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Electroencephalography (EEG) – Explained

Electroencephalography (EEG) is a diagnostic method used to measure and record the electrical activity of the brain. It is commonly used in the diagnosis of epilepsy and sleep disorders.

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Things worth knowing about "Electroencephalography"

Electroencephalography (EEG) is a diagnostic method used to measure and record the electrical activity of the brain. It is commonly used in the diagnosis of epilepsy and sleep disorders.

What is Electroencephalography?

Electroencephalography (abbreviated as EEG) is an electrophysiological monitoring method used to record the electrical activity of the brain. It detects voltage fluctuations produced by the activity of neurons (nerve cells). The EEG is one of the most important diagnostic tools in neurology and sleep medicine.

How Does an EEG Work?

During an EEG, multiple small electrodes are attached to the scalp using a conductive gel or an electrode cap. These electrodes detect tiny electrical signals generated by the cerebral cortex. The signals are amplified and displayed as brain waves on a screen or paper printout.

The recorded brain waves are categorized into different frequency bands:

Delta waves (0.5–4 Hz): deep sleep, coma
Theta waves (4–8 Hz): light sleep, relaxation
Alpha waves (8–13 Hz): resting state with eyes closed
Beta waves (13–30 Hz): active thinking, concentration
Gamma waves (>30 Hz): cognitive processing, perception

When is an EEG Used?

The EEG is used in a wide range of clinical situations. The most common indications include:

Epilepsy: detection and classification of seizures and epileptiform patterns
Sleep disorders: diagnosis of sleep apnea, narcolepsy, and other sleep conditions (polysomnography)
Disorders of consciousness: assessment of coma or suspected brain death
Encephalitis and encephalopathies: detection of diffuse or focal brain dysfunction
Dementia evaluation: support in diagnosing certain dementias, such as Creutzfeldt-Jakob disease
Intraoperative monitoring: real-time monitoring of brain function during neurosurgical procedures

How is an EEG Performed?

A standard EEG examination typically takes 20 to 40 minutes. The patient sits or lies in a relaxed position while the electrodes record brain signals. Activation procedures are often used to provoke abnormal patterns:

Hyperventilation: deep, rapid breathing for several minutes
Photic stimulation: flashing lights at various frequencies
Sleep deprivation: deliberate lack of sleep before the examination

For specific clinical questions, extended methods are available, such as long-term EEG (recording over several hours to days), ambulatory EEG, or video-EEG (simultaneous video recording to observe seizure behavior).

What Do the Results Mean?

A normal EEG shows age-appropriate background activity without abnormalities. Pathological findings may include:

Epileptiform discharges (spikes, sharp waves, spike-and-wave complexes): indicator of epilepsy
Diffuse slowing: indicator of generalized brain dysfunction
Focal slowing: indicator of localized brain damage
Suppression or flat line: possible indicator of deep coma or brain death

Safety and Risks

The EEG is a non-invasive and painless procedure with no radiation exposure. There are no significant known risks. The electrodes only record brain activity and do not transmit electrical current into the brain. In sensitive patients, the electrode gel may cause mild skin irritation.

References

Niedermeyer E., da Silva F. L. (Eds.): Electroencephalography: Basic Principles, Clinical Applications, and Related Fields. 5th edition, Lippincott Williams & Wilkins, 2005.
Rosenow F., Luders H. O.: Presurgical evaluation of epilepsy. Brain, 124(9):1683–1700, 2001. PubMed PMID: 11522572.
World Health Organization (WHO): Epilepsy – Fact Sheet. www.who.int/news-room/fact-sheets/detail/epilepsy (2023)

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