When a newborn with HIE is finally discharged from the NICU, the parents often leave with a stack of reports. Among the more intimidating is the EEG report. It is written in a specialized vocabulary (background activity, state cycling, burst suppression, electrographic seizures) that radiologists and neurologists take for granted but that is rarely translated for families. This guide walks through each part of a typical neonatal EEG report and explains what the findings mean for your baby.
What a Standard EEG Measures
A standard (conventional) EEG records the brain’s electrical activity using a full montage of 16 or more scalp electrodes placed according to neonatal standards. The signal is recorded in detail and interpreted by a pediatric neurologist or clinical neurophysiologist, usually with simultaneous video so that any movements can be correlated with the electrical record.
For babies with HIE, conventional EEG is used alongside continuous aEEG (amplitude-integrated EEG). aEEG gives a continuous bedside read; conventional EEG gives detail and is the definitive test for seizure confirmation and background characterization. Most HIE protocols use both, and many centers now do continuous video EEG (cEEG) for 24 to 72 hours to catch subclinical seizures.
Reading the Background Activity Section
Background activity is the general, non-seizure electrical activity of the brain. It is the most prognostically informative single feature of a neonatal EEG. Reports describe background in terms of:
- Voltage: normal, low voltage (below 10 to 25 μV), or inactive (electrocerebral inactivity).
- Continuity: continuous (activity present throughout the recording), discontinuous (alternating active and quiet periods), or burst suppression (high-voltage bursts separated by near-flat intervals).
- Symmetry: symmetric between the two hemispheres, or asymmetric (one side noticeably different).
- Synchrony: whether activity in the two hemispheres occurs at the same time.
- State cycling: the presence or absence of normal alternation between sleep and wakefulness patterns.
The most reassuring background in a term newborn is continuous, normal voltage, symmetric, synchronous, and shows clear state cycling. The most concerning is low voltage or inactive, discontinuous or burst suppression, and lacking state cycling.
What Burst Suppression, Low Voltage, and Inactive Mean
These three abnormal background patterns are important because they directly map onto severity of encephalopathy and predict outcomes.
| Pattern | Description | Clinical Implication |
|---|---|---|
| Normal continuous | Normal voltage, continuous, with state cycling | Reassuring |
| Discontinuous | Periods of activity alternating with quieter intervals | Mild to moderate |
| Burst suppression | High-voltage bursts separated by near-flat intervals | Moderate to severe encephalopathy |
| Continuous low voltage | Persistent low-amplitude activity | Severe |
| Electrocerebral inactivity (flat) | No discernible activity | Most severe |
Medications, particularly sedatives and anticonvulsants, and therapeutic hypothermia itself, can suppress background activity to some extent. An experienced reader accounts for these effects. A persistently burst-suppressed or inactive background in the absence of heavy sedation, and especially one that does not improve over the first 24 to 48 hours, is a strong negative prognostic sign.
Why EEG readings can look different to different clinicians
Parents sometimes see two EEG reports on the same baby with slightly different descriptions, or hear the bedside team summarize the tracing differently from the formal neurology read. A few reasons for this: EEG interpretation involves judgment, especially in the newborn period where normal patterns vary by gestational age; short-term sedation or a feeding can change the tracing; muscle and movement artifact can mimic or obscure real findings; and the formal report often includes detail the bedside summary simplifies. When reports disagree in meaningful ways, the interpretation by a board-certified pediatric neurologist or clinical neurophysiologist carries the most clinical weight.
The terms in a neonatal EEG report are specific and often not explained. A case review can help you read the report in the context of the full record.
Electrographic Seizures: Often Invisible, Always Important
Electrographic seizures are seizures seen on the EEG that may or may not have physical signs. In HIE, many seizures are subclinical: the baby looks still, but the EEG shows rhythmic, evolving discharges characteristic of seizure activity.
Shellhaas and colleagues (2011, Journal of Clinical Neurophysiology) documented that electrographic seizures are common in HIE, with published rates up to approximately 50 percent in moderate-to-severe cases. Most current protocols treat electrographic seizures with anticonvulsants (most commonly phenobarbital, sometimes levetiracetam or midazolam) even in the absence of outward signs, because seizure burden itself contributes to brain injury.
State Cycling and Why It Matters
A healthy full-term newborn cycles between active sleep, quiet sleep, and brief wakeful periods. Each state has a characteristic EEG pattern, and the alternation between them is a sign of normal neurological organization.
In a baby with HIE, absence of state cycling on the EEG can indicate that the brain is not functioning through normal sleep-wake architecture. Return of state cycling during or after cooling is a favorable sign. Persistent absence of state cycling, especially paired with abnormal background and seizures, is concerning.
How EEG Findings Inform Prognosis
EEG is one of the most prognostically informative tests after HIE, but it is not used in isolation. Murray and colleagues (2009, Archives of Disease in Childhood) found that EEG grade at birth, combined with clinical exam, predicts 5-year neurodevelopmental outcome in newborns with mild HIE. More severe HIE requires integration with MRI findings.
The modern approach combines several data streams: Sarnat stage on clinical exam, early and continuous EEG (often with aEEG), cord blood gases and neonatal clinical course, response to cooling, brain MRI typically around day 4 to 7 after cooling, and serial developmental assessments. Each contributes, and no single test is sufficient on its own.
Follow-up EEGs after discharge
Babies discharged after HIE often need outpatient EEG follow-up, particularly if they had electrographic seizures during the NICU stay. The typical plan includes: a follow-up EEG before discharge to confirm that any seizure activity has resolved on current medication, outpatient neurology clinic visits with serial EEG as needed, and trial weaning of anticonvulsants once the baby has been seizure-free for a period (often 3 to 6 months). Some children develop post-neonatal epilepsy even after a quiet discharge period, so ongoing neurology follow-up is standard for any baby who had moderate or severe HIE, regardless of whether seizures were seen in the NICU.
What an abnormal EEG can and cannot tell you about cause
An EEG report documents what the brain is doing; it generally does not directly identify why. A severely abnormal background can be caused by HIE, but also by severe infection (meningitis, encephalitis), metabolic disease, genetic conditions, stroke, medication effects, or severe hypoglycemia. When neonatal encephalopathy has no clear intrapartum explanation, neonatologists will often pursue a broader workup including metabolic screens, genetic testing, and infection evaluation. A case review in a family with both an abnormal EEG and an unclear cause will examine whether that broader workup was appropriate and timely.
What to Ask About Your Baby’s EEG Report
When you receive the formal EEG report from the NICU or from a pediatric neurologist, useful questions include:
- Was this a continuous EEG or a short recording, and for how long?
- What was the background activity like (voltage, continuity, symmetry, state cycling)?
- Were any electrographic seizures seen, and how many?
- Were the seizures treated with anticonvulsants, and did they resolve?
- Did the background pattern improve, stay the same, or worsen over the recording?
- How does the EEG compare with the aEEG, the MRI, and the clinical exam?
- Is a follow-up outpatient EEG recommended, and when?
- Who interpreted the EEG, and is the report available in writing?
Neonatal EEG reports can be hard to interpret. A case review can place the findings in the full clinical context and help identify whether care met the standard.
Related reading for parents
- What is aEEG? Continuous brain monitoring in the NICU explained for parents
- 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
Our team helps families in 38 states understand their baby’s EEG, aEEG, and MRI reports in the full clinical context. No cost. Answers first.