The normal EEG from neonates to adolescents

Continuity

The normal EEG evolution is one of persistent discontinuity in the premature infant to one of continuity in a fully mature infant. A pre­mature infant of less than 29 weeks CA may have an EEG that is entirely flat or flat with medium to high amplitude bursts (50–300 µV). Between 29–32 weeks CA, the interburst interval (IBI) is typically 6–8 seconds but can be as long as 35 seconds. The amplitude of the IBI is less than 25 µV. This pattern is known as tracé discontinu ( Figure 3-1 ). Rare periods of continuous activity may be seen in wake and active sleep. Between 32 and 35 weeks CA the IBI becomes shorter and is rarely greater than 10 seconds. At this time continuous activity may be seen in wake and in active sleep. At 35 weeks CA the tracé discontinu is shifting into a less discontinuous pattern called tracé alternant ( Figure 3-2 ). In tracé alternant, the periods of relative discontinuity are shorter (typically 4–6 seconds) and higher in amplitude (>25 µV). The EEG shows a tracé alternant pattern in quiet sleep and continuous activity in wake and active sleep. Between 37 and 44 weeks CA, the EEG should be continuous except for quiet sleep, which can maintain a tracé alternant pattern. After 44 weeks, the EEG should be continuous at all times.

Interhemispheric Synchrony

The hemispheres are defined as synchronous if there is less than a 1.5-second difference in the onset of EEG activity during a discontinuous background. The development of the neonatal EEG is interesting because synchrony is initially abundant ( Figure 3-1 ) and then decreases and increases again. Specifically, premature infants less than 29 weeks have a high level of synchrony (90–100%). Synchrony nadirs between 31 and 32 weeks CA with approximately 50–70% of bursts being synchronous ( Figure 3-3 ). After this, synchrony gradually increases ( Figure 3-2 ). Between 37 and 44 weeks CA, nearly 100% of bursts (seen during quiet sleep when there is a tracé alternant pattern) are synchronous.

EEG Features

At different neonatal ages, there is the development of certain characteristic waveforms. These are either not seen at all in adult life (delta brush) or seen in adult life but may have an entirely different significance (sharp waves/transients). The following background elements appear, peak and then fade during particular periods of neonatal development.

The first characteristic waveform to be seen is the delta brush pattern, which can be present as early as 24 weeks CA. This is a slow moderate- to high-amplitude delta wave with superimposed lower amplitude fast frequencies. Between 24 and 29 weeks CA delta brushes are seen mostly over the central and midline areas but by 32–35 weeks CA delta brushes are seen mostly in the occipital and temporal regions ( Figure 3-2 ). Prior to 33 weeks CA, the delta brushes are seen primarily in active sleep. After 33 weeks CA, the delta brush pattern is seen primarily in quiet sleep. Delta brushes are infrequent by 37 weeks and, if abundant, should be taken as evidence of possible dysmaturity.

Monorhythmic occipital delta activity consists of runs of high amplitude posterior delta. This activity occurs symmetrically and synchronously usually in the bilateral occipital regions. It has a similar time course as delta brush, first appearing at 24 weeks CA, peaking between 31 and 33 weeks, and fading by 35 weeks. In an infant less than 29 weeks, CA monorhythmic occipital delta activity rarely lasts more than a few seconds in duration. By 31 weeks CA runs of monorhythmic occipital delta can last for more than 30 seconds. This is often admixed with delta brush.

Theta bursts, also known as temporal sawtooth waves, are seen starting at 26 weeks CA and maximal in the relatively narrow CA bandwidth of 29 to 32 weeks CA. These occur in the temporal electrodes independently for 1–2 seconds and consist of sharply contoured rhythmic theta waves, with amplitudes of up to 200 µV.

Starting at 32 weeks CA, during continuous portions of EEG, there is the development of a rarely present amplitude gradient with higher amplitudes posteriorly (in the delta range) and lower-amplitude, faster activity anteriorly. This gradient is maintained in adult life (though the frequencies are different) and becomes the cornerstone of an organized adult EEG.

Multifocal sharp transients ( Figure 3-4 ) are most frequent between 32 and 34 weeks CA but can persist and are considered normal up to 46 weeks CA. These are sharp waves, which can be maximal in essentially any location.

After 34 weeks, frontal sharp waves (also known as encoches frontales) become more frequent as multifocal sharp transients become less frequent. Frontal sharp waves usually occur in isolation or in brief runs and are typically synchronous and symmetric ( Figure 3-5 ). They may appear as early as 26 weeks CA but are polyphasic with high amplitudes. Typically there is a small initial negative deflection and a larger positive deflection. They diminish at 44 weeks CA, rarely seen during sleep after 46 weeks CA and disappear by 48 weeks CA. Frontal monorhythmic delta is seen around 34 weeks CA. This pattern often appears with ad­mixed frontal sharp waves. Both multifocal sharp transients and frontal sharp waves can occur in any state. As these have a morphology similar to adult sharp waves, it is a common rookie mistake to report these as abnormal and as a marker for a possible seizure disorder. Even if these persist past 46 weeks CA, they are a more non-specific sign of cerebral dysfunction and may not be secondary to cortical hyper-excitability. If sharp waves are overly frequent at any one location, occur for long periods of time, and/or have persistent asymmetry, it is abnormal at any age.