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Electroencephalography (EEG) records the electrical activity generated by the brain. It is a safe, readily available, and inexpensive tool to provide real-time information on brain functioning. It can provide valuable clinical information on patients with focal brain lesions, coma, and encephalopathy. It is an essential test for the evaluation and management of patients with epilepsy. It may provide definitive diagnostic information on patients with seizure-like episodes or alteration of the level of consciousness that is psychogenic in nature.

Prolonged EEG monitoring is a valuable tool in the hospital setting. Video-EEG monitoring is used for the characterization of spells of indeterminate mechanism as to whether epileptic or nonepileptic, for seizure-type classification, for the localization of a seizure focus in the evaluation of candidates for epilepsy surgery, and for characterization of the interictal epileptiform discharges. Prolonged EEG monitoring in the ICU setting is able to recognize subtle or electrographic seizures and is an essential tool in the management of status epilepticus.


Electroencephalography EEG records the electrical activity of the brain, measuring the spatial distribution of voltage fields on the scalp and their variation over time. The activity detected is considered the result of the sum of excitatory and inhibitory postsynaptic potentials generated primarily in the apical dendrites of the pyramidal cells in the outer layers of the cerebral cortex (mainly layers III, IV, and VI). The cortical activity is modulated by subcortical structures such as the thalamus and other brainstem centers, mainly through the ascending projections of the reticular activating system. The cortical activity is synchronized by these subcortical pacemakers in either physiologic (ie, sleep) or pathologic (ie, generalized spike-wave discharges) states. EEG, by providing continuous, real-time information on the fluctuations of brain electrical activity, is the only readily available, practical, and inexpensive ancillary test to assess brain function in the hospital setting.

The EEG activity is recorded by a set of 21 electrodes placed on the scalp in accordance to the International 10–20 system. These electrodes are distributed strategically to cover all areas of the cortex accessible from the scalp. The EEG hardware consists of multiple differential amplifiers (channels) that record differences in potential between two electrodes. The information from these multiple channels is arranged in a specific order called a montage, providing information on the topography of voltage differences. Multiple montages allow for a systematic visualization of the field of electrical activity of the brain. The EEG is graphically displayed in multiple channels (typically 21–24), each carrying information from different locations of the scalp overlying the cerebral cortex. This activity is analyzed in terms of its frequency, voltage, morphology, and topography.

It is important to recognize that the scalp EEG has significant limitations. Large areas of the cortex, such as the interhemispheric or basal regions, are located far from the surface and beyond the detection ...

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