How Are Silent Seizures Detected in Newborns With HIE?

When a newborn is diagnosed with hypoxic-ischemic encephalopathy (HIE)—a type of brain injury caused by reduced oxygen or blood flow around birth—they are often treated with therapeutic hypothermia (cooling). During cooling, your baby may look perfectly still, sleepy, or sedated. Naturally, parents often wonder: If my baby is sleeping or sedated, how will I know if they are having a seizure?

The short answer is that doctors use specialized brain monitors to detect seizures even when there are no outward physical signs. This is critical because a substantial proportion of seizures in babies with HIE are “silent”—meaning they involve abnormal electrical seizure activity in the brain without causing physical jerking or stiffening ^[1][2]. In some studies of HIE, over 40% of seizures detected were purely electrical (subclinical) ^[1][3].

Why Are Silent Seizures So Common?

In newborns with HIE, a subclinical seizure (or silent seizure) occurs when there is abnormal, overactive electrical activity in the brain, but the baby’s body does not move ^[1][3]. It is important to know that the cooling treatment does not cause the seizures—the brain injury itself is the underlying cause. However, a few factors make seizures harder to see during this time:

• Suppressed Reflexes: The HIE injury itself and the lowered body temperature can suppress a baby’s normal physical reflexes and outward movements ^[3].
• Sedation: Many babies undergoing cooling receive pain or sedating medicines to keep them comfortable, which also keeps them physically still.
• Electroclinical Uncoupling: If a baby starts having physical seizures and is given anti-seizure medication, the medication might stop the physical movements, but the brain may continue to have electrical seizures hidden from plain sight ^[4][1].

Because clinical observation (simply watching the baby) can miss ongoing seizures, doctors cannot rely on their eyes alone ^[5]. You are not expected to diagnose a seizure just by watching your baby. If you see repetitive movements (like lip smacking or bicycling legs), tell your nurse, but remember that many movements are completely normal and not seizures.

How Doctors Monitor for Silent Seizures

To look for silent seizures, the Neonatal Intensive Care Unit (NICU) team uses brainwave monitoring. These monitors only record electrical activity; the wires do not deliver electricity or shock your baby. Staff will routinely check your baby’s skin and the leads.

There are two main types of monitors used:

Continuous Video EEG (cEEG)

Continuous electroencephalography (cEEG) is considered the reference standard (the best available clinical test) for detecting neonatal seizures ^[3][2].

• How it works: Small sensors (electrodes) are placed on your baby’s scalp to measure electrical activity across the brain. A video camera records the baby at the same time so doctors can match any physical movements with what is happening in the brain.
• Why it’s used: It provides the most comprehensive picture of the brain’s activity. While it is not infallible (it can sometimes be affected by movement artifact), using cEEG helps doctors reliably detect subclinical seizures and guides decisions about whether anti-seizure medications are needed or can be adjusted ^[6][5].

Amplitude-Integrated EEG (aEEG)

An amplitude-integrated EEG (aEEG) is a simplified bedside brain monitor ^[3].

• How it works: It uses fewer sensors to compress complex brainwave data into a slow-moving trend line on a screen.
• Why it’s used: It is a valuable screening tool that nurses and doctors can read quickly to track the general background health of the brain ^[2].
• Limitations: Because it provides a simplified view, aEEG can sometimes miss brief or low-level seizures, or misinterpret standard care movements as a seizure ^[3][2]. If the care team sees something suspicious on an aEEG, they will usually try to review the raw EEG data or order a full cEEG when available ^[2].

How Long Are Babies Monitored?

Monitoring practices depend on the individual NICU’s guidelines, the availability of cEEG equipment, and your baby’s specific risk. Many guidelines recommend that continuous brain monitoring begins as soon as possible after HIE is diagnosed ^[7][4].

Many centers continue monitoring throughout the full 72 hours of cooling and through the rewarming phase (when the baby is slowly brought back to normal body temperature). Rewarming is a critical period because the brain’s electrical activity changes as it warms up, and seizures can sometimes emerge during this time ^[8]. The exact length of monitoring may be tailored to your baby; often, monitoring continues until there has been an appropriate seizure-free interval (such as 24 hours without a seizure) ^[9][7].

Why Detecting Silent Seizures Matters

If the EEG shows a seizure, the team will review the tracing, confirm it, and determine the “seizure burden” (the total amount of time a baby spends having seizures). When indicated, they will give or adjust treatments.

Research shows that a high seizure burden is associated with a higher risk of more severe brain injury ^[1][10]. By using advanced monitoring, doctors aim to reduce ongoing electrical seizure activity and guide care ^[11]. While treating seizures helps reduce this burden, the long-term impact of treatment on a baby’s overall development is still an area of ongoing medical research. Furthermore, an EEG background pattern is just one piece of the puzzle—neither a reassuring nor an abnormal EEG by itself guarantees a specific future outcome. Clinicians look at the EEG, MRI results, and the baby’s clinical exam together to understand their overall health ^[12].