Neonatal EEG

Anatomy and Physiology

EEG measures the postsynaptic potential of neuronal axons. Neonatologists use this valuable tool to gauge if there are abnormalities present in the normal background brain activity of the neonate.[3][2] This background pattern undergoes expected changes with increasing gestational age that signal normal neonatal brain maturity.[2] Deviation from the normal background pattern of an EEG can indicate cerebral impairment.[7] Although the basic electrophysiological principles for neonatal EEG interpretation are similar to older adults, the EEG interpretation process in neonates is unique. The belief is that the generators of EEG are the cortical neurons, which are, in turn, mediated by the extensive thalamocortical connections.[8]

Indications

Neonatal electroencephalography (EEG) is a diagnostic procedure utilized to assess neurologic conditions (eg, encephalopathy and seizure disorders) and an objective method of measuring the functional integrity of the maturing newborn brain.[1][2] Neonatologists use this valuable tool to gauge the behavioral state of newborns. Moreover, neonatal EEG helps pediatric neurology clinicians understand the impact of neurological insults on the infant's brain and identify and confirm the presence of seizures. Indications for neonatal EEG include:

• Assessment of clinical signs of seizure (eg, intermittent horizontal gaze deviation, tonic posturing, and unexplained episodes of elevated blood pressure or tachycardia)
• Neonatal seizures
• Evaluation of hypoxic encephalopathy severity
• Hypotonia
• Altered mental status
• Infants at high risk for brain injury (eg, central nervous system infection or trauma, intracranial hemorrhage, cerebral malformation, persistent pulmonary hypertension, hypoxic-ischemic encephalopathy, perinatal stroke, and inborn errors of metabolism) [1][2][9]

Contraindications

Neonatal EEG has no absolute contraindications. However, the modality should be used cautiously in infants with anencephaly and significant scalp injury with edema and premature neonates with very low birth weight.[10]

Equipment

The American Clinical Neurophysiology Society’s Guidelines have modified the procedure to pediatric EEG compared to adults due to the smaller head size of neonates and a relative lack of EEG activity noted in the extreme frontopolar head regions. Therefore, the international 10 to 20 electrode placement system has undergone modification for neonatal EEG recordings. The standard neonatal montage includes 8 scalp electrodes (ie, FP1 and 2, Cz, Fz, Tz, C3, C4, T4, O2, T3, O1, P3, P4), EKG, and respiration. An additional 3 electrodes, including Fz, Cz, and Pz, may be added to improve coverage. Other electrodes may be used to monitor eye movements and EMG. Needle electrodes are not recommended for neonates. Synchronized video monitoring is also recommended to help characterize any events noted during an EEG.[9]

Personnel

The personnel involved with neonatal EEGs include nurses, EEG technologists, neonatologists, pediatricians, neurologists, and clinical neurophysiologists. EEG technicians and neonatal nurses are the primary personnel required to obtain an EEG, and pediatric neurologists and neurophysiologists assist with interpreting the results.[9] The technician should have prior training in electrode placement and polygraphy of newborns to obtain a useful and technically satisfactory recording.[11] The technologist should be familiar with the artifacts in newborns and have the ability to troubleshoot with minimal disruption to other neonatal intensive care unit patient care. At times, there may be difficulty in adequately accessing the newborn because of concurrent non-neurological monitoring and overcrowding due to equipment associated with ventilators and extracorporeal membrane oxygenation (ECMO) procedures. The technician should work with the neonatal nurses, physician team, and respiratory therapists to obtain a satisfactory recording without compromising the care of the infant. A bedside observer may be required if video monitoring is not available to record any relevant activity affecting the EEG's appearance.[9]

Preparation

When preparing for a neonatal EEG, clinicians should be aware of the following issues:

• Any potential disturbances to the neonate
• Appropriate equipment preparation and positioning
• Documentation of the infant's gestational age (GA), conceptual age (CA), status, and other clinical data
• Any possible interference and artifacts that may affect the EEG 
• Preparation of the infant's skin is of utmost importance, including adequate use of conducting paste to reduce the impedance below 5 to 10 kOhms.[12][13][12]

