When a baby is admitted to the NICU with suspected HIE, one of the first things the team sets up is often a small strip of electrodes on the baby’s scalp connected to a bedside monitor displaying a scrolling, colored band. That is amplitude-integrated EEG (aEEG), sometimes called cerebral function monitoring or CFM. Parents are rarely told what the numbers and colors mean, but aEEG is one of the most important continuous measurements being made of the baby’s brain, and the patterns recorded in the first hours often influence decisions about cooling, seizure treatment, and prognosis. This guide walks through how aEEG works, the five classic patterns neonatologists look for, how aEEG compares with a conventional full-montage EEG, and what parents should ask their team.
What aEEG Is
Amplitude-integrated EEG is a simplified, processed form of electroencephalography that presents brain electrical activity as a compressed trend band rather than as detailed waveforms. Instead of the 16 or more electrodes used in a conventional EEG, aEEG typically uses 2 to 5 scalp electrodes. The raw signal is filtered, rectified, and displayed as a time-compressed trace in microvolts, so that many hours of brain activity fit on a single screen.
The result is a continuous picture that a neonatologist can interpret at the bedside, without waiting for a specialized EEG technologist or a neurology read. It does not replace a conventional EEG for detailed analysis, but it gives the team a running sense of the baby’s overall brain state and whether it is stable, improving, or deteriorating.
How aEEG Is Set Up on a Newborn
The setup is deliberately simple so it can be used continuously in a NICU. Typical practice:
The Five Classic aEEG Patterns
Hellström-Westas and colleagues standardized a pattern-based classification that is now widely used. The five patterns in term newborns, listed from most reassuring to most severe:
| Pattern | Appearance | Clinical Meaning |
|---|---|---|
| Continuous normal voltage (CNV) | Upper margin above 10 μV, lower margin above 5 μV | Normal or near-normal |
| Discontinuous normal voltage (DNV) | Variable lower margin, periods below 5 μV | Mildly abnormal |
| Burst suppression (BS) | Alternating high-voltage bursts and low-voltage periods | Moderate to severe encephalopathy |
| Continuous low voltage (CLV) | Both margins consistently below 5 μV | Severe |
| Flat trace (FT) | Essentially inactive, both margins below 2 μV | Most severe |
In practice, the pattern at 3 to 6 hours of life in a term newborn with HIE is one of the earliest strong signals of injury severity. A baby whose aEEG recovers toward normal during the first 24 to 48 hours of cooling often has a better outcome than one whose pattern remains flat or burst-suppressed. Toet and colleagues’ 1999 study in Archives of Disease in Childhood was among the first to quantify the prognostic value of this early recording.
How aEEG informs cooling eligibility
Many HIE cooling protocols use aEEG as an additional gate when clinical criteria are borderline. A typical two-step assessment looks like this: Step 1 is the presence of moderate or severe encephalopathy on clinical exam plus evidence of a perinatal hypoxic-ischemic event (low cord pH, low Apgars, need for resuscitation, or a sentinel event). Step 2, where available, is an aEEG showing at least moderate abnormality (discontinuous normal voltage, burst suppression, continuous low voltage, or flat trace) recorded for at least 30 minutes within the first 6 hours of life, or clear electrographic seizures. When the clinical exam is equivocal, an abnormal aEEG can tip the decision toward starting cooling; a reassuring aEEG can support observation. Not every NICU requires aEEG for cooling decisions; the original NICHD cooling trial did not, but aEEG has become a standard adjunct in many centers and is written into many hospital protocols.
What aEEG looks like during cooling and rewarming
Therapeutic hypothermia itself changes the aEEG pattern. Cooling to 33.5°C tends to slightly reduce overall voltage and produce a more discontinuous-looking trace, which is a known effect and does not necessarily indicate worsening. As the baby is rewarmed over approximately 6 hours at the end of 72 hours of cooling, the aEEG pattern typically returns toward the baby’s baseline. Teams often continue aEEG monitoring for 24 to 48 hours after rewarming because post-rewarming seizures can occur and the pattern of recovery is prognostically meaningful. A pattern that improves through cooling and rewarming is more reassuring than one that remains severely abnormal.
The pattern recorded in the first hours often shapes decisions about cooling and medications. A case review can examine whether the monitoring and responses met standard of care.
aEEG vs Conventional EEG: What’s the Difference?
Both aEEG and conventional EEG record the brain’s electrical activity, but they serve different purposes. Most NICUs that care for HIE babies use both.
| Feature | aEEG (Amplitude-Integrated) | Conventional EEG |
|---|---|---|
| Electrodes used | 2 to 5 | 16 or more (full montage) |
| Interpretation | Bedside, by neonatologist in real time | By neurologist, often offline |
| Detail level | Compressed trend; broad patterns | Detailed waveforms; all frequencies |
| Continuous use | Yes, often for days | Shorter epochs, typically 30 to 60 minutes |
| Best for | Overall brain state, trend, gross seizures | Detailed seizure characterization, focal findings |
| Used in HIE? | Yes, continuously | Yes, intermittently or to confirm seizures |
What aEEG Can and Cannot Tell You
aEEG is powerful but has specific limitations. Understanding both sides helps make sense of what the NICU team is saying:
- It can detect electrographic seizures that are not clinically visible. In HIE, many seizures have no outward physical sign, and aEEG (together with conventional EEG) is the main way they are found and treated.
- It gives an early prognostic estimate, especially when the pattern at 3 to 6 hours is severely abnormal and does not recover.
- It provides continuous feedback during cooling. The team can see whether the brain is stabilizing or deteriorating without having to order a new test each time.
- It cannot replace conventional EEG for short, focal, or low-amplitude seizures.
- It is affected by artifacts: movement, ECG, high-frequency ventilator noise, and loose electrodes can all distort the trace. Interpretation requires experience.
- Medications affect the pattern. Sedatives, anticonvulsants, and cooling itself can change the aEEG, and the team has to factor that in when reading a trace.
What Parents Should Ask Their NICU Team
If your baby is on aEEG, these are reasonable and informative questions:
- What aEEG pattern is my baby showing today, and how does it compare with yesterday?
- Has the trace ever shown burst suppression, continuous low voltage, or flat trace?
- Has aEEG suggested any electrographic seizures, and has a conventional EEG confirmed them?
- Are any medications (anticonvulsants, sedatives) currently affecting the trace?
- How does the aEEG fit with the MRI and the neurological exam?
- Will the monitoring continue through rewarming, and for how long after?
- Who is interpreting the aEEG, and is pediatric neurology involved?
Keeping your own notes about the answers, and asking for printouts of significant aEEG epochs, can be valuable when reading the records later or discussing care with another clinician.
aEEG findings are part of the record that follows your baby for life. We can help you understand what the monitoring showed and whether care met the standard.
Related reading for parents
- Standard EEG for newborns with HIE: what the results actually mean
- When should an MRI be done after HIE? Timing, types, and what each shows
- Why an early MRI after HIE can look normal: understanding diffusion-weighted imaging
- Basal ganglia injury on an HIE MRI: what this finding means for your child
- Neonatal encephalopathy vs HIE: understanding the terminology doctors use
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