1. Neonatal EEG Montage

The neonatal EEG montage is designed to be less invasive and more suitable for the small head size of neonates, which makes a full 10-20 system impractical due to the risk of salt bridges between closely spaced electrodes. Therefore, a reduced montage is used, focusing on regions of the brain that show significant activity in neonates.

Key Points:

  • Simplified Montage: A central-focused montage with a transverse chain from T3 to T4.
  • Electrode Placement: Electrode placement avoids areas that are difficult to reach or overly sensitive in neonates, such as those near the fontanelle (soft spot). The focus is on central electrodes, which are more active in neonates.
  • Monitoring Until Full Term: This montage is used until the baby reaches full term or a PMA of 46 weeks. After this period, a more adult-like montage may be introduced.

2. Additional Electrodes and Devices

In addition to the brain electrodes, neonates have several other electrodes attached to monitor physiological parameters. These additional sensors are critical in assessing various bodily functions simultaneously during an EEG test.

  • ECG Electrodes: Measure electrical activity of the heart, providing insight into any cardiac irregularities.
  • Extraocular Electrodes (EOG): Positioned laterally near the eyes to record eye movements and identify abnormal ocular activities.
  • EMG Electrodes: Placed on the chin to monitor muscle activity, which can help differentiate between genuine EEG activity and muscle-related artifacts.
  • Pneumograph Electrodes: These electrodes measure the chest movements to monitor breathing patterns, essential in neonates due to their developing respiratory systems.

The placement of these electrodes helps in providing a comprehensive picture of the neonate's health, from brain activity to respiratory function.

3. Technical Settings for Neonatal EEG

Neonatal EEGs have specific technical settings that distinguish them from adult EEGs, primarily to accommodate the rapidly developing brain and the unique characteristics of neonates' brain waves.

  • Page Speed: Neonatal EEG recordings are typically read at a faster speed of 15 mm/sec. This allows for more detailed visualization of rapid brain wave changes that are characteristic in neonates.
  • Low Frequency Filter: The low-frequency filter is set between 0.01 Hz and 0.5 Hz to allow for the detection of slower brain wave activity without the interference of higher-frequency noise.
  • High Frequency Filter: The high-frequency filter is typically set at 70 Hz or higher to eliminate high-frequency artifacts and noise that are not relevant to the neonatal brain's activity.
  • Impedance: Electrode impedance should be kept low (<1 kΩ) to avoid signal degradation or artifact, ensuring a clearer EEG signal.

4. Sleep Stages in Neonates

Neonatal sleep differs greatly from adult sleep patterns. Neonates experience two primary stages of sleep, instead of the four stages seen in adults. These stages evolve as the brain develops, and understanding this progression is crucial for interpreting EEG data accurately.

The two stages of neonatal sleep are:

  • Quiet Sleep: Similar to non-REM sleep in adults, quiet sleep is characterized by low-amplitude, high-frequency brain waves and a relaxed, still body.
  • Active Sleep: This stage, similar to REM sleep in adults, features rapid eye movements (REM), irregular breathing, and more pronounced brain activity, often mixed with high-amplitude slow waves.

Example: Active sleep in neonates often presents with rapid eye movements and irregular limb movements, which can be mistakenly interpreted as seizures if not recognized as a normal sleep stage.

5. Understanding Postmenstrual Age (PMA)

Neonatal EEG interpretation is closely tied to the concept of Postmenstrual Age (PMA) rather than chronological age. This accounts for prematurity or delayed delivery and helps clinicians interpret brain development milestones more accurately.

  • Gestational Age: The length of the pregnancy, calculated from the mother's last menstrual period.
  • Chronological Age: The age of the baby since birth.
  • PMA Calculation: PMA = Gestational Age + Chronological Age. This calculation gives a more accurate reflection of the baby's brain development than chronological age alone.

Example: A baby born at 30 weeks gestation and now 6 weeks old would have a PMA of 36 weeks (30 weeks + 6 weeks). This helps clinicians understand the expected developmental stage of the baby's brain.

6. Development of EEG Patterns

The neonate's brain exhibits significant changes during the first few months, and these changes directly affect the EEG patterns. As the brain matures, EEG patterns evolve from those that may seem abnormal (such as diffuse slowing and discontinuity) to more typical patterns seen in infants, children, and eventually adults.

  • Week 1-2: The EEG in neonates may show a high degree of asynchrony, with minimal reactivity to stimuli.
  • Week 3-4: Increased synchronous activity, with more defined slow waves appearing. Sleep cycles begin to show more distinct characteristics of quiet and active sleep.
  • Month 1-2: By the second month, the EEG starts to resemble that of a more mature infant, with further differentiation in sleep states and more reactive responses to external stimuli.

Crucial Understanding: EEG interpretation in neonates is highly age-dependent. What may be considered abnormal at one age could be entirely normal just a few weeks later.