Physiological variables like eye movements, ECG, breathing pattern, and muscle activity maximize information about different stages of arousal or sleep, as well as paroxysmal phenomena like seizures. Eye movements [EOG channel] are recorded using two surface electrodes, placed diagonally, one centimeter above and below the outer canthus of the left and right eyes. Muscle tone is registered using a surface electrode placed over the submental region (chin EMG). ECG is recorded by two electrodes kept over the right and left side of the anterior chest wall. Another option is to place these electrodes over the right and left
Neonatal electroencephalogram (EEG), though often perceived as being difficult to record and interpret, is relatively easy to study due to the immature nature of the brain, which expresses only a few well-defined set of patterns. The EEG interpreter needs to be aware of the maturational changes as well as the effect of pathological processes and medication on brain activity. It gives valuable information for the treatment and prognostication in encephalopathic neonates. In this group, serial EEGs or EEG monitoring often gives additional information regarding deterioration/improvement of the brain function or occurrence of seizures.
Keywords: Amplitude integrated EEG, electroencephalogram monitoring, hypoxic ischemic encephalopathy, neonatal electroencephalogram, neonatal seizures
Electroencephalogram (EEG) offers a window to the brain function and has a unique place in the care of sick newborn babies. Its value is increased when neurological examination is confounded by the use of sedatives and neuromuscular blocking agents (even though EEG itself may be influenced by sedatives). It is done in a neonate commonly for the following indications: a) assess the severity of brain dysfunction, b) detect (subclinical) seizures, c) assess cerebral maturation, and d) determine prognosis.
To derive maximum benefit from an EEG, the neurologist needs to be familiar with maturational changes as well as the effect of various pathological conditions on the EEG. Often, the recording has to be made in the NICU, with interference from monitoring equipment, indwelling lines, incubator, and frequent nursing care. In this article, we highlight those technical details that are relevant for neonatal EEG registration and describe normal and abnormal EEG phenomena in the preterm and the term neonate.
Recording the EEG: Preparation, precautions, and technical aspects
Age of the neonate: Knowledge of the exact age of the baby undergoing EEG is important for a proper interpretation. Maturational changes occur fairly rapidly in the 25–48 week period [Table 1] and any discrepancy in the age-related EEG findings of more than two weeks is abnormal. The gestational age (GA) is defined as the time elapsed between the first day of the last menstrual period of the mother and the birth of the baby. A baby born prematurely at 32 weeks and having a chronological age of four weeks at the time of EEG recording is considered to have a postmenstrual age (PMA) of 36 weeks. This corresponds to a conceptional age (CA) of 34 weeks, assuming that conception occurred two weeks after the first day of the last menstrual period. However, in practice, the term CA is used interchangeably with PMA by many authors.
Salient normal electroencephalogram features at different conceptional ages
Clinical information: Time of birth, history of birth asphyxia, Apgar scores, occurrence of convulsions, etc. need to be noted by the technician before starting the recording.
Medication use: Medications like morphine, barbiturates, and benzodiazepines may influence the EEG findings, especially by lowering the voltage of the background activity. Their dosages, time of administration, and serum levels, if known, should be noted.
State of the patient: Noting the condition of the neonate, whether awake or asleep, on a ventilator, lying in an incubator, etc. is relevant. This helps not only in relating EEG phenomena to the state of the patient, but also in recognizing artifacts like those arising from high-frequency ventilation. Changes happening in the environment like loud noises, flashes of bright light, nursing care, etc. should also be noted, as these may produce transient attenuation of the background activity or produce movement artifacts.
Recording the EEG
The timing of registration after birth: If the EEG is done to assess the degree of brain maturation, it can be done at least 24 hours after birth to ensure that transient EEG abnormalities caused by birth itself are not recorded. If however, the goal is to assess the degree of encephalopathy and detect subclinical seizures, in situations like perinatal asphyxia, it is advisable to start the EEG registration (as a part of serial EEGs or EEG monitoring) as early as possible (at least within 24 hours), after respiratory and hemodynamic stabilization.
The duration of recording: Recording with the child asleep as well as awake is needed for the proper interpretation of neonatal EEG. A record made after feeding the baby is likely to succeed in this. The preparation for recording, like pasting of electrodes, can be done toward the end of the wake period so as to ensure that the neonate is not disturbed during sleep. The registration time should be 45 minutes or longer. Abnormal or absent sleep-wake cycling[4–6] is sometimes the earliest indicator of brain dysfunction, and in such situations, it is required to record the EEG for a longer period (at least two to three hours).
