending on whether the patient has their eyes open or not, whether they’re awake or not, and if they’re asleep, which stage of sleep they’re in. When the patient is awake with their eyes closed, alpha waves (8 – 12 hz) are present.

Superimposed on the background electrical activity are certain EEG patterns. Some are physiological, but in the evaluation of disease, the pathological EEG patterns are most important. Interictal epileptiform discharges (IEDs) are pathological EEG patterns which occur between epileptic seizures (i.e., while the patient does not have a seizure), and detection of these is important in the evaluation of epilepsy. IEDs can be sharp waves or spikes.

If an EEG is made during an epileptic seizure, paroxysmal epileptiform discharges can be seen. Certain types of epilepsy have specific epilepticform discharges. A slow spike-and-wave complex is typical for Lennox-Gastaut syndrome, for example.

Slow-wave activity and slowing of the background rhythms is nonspecific, as it can occur in both epilepsy and in case of focal structural lesions.

Special forms

  • Video-EEG – continous video and EEG recording simultaneously, allows for evaluation of the clinical features during the seizure
    • Especially good for differentiating epileptic seizure from PNES
  • Ambulatory EEG – EEG while the patient is up and walking
  • 24 hour EEG – when regular EEG doesn’t detect abnormalities
  • Sleep-deprived EEG – when regular EEG doesn’t detect abnormalities
    • EEG is made both while the patient is sleep-deprived and while they fall asleep after the deprivation
    • Sleep deprivation can provoke seizures or IEDs

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