Neonatal EEG (electroencephalogram) can present various patterns that may appear abnormal to inexperienced eyes but are actually benign developmental features. However, distinguishing normal transients from epileptiform abnormalities is crucial for correct diagnosis and management, as many neonatal seizures are clinically silent and only detectable on EEG.

1. Understanding Epileptiform Activity in Neonates

Polarity and Morphology:
Epileptiform discharges in neonates tend to be negative in polarity (similar to adult epileptic discharges). The most common pattern is a spike and slow-wave morphology, where a sharp, brief spike is followed by a slower wave. These discharges should be distinguished from normal sharp transients, which often lack the same rhythmicity and field of spread.

Disturbance of Background Activity:
Epileptiform activity typically disturbs the background brain activity. In neonates, the normal background consists of rhythmic patterns of activity, and any epileptiform discharge that disrupts this pattern is considered abnormal. This disturbance can be seen as sudden changes in amplitude or the appearance of new sharp waves that distort the underlying rhythms.

Evolution in Time and Space:
A key feature of epileptiform activity is its evolution: these discharges will often change over time and may spread to different regions of the brain. This spread is not typical of normal neonatal patterns, which tend to be more localized and transient. If sharp waves appear to evolve or spread, especially over several seconds, this should raise suspicion for seizure activity.

2. Sharp Waves and Their Location

Frontal and Centrotemporal Sharps:
Frontal and centrotemporal sharp waves are common and typically benign in neonates. These can be seen in both full-term and preterm infants, often occurring during sleep and rarely causing concern. As these are part of normal brain development, they don’t disrupt the overall background rhythm significantly and are considered normal transients.

Midline and Occipital Sharps:
Sharps in the midline (central region) and occipital lobes are more likely to be epileptiform. These regions are typically more involved in seizure activity, especially if the sharp waves persist or increase in frequency. Occipital sharp waves, for instance, could indicate a focal seizure originating in the visual cortex or surrounding areas.

Multifocal Spikes:
Occasional multifocal spikes (spikes occurring in different regions of the brain) can be normal in neonates, especially if they occur infrequently. However, an increase in the frequency or intensity of these spikes should raise concern. Excessive multifocal spikes could signal pathological conditions like seizures or other forms of brain dysfunction.

3. Rhythmic Activity and Persistent Rhythmicity

Normal Rhythmic Activity:
Neonates can exhibit rhythmic activity, often in bursts, as part of normal brain development. These rhythmic bursts may occur in regions such as the centrotemporal areas, and in most cases, these are not associated with seizures or other abnormalities. The rhythmicity typically lacks the consistency or pattern seen in epileptiform activity.

Persistent Rhythmicity:
If rhythmic activity becomes persistent or localized, particularly in a specific brain region, it could be a sign of an abnormality. This could indicate a seizure focus or a seizure-like event, and further evaluation is warranted. If rhythmic discharges become confined to one hemisphere or specific area (e.g., occipital or frontal lobes), this should be taken seriously.

4. Neonatal Seizures

Neonatal seizures are common but are often clinically silent, meaning they may not present with obvious signs like in adults. EEG is essential for detecting these seizures, as it can capture subtle changes in brain activity that are not visible clinically.

Types of Seizures:

Focal Seizures: These are much more common in neonates than generalized seizures. Focal seizures often start in one specific area of the brain and may evolve, spreading to other regions over time. The sharp wave morphology may remain in the same location or spread during the seizure.

Generalized Seizures: These are rare in neonates but can occur. Generalized seizures usually affect both hemispheres simultaneously. These seizures are often more difficult to detect but can be identified by widespread spike-wave complexes in the EEG.

Seizure Characteristics:

Low-Amplitude and Long Sharps: Seizures in neonates can appear as low-amplitude, long-duration sharp waves, which might be easy to overlook. These waves are typically rhythmic and periodic, which sets them apart from normal transients that are less rhythmic and more variable.

Desaturation Events: Seizures may be accompanied by brief drops in oxygen saturation, known as desaturation events, which are subtle clinical findings that can help identify a seizure. Monitoring oxygen levels alongside EEG is important in detecting silent seizures.

5. Example of a Right Hemispheric Onset Neonatal Seizure

An example of a right hemispheric onset neonatal seizure would show sharp waves or spikes originating from the right hemisphere. Initially, these waves may be localized to a small region, but as the seizure progresses, they may spread to the surrounding areas and across the midline to the opposite hemisphere. This spread of activity is a hallmark of seizures and can help differentiate them from normal developmental transients.

Key Points to Keep in Mind:

  • Normal vs. Abnormal: While sharp waves and rhythmic activity are common in neonates, especially in the first months of life, certain features—like excessive or asymmetric discharges, persistent rhythmicity, and evolution over time—may indicate a seizure or pathological condition. Properly distinguishing between normal transients and epileptiform activity requires careful consideration of the timing, location, and progression of the discharges.
  • Clinical Monitoring: Because neonatal seizures can be silent, monitoring both EEG and clinical signs is essential. Subtle seizures may go unnoticed without EEG monitoring, and early detection is important for timely intervention.
  • Awake vs. Sleep States: Be cautious about sharp waves seen during the awake state or active sleep after 42 weeks PMA, as they are more likely to be abnormal. In younger neonates, these sharp waves are often part of normal development, but after this point, they should be more closely scrutinized.