Electroencephalography (EEG), a diagnostic test invented over a century ago, is still widely used today in the evaluation of patients with paroxysmal neurologic disorders such as seizures and epilepsy. Although brain electrical activity is very low in voltage (on the order of microvolts) in comparison with ambient noise (on the order of volts), EEG uses the technique of differential amplification to cancel out noise and increase the amplitude of the waveforms of interest. EEG compares the voltages recorded from two different brain regions and plots this result over time. A standard array of metal electrodes is placed on the scalp of the patient, and over a 30-minute period, brain electrical activity sampled from different regions of the cortex is recorded simultaneously. EEG thus provides both spatial and temporal information about brain activity.
In the past, EEG was recorded on paper, and the electrical activity was displayed in a static manner. Today, the activity is recorded digitally, allowing the data to be displayed in multiple ways after the recording has been completed. EEG recordings use standard montages, which allow the comparison of recordings from individual electrodes with either adjacent electrodes or distant electrodes (Figure 1–1). Montages provide a means of viewing the data in an organized fashion; some montages enhance localized findings, whereas others highlight global or diffuse findings.
Two commonly used EEG montages: longitudinal bipolar and transverse bipolar. (C = central; F = frontal; Fp = frontal polar; O = occipital; P = parietal; T = temporal. Odd numbers denote “left”-hemisphere electrodes and even numbers denote “right”-hemisphere electrodes.)
For routine outpatient EEGs, an ideal recording environment is quiet, allowing the patient to achieve relaxed wakefulness and to fall asleep (Figure 1–2). During the EEG recording, hyperventilation (having the patient exhale repeatedly and deeply for 180 seconds) and photic stimulation (strobe light flashes for 10 seconds at a time, at different frequencies ranging from 1–25 Hz) are also performed, as both techniques can elicit abnormal EEG activity in certain patients.
Normal awake EEG of a 7-year-old child (longitudinal bipolar montage). This 11-second epoch is presented using the longitudinal bipolar montage with the first four channels representing the left parasagittal electrodes and the next four channels representing the right parasagittal electrodes. Channels 9 through 11 are left temporal electrodes; channels 13 through 16 are right temporal electrodes. Channels 17 and 18 are over the vertex of the head. Note the V-like deflections in the bifrontal channels, which are secondary to eye blinks and the 8–9 Hz “alpha” rhythm in the occipital channels.
The EEG has multiple clinical applications. It can be used to confirm the diagnosis of seizures or epilepsy, either ...