The electroencephalogram (EEG) is a non-invasive medical test that measures the electrical activity produced by the brain. In newborns, this procedure is known as a Neonatal EEG (nEEG) and is a fundamental tool for evaluating neurological function, especially in the Neonatal Intensive Care Unit (NICU). The nEEG captures electrical impulses, providing a functional assessment of the newborn’s brain health and maturity.
The Clinical Necessity of Neonatal EEGs
Physicians order an nEEG when there is concern about a newborn’s neurological status. The test is frequently used to evaluate infants who have experienced conditions known to affect the brain, such as Hypoxic-Ischemic Encephalopathy (HIE)—brain injury resulting from a lack of oxygen or blood flow. The EEG helps determine the severity of this injury and provides information for predicting the long-term neurological outcome.
The primary role of the nEEG is the detection of seizures, which can be particularly subtle or silent in newborns. Unlike older children, a neonate’s seizure may manifest only as slight eye fluttering, staring, or subtle chewing movements. Since these clinical signs are difficult to distinguish from normal neonatal behaviors, the EEG is the necessary tool for confirmation.
The nEEG also monitors the brain development of high-risk premature infants. Since brain activity changes rapidly with age, the test helps clinicians determine if electrical patterns are progressing appropriately for the baby’s gestational age. This evaluation helps identify and address any signs of delayed neurological organization.
How the EEG Procedure is Administered
The neonatal EEG is performed by a trained neurodiagnostic technologist, often at the baby’s bedside in the NICU. The technician prepares the scalp by gently cleaning small areas where the electrodes will be placed. A reduced array of small, disc-shaped electrodes, using a modified 10-20 placement system, are then secured to the scalp with a temporary conductive paste or glue.
The sensors measure electrical voltage changes on the skin surface. The test is often supplemented with additional electrodes to monitor physiological functions, such as eye movements, muscle tone, respiratory effort, and heart rate. These extra leads help distinguish true brain activity from movement artifacts.
Routine nEEGs require a minimum of one hour of recording, though continuous monitoring for high-risk infants can last between 24 and 96 hours. Extended monitoring is necessary to capture the baby in both awake and sleep states, as these behavioral states yield distinct electrical patterns. Technologists often schedule the recording after a feeding to encourage sleep, which is crucial for a complete assessment. Throughout the procedure, the technologist annotates the recording, noting any physical movements or changes to aid the interpreting physician.
Recognizing Typical Newborn Brain Activity
Interpreting a neonatal EEG requires specific knowledge of how electrical activity changes based on the baby’s postmenstrual age (PMA: gestational age plus chronological age). Unlike the continuous background seen in adults, the electrical activity of a newborn, particularly a premature one, is often discontinuous. This normal pattern, sometimes called tracé alternant or tracé discontinu, consists of bursts of high-voltage activity alternating with periods of low-voltage activity.
As the brain matures, discontinuity lessens, becoming confined mostly to quiet sleep by the time the baby reaches full term. A significant indicator of neurological maturity is the establishment of a clear sleep-wake cycle, one of the first signs of organized brain function. This cycling involves clear transitions between active sleep (similar to REM sleep) and quiet sleep (similar to non-REM sleep), each having its own characteristic EEG pattern.
The presence of specific waveforms, such as delta brush or encoches frontales (frontal sharp transients), are also considered normal findings in the developing brain at certain ages. Delta brush, for instance, appears as a burst of rapid activity superimposed on a slow wave, typically seen between 28 and 40 weeks PMA. The predictable appearance and disappearance of these patterns confirms that the brain is developing on track.
Identifying Abnormal Electrical Patterns
Physicians look for deviations from these expected maturational patterns. The most urgent deviation is the presence of electrographic seizures, identified as rhythmic, stereotyped waveforms that evolve in frequency and amplitude over time. The EEG is the definitive diagnostic tool because a large percentage of neonatal seizures do not have clear clinical signs, meaning the baby may be seizing electrically without noticeable physical movements.
Severe abnormalities in the background activity often point to diffuse brain injury. This can manifest as background suppression, where brain activity is persistently low in voltage or isoelectric (flat), indicating severe functional impairment. Another severe pattern is burst suppression, characterized by periods of highly abnormal, high-voltage activity alternating with long periods of near-flatness, typically associated with a poor prognosis.
The third category relates to organizational or maturational deficits, where electrical patterns are inconsistent with the baby’s age. This includes a failure to establish the expected sleep-wake cycling or the persistence of discontinuous patterns past the age when continuous activity should be present. Such findings suggest a delay in the organization of the brain’s neural networks, guiding the medical team in diagnosing neurological disorders and planning intervention.