Adequate skin preparation is required to achieve a scalp impedance of <5 kΩ. Mildly abrasive pastes (like NuPrep™ gel) applied using a cotton bud, followed by alcohol swabs generally give a good result. Enough time (at least 90 minutes) has to be scheduled for the EEG recording to avoid stressing the technician and the parents. This is preferable to registering an artifact-filled EEG, and later trying to minimize their influence, for instance by changing the filter settings.
Electrodes and montages
At our center, we use the full 10–20 system of electrodes for all neonatal EEGs done during working hours and the restricted system of electrode placement during emergency EEGs done outside the working hours. The restricted 10–20 system of electrode placement uses nine active scalp electrodes – Fp1-2, Cz, C3-4, T3-4, and O1-2 electrodes. Important spatial information is lost by using lesser number of electrodes. However, most clinical indications for an emergent EEG at this age do not call for a high degree of spatial resolution. Assessment of background activity is not affected by the reduced number of electrodes. The reduced montage has been shown to have a high sensitivity (96.8%) and 100% specificity when compared to a full 10–20 montage in detection of neonatal seizures. Silver–silver chloride EEG electrodes with conductive adhesive electrode paste (like 10–20™ paste) are used. In addition, at our center, we prefer using 3% collodion with small pieces of cotton or gauze for optimal fixation of the electrodes. This gives a better quality registration and also ensures that there is no deterioration of electrode contact if the recording has to be extended to continuous EEG monitoring. In a period of more than 25 years of using this method, we have not encountered any allergic skin reactions or hazards due to flammability. Compressed cold air is used for drying. Fire hazard due to the flammable collodion has to be borne in mind, and a hair dryer should never be used. Removal of the electrodes is done using an acetone-free solvent (we use collodion-remover from Mavidon™ Medical Products). Acetone works equally well, but its fumes are quite irritating. We use the same technique in a baby lying in an incubator. However, the incubator is kept open for sometime after fixing the electrodes.
Many EEG transients have a preponderance at the vertex, and hence, inclusion of the Cz electrode as well as a coronal bipolar electrode derivation in addition to the bipolar anteroposterior derivation is meaningful [Figure 1]. With digital EEG equipment, montage selection during registration of EEG is no longer very important.
Electrode caps: They help to save time, especially when a full 10–20 system of electrodes, or a more extensive recording (e.g., high-density EEG) has to be made. They also help to keep the electrode positions fairly accurate, especially in centers where there are no experienced technicians. The quality of the record depends on a snug fit, and thus, a wide range of cap sizes fitting different head sizes would have to be available. The electrode positions may show some change over time and with repeated use, much more than with conventional electrode application.
Filters: Very slow waves (0.2–0.5 Hz) occur in the neonate and hence the low-frequency filter is set at 0.005–0.01 Hz. This very low filter setting may make the EEG more vulnerable to slow artifacts like sweat potentials. The high-frequency filter is set at 70 Hz.
Polygraphy: Polygraphic registrations are the rule while recording neonatal EEG. Simultaneous recording of video is also becoming a standard practice. Video helps in assessing clinical seizures and in recognizing artifacts. Physiological variables like eye movements, ECG, breathing pattern, and muscle activity maximize information about different stages of arousal or sleep, as well as paroxysmal phenomena like seizures. Eye movements [EOG channel] are recorded using two surface electrodes, placed diagonally, one centimeter above and below the outer canthus of the left and right eyes. Muscle tone is registered using a surface electrode placed over the submental region (chin EMG). ECG is recorded by two electrodes kept over the right and left side of the anterior chest wall. Another option is to place these electrodes over the right and left arms, in which case they can, in addition to the ECG, also record the limb movements. Extra channels for registering movements (transducers or surface EEG electrodes) may be added, for instance in the presence of clonic movements, tremor, hiccups, etc. Respiration is monitored by a transducer kept over the abdomen or the chest. Cessation of breathing for 6-7 seconds may be normally seen in neonates[9,10] and are considered to be pathological apneas meriting treatment only if they last longer than 10 seconds or occur frequently, or are associated with hemodynamic disturbances (like bradycardia or desaturations). Healthy premature infants may show short apneas as well as episodes of periodic