The neonatal EEG (electroencephalogram) reflects the electrical activity of the brain, which evolves as the infant develops, particularly in the early weeks and months after birth. This development is marked by changes in several key EEG features, such as continuity, synchrony, reactivity, and the structure of the waves themselves. Here's a deeper dive into these concepts:

1. Neonatal EEG States (Awake, Quiet Sleep, Active Sleep)

  • Awake: When the infant is awake, the EEG shows a more variable pattern, with activity that is more disorganized compared to sleep states. The defining feature is the eyes being open.
  • Quiet Sleep: This stage is akin to deep sleep, where there is minimal movement. The EEG in this state is characterized by higher amplitude slow waves. The eyes are closed, and there is less body movement.
  • Active Sleep: Similar to REM (Rapid Eye Movement) sleep in adults, active sleep is associated with eye movements, muscle twitches, and irregular breathing. The EEG in this state shows mixed high-frequency activity. Eyes are closed, but the body is more active.

Key Point: Up until 29 weeks PMA (Postmenstrual Age), the EEG is relatively similar across all these states, showing little distinction between them. As the neonate matures, the EEG becomes more differentiated, and you can begin to observe clearer differences between these states.

2. Key Terms in EEG Analysis

  • Continuity: A continuous EEG means that the brain waves do not exhibit large periods of inactivity. In neonates, discontinuity (brief flat periods) is normal, particularly at early stages of development.
  • Synchrony: This term describes whether the brain’s activity occurs simultaneously on both sides of the brain. In synchronous activity, both hemispheres exhibit bursts of activity at the same time. This is important as it indicates the development of coordinated brain function. As the neonate matures, the EEG shows increasing synchrony.
  • Reactivity: This refers to changes in the EEG in response to external stimuli, such as sound or touch. Reactivity indicates the developing brain's ability to respond to its environment. Early in life, the brain may not show significant reactivity, but as the child matures, reactivity becomes more apparent.
  • Amplitude: This refers to the height of the EEG waves. In neonates, higher amplitude waves are typically seen in quiet sleep, while lower amplitude and more mixed patterns are observed in active sleep and awake states.
  • Frequency: The number of cycles per second (measured in Hertz, Hz) that the brain’s electrical activity oscillates. Newborns tend to have slower frequency waves initially, but as they mature, higher-frequency activity becomes more prominent.
  • Morphology: This describes the shape of the EEG waves. In neonates, these waves can appear irregular or have a more chaotic morphology, which becomes more defined and organized over time.

3. Discontinuity and Interburst Intervals (IBI)

Discontinuity is a hallmark feature of neonatal EEG and refers to periods where there is little or no electrical activity (flat periods) between bursts of activity. These periods are known as interburst intervals (IBI).

In adults, such periods of flat activity would be considered abnormal, but in neonates, they are part of normal development, reflecting the immature and developing state of the brain.

The duration of the IBI shortens over time as the infant’s brain matures:

  • Up to 24 weeks PMA: The IBI can be as long as 60 seconds.
  • At 26 weeks PMA: The IBI shortens to 40 seconds.
  • At 28 weeks PMA: The IBI should be around 20 seconds.
  • At 34-36 weeks PMA: The IBI is expected to shorten to 10 seconds.
  • At 37-40 weeks PMA: The IBI is expected to be around 6 seconds.

After 40 weeks PMA, the EEG becomes largely continuous, with very brief periods of IBI that eventually disappear by around 46 weeks PMA.

4. Maturation of Synchrony and Reactivity

  • Synchrony: Refers to both hemispheres of the brain exhibiting bursts of activity at the same time. In the early weeks, the EEG is asynchronous, meaning the two hemispheres show activity at different times. As the brain matures, synchrony increases. By 30 weeks PMA, about 80% of the EEG shows synchronous activity. By 38 weeks PMA, the EEG should be fully synchronous, indicating a well-organized and coordinated brain.
  • Reactivity: Initially, neonates show little or no reactivity to external stimuli, but as they develop, the brain begins to respond to sensory input. By 30 weeks PMA, you may begin to observe some reactivity to stimuli. Between 32-40 weeks PMA, most infants will show full reactivity to external stimuli, such as sound or touch.

Summary of Developmental Timeline

  • Before 29 weeks PMA: The EEG is relatively disorganized, with little differentiation between awake, quiet sleep, and active sleep states.
  • 28 weeks PMA: The IBI starts to shorten and synchrony begins to increase.
  • 30 weeks PMA: Reactivity to external stimuli begins to appear.
  • 34-36 weeks PMA: The EEG shows significantly more synchrony, and the IBI is about 10 seconds.
  • 38 weeks PMA: Full synchrony is achieved.
  • 40 weeks PMA and beyond: The EEG should be largely continuous, with only brief periods of IBI remaining.

The neonatal EEG provides a window into the maturation of the brain. As the infant develops, the EEG transitions from a more chaotic and asynchronous pattern to one that is organized, synchronized, and responsive to external stimuli, signaling a brain that is increasingly capable of more complex functions.