What are the characteristics of a normal neonatal EEG?

Updated: Oct 03, 2019
  • Author: Rosalia C Silvestri-Hobson, MD; Chief Editor: Selim R Benbadis, MD  more...
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In the full-term born infant, ultradian sleep and waking cycles are well defined and easy to detect with polygraphic and behavioral criteria. On EEG, wakefulness characterized by eye opening, crying, and vigorous motor behavior accompanied by irregular vital signs on recordings (ECG, respiration) is marked by a low-amplitude activity and discontinuous 4–7 cps theta activity interspersed with low-voltage delta rhythms. Hence, the French name activité moyenne.

Active sleep (AS), the antecedent of rapid eye movement (REM) sleep, is usually indicated by irregular respiratory patterns with interspersed, brief apneic episodes that often precede clusters of eye movements. Contrary to adult physiology, prominent, subtle motor activity, especially of the face (eg, grimacing, smiling), accompanies this state. These results are often interpreted as seizure activity by the inexperienced reader. On EEG, 2 patterns are observable, as follows:

  • A continuous, low- to medium-voltage background with theta and delta activity and occasional anterior sharp-waves that occur primarily at sleep onset, or

  • A lower amplitude, more continuous theta background is mostly seen between periods of quiet sleep (QS). The latter non-REM sleep is marked by a discontinuous pattern (tracé alternant) that is characterized by bursts of high-amplitude (50-20 mV) synchronous delta activity and separated by intervals of lower mixed activity that resemble wake or AS activity (see image below).

    Quiet sleep non–rapid eye movement tracé alternant Quiet sleep non–rapid eye movement tracé alternant.

Sleep state cyclicity is certainly achieved after 30 weeks' postconceptional age (PCA), with stability over multiple cycles only at 36 weeks' PCA.

Lately, the increased survival of extremely young premature babies has allowed to assess very early expression of sleep cyclicity by combining measures of REM and EEG discontinuity [3] between 25 and 30 weeks' PCA. Early forms of "transitional" sleep akin to "seismic sleep" in the rat represents an immature form of paradoxical sleep with mixed features of active and quiet sleep. It probably corresponds to a primitive form of brain activity characterized by a low level of inhibition progressively declining toward term. [4]

Near the end of the first month, a more diffuse pattern usually appears, consisting of continuous, high-to-moderate amplitude, slow activity that is not seen in the preterm infant. A high degree of synchrony between burst and interburst activity is desirable at term. This usually confirms normal maturational patterns.

Several morphological figures may occur with variable frequency. Random sharp-waves, most commonly temporal or rolandic, are sporadically seen in QS. Nonrolandic, repetitive, highly focal spikes confined to a single location that occur during wakefulness usually indicate abnormalities. A burst of frontal delta and synchronous, frontal sharp waves are still abundant in the full-term born infant during AS. Spindle delta bursts (brushes) are seen with decreasing frequency in the full-term born infant and are usually confined to the central and temporal leads during QS. This state is the most vulnerable, being susceptible to various minor CNS insults that are only transiently apparent, depending on their expression. It is important to perform prolonged recordings, especially in stressed infants as they are likely to express less QS.

Several important milestones characterize EEG maturation patterns during the first months of life. The newborn progressively develops a circadian rhythm, resulting from the interaction of endogenous factors with external synchronizers such as light, eating, and sensory stimulation over the course of a day. At approximately the third month, sleep efficiently occurs in nocturnal intervals of at least 8 hours, reflecting mother-child interactions and the established activity of endogenous pacemakers.

With regard to EEG results, several important changes accompany this phase. From the second week of life, slow and continuous background activity (consisting of increasing amplitude delta waves whose frequency also decreases with the approaching first month of life) progressively replaces the discontinuous pattern (tracé alternant) that is typical of QS. Typical EEG characteristics disappear within the second month of life, including slow frontal biphasic spikes (encoches frontales) and negative rolandic spikes. The newborn still falls asleep in AS until the end of the third month. AS decreases from 50% to 40% by the end of the fourth month; likewise, QS progressively increases and becomes more defined due to the appearance of EEG hypnic features that are typical of adults. Vertex waves can be noted in the rolandic regions after the third month; sleep spindles appear earlier, at about the sixth week, over the central regions.

The first sleep spindling samples are slower in frequency and more anteriorly distributed in newborns compared with older infants. These infrequently appear at the beginning of QS as rudimentary, low-voltage (< 25 mV), immature, asymmetric, and asynchronous 10–16 Hz EEG waveforms. The length, amplitude, and synchrony of these spindling samples increase during the first year of life and are more prominent in females and small for gestational age newborns, especially those with neonatal respiratory distress. Spindling maturation is prognostically valuable: their absence at the third month indicates abnormal maturation (eg, hypothyroidism, severe patterns of mental retardation). At the same time, the sawtooth waves that are typical of adult REM sleep make their first appearance in AS.

Around the sixth postnatal week, 75 mV occipital sharp waves characterize AS and increase in frequency from 2 to 4 Hz toward the end of the third month. At 3 months, a clearly defined 3–4 Hz, 50–75 mV occipital rhythm appears during wakefulness; this is interrupted by eye opening. It progressively evolves at about 5 months to a faster frequency of 6–7 Hz.

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