EEG is a method used to measure the brain’s electrical activity. It records the natural voltage fluctuations of the cerebral cortex.

Metal plates (electrodes), usually 21, are placed on the scalp. The electrical voltage differences between each pair of electrodes are measured in various combinations, amplified by a special device, and finally recorded as brain waves. This electrical voltage is generated by the nerve cells themselves and is a sign of their activity and functionality.

The doctor evaluates the brain waves based on their frequency, amplitude, steepness, and location on the brain’s surface. The symmetry between the two hemispheres of the brain is also important.

EEG can detect pathological changes in the brain’s electrical activity. These include:

• Epilepsy-specific potentials: These are characteristic EEG changes that can occur in individuals with a tendency to experience epileptic seizures (brain seizure tendency). EEG can assist in diagnosing epilepsy. However, it often shows no abnormalities between seizures. Therefore, it may need to be performed multiple times and under conditions that increase seizure susceptibility (e.g., sleep deprivation the night before the examination).
• General changes in the EEG pattern: These affect the entire cerebral cortex and can occur, for example, in inflammatory brain diseases (encephalitis), metabolic disorders, or intoxications. They can provide clues about the severity of the disease.
• Focal findings: These are localized changes in brain wave activity that occur mainly in local brain diseases such as stroke, tumors, or localized inflammatory processes.

Today, EEG is most commonly used in the diagnosis when epilepsy is suspected.

Usually, 21 electrodes are evenly placed on the patient’s scalp to record brain activity. To keep them from slipping, the electrodes are hooked into a cap that covers the forehead and the hair-covered part of the head like a hairnet.

A routine resting EEG recording takes about 30 minutes. Sometimes, this resting recording with closed eyes is supplemented by a flashing light test, in which the patient must look at a rapid light–dark sequence (stroboscope). This can reveal a previously hidden tendency toward epilepsy. In special cases, a 24-hour EEG recording may be necessary. The EEG examination is safe, painless, and can be repeated as often as needed.