How is neonatal electroencephalogram (EEG) performed?

Updated: Nov 08, 2018
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While the principles of electroencephalography are the same in neonates as in older children and adults, successfully recording and interpreting neonatal EEGs require additional skills. Considerable experience and patience are required to obtain technically satisfactory EEGs from neonates in a busy intensive care nursery. Sick neonates often have multiple organs being monitored by a variety of devices that can lead to unusual artifacts and may make reaching the infant's head difficult.

The number of electrodes used in recording the neonatal EEG is reduced owing to the small head circumference of the newborn. A standard 10-20 system is used with a single combined longitudinal and transverse montage. The frontal-temporal, frontal-central, temporal-occipital, and central-occipital longitudinal measurements are double-distance recordings. Instead of each interelectrode distance being 20% of the diameter of the head, they are 40%. Transverse and midcentral interelectrode distances are the standard 20% distances. See the image below.

Electrode map and montage. Electrode map and montage.

Additional physiological leads that consist of 2 eye-movement monitors, electromyogram (EMG), and ECG and respiratory monitor electrodes (chest wall and/or nasal airflow) are placed to aid in determination of state. One eye electrode is placed 0.5 cm below the outer canthus, and the other electrode is placed 0.5 cm above the outer canthus of the other eye. Differential field effects of the retina-to-cornea dipoles recorded in these opposing electrodes provide data on the types of eye movements. EMG electrodes are useful in differentiating subcortical or peripheral myoclonus from movements associated with epileptiform activity recorded at the surface of the brain.

Recording settings are chosen to acquire the necessary spectrum of data and to eliminate artifact. A time constant of 0.3 seconds typically is chosen to record the slower frequencies present in the neonatal EEG. A low-pass filter of 70 Hz is recommended. Filtering of high-frequency muscle artifact with 35 Hz (or other low-pass filter) is not recommended because EEG activity can be distorted and may be misinterpreted as sharp activity. Sensitivity typically is set at 7 µV/mm and adjusted, if necessary. ECG, EMG, and eye electrodes are set at sensitivities of 50 µV/mm, 50 µV/mm, and 7 µV/mm, respectively, and adjusted as necessary.

The EEG test typically is run for 60 minutes or more to ensure the recording of at least one change in sleep state (a full sleep cycle in the neonate is typically 50-60 min).

Traditionally, neonatal EEGs were performed on paper EEG writers, and a consensus regarding paper speed was not established. Recording speed often was chosen to be half the speed of the typical adult record (ie, 15 mm/s instead of 30 mm/s).

Detection of asymmetries or asynchronies may be enhanced by slow speed. This becomes less of an issue as digital EEG acquisition and review allows for postrecording change of the time base of the recording. Review at different speeds is available, when necessary. Unfortunately, waveform morphology is different in records that are recorded at different paper speeds. At 15 mm/second, sharp waves and spikes are half the width in the EEG record and look sharper. Sharp waves and spikes must stand out from the background, be sharply contoured, and be 70-200 milliseconds (ms) and 20-70 ms in duration, respectively.

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