Technique or Treatment

Before a neonatal EEG is performed, technicians should obtain relevant clinical information, which is critical for accurate EEG interpretation. Clinical history should include the infant's gestational age at birth, chronologic age (ie, age since delivery), age since the maternal last menstrual period (ie, postmenstrual age), laboratory results (eg, blood gas and serum electrolyte values), current medications, and vital signs. The various ages should be calculated on the day of recording, using weeks as the units of time. EEG technicians should also ensure that the neonate's condition is stable enough to allow the procedure to be performed and ascertain whether any restrictions are present.[14]

In neonates, disk electrodes are typically used and applied with electrolyte paste in a reduced or full 21-electrode array, depending on clinical preference. The neonate's skin should be prepped until the electrode impedance is 5 to 10 kOhms.[12] Electrode impedances of less than 10 kOhms are permissible to prevent excessive abrasion of neonatal skin, but marked differences between electrode impedances should be avoided.[14] Needle electrodes are not recommended for neonatal EEG. Various electrode placements can be used to permit a combination of longitudinal and transverse bipolar arrangements. However, using any of the arrangements suggested by the American Clinical Neurophysiology Society is recommended to help with standardization.[14] At least 12 electrodes are recommended, along with electrodes to record non-EEG variables (eg, electrocardiogram, eye movements, and respiration). Wake and sleep cycles are not as clearly differentiated by EEG patterns in the neonates; therefore, non-EEG variables are helpful for EEG interpretation and evaluating artifacts. Eye movements can be recorded by placing an electrode 0.5 cm superior and lateral to the outer corner of one eye and another electrode 0.5 cm below and lateral to the outer corner of the other eye.[14]

A sampling frequency of 256 Hz is typically used. Commonly used screen display settings include a gain of 10µV/mm, a minimum time constant of 0.3 sec, and filter settings of a high pass at 0.5 Hz and a low pass at 70 Hz. Paper speeds may be set at the standard of 30 mm/sec. Slower paper speeds may occasionally be used to compress the record and facilitate visual recognition of delta activity, which is the dominant frequency seen in neonates.[15] A recording of both active and quiet sleep should be performed during an EEG. Typically, at least 60 minutes is needed to obtain an adequate recording. Though a full period of quiet sleep is required, sedation should not be used to induce this state. Throughout the EEG recording, technicians should document the neonate's status and head and eyelid position.[14]

Complications

Neonatal EEG is not associated with many adverse effects. Due to the fragility of neonatal skin, the primary complications involve dermal issues. Long-term monitoring is avoided in newborns, especially in very premature infants, due to the following:

• Skin maceration: Skin breakdown and maceration can occur because of the electrode adhesives and traction on the electrodes. 
• Chance of infections: Newborn infants are prone to infections due to their delicate skin. Contamination should be avoided, and leads should be placed away from scalp lacerations or sutures.[16][6] 

Clinical Significance

Temporal Maturational EEG Changes with Neonatal Age

Interpretation of neonatal EEG depends on clinical factors (eg, gestational and chronological age) and the wake-sleep state because general patterns of background activity change by variables. A term newborn EEG activity will normally demonstrate characteristic changes, especially in the first few weeks after birth. These changes in brain activity are indications of maturing brain function that help guide estimations of a neonate's actual functional brain age. Many experts believe that a trained clinician should be able to estimate a neonate's functional brain age within a range of 2 weeks based on EEG findings. Deviations from typical EEG findings of brain maturation, sometimes termed dysmaturity, may indicate an increased risk for abnormal neonatal brain development. Observable changes with progressing gestational age include increasing amplitude during the first year of life, dominant frequency, decreasing periods of discontinuity, synchronicity of bursts progressing to greater degrees of continuity, shortening of interburst intervals, and the presence of lability. With the appearance of observable eye movements and changes in background activity, sleep state differentiation typically begins at 25 weeks and is completed by 30 weeks.[4][5]

Behavioral States and Neonatal Neurodevelopment Milestones

State differentiation is an essential aspect of the neurophysiological assessment of neonates. The waking state is divided into active (AW) and quiet (QW) types. Active wakefulness is associated with agitation, while quiet wakefulness is not. Similarly, the sleep state is comprised of active sleep (AS), which is associated with rapid eye movements (REMs), and quiet sleep (QS), during which REMs do not occur. Between these states, there can be other states, termed indeterminate or transitional sleep, with discordant features. The following developmental changes in neonatal brain activity are typically observed according to gestational age.[17]

• 24 to 28 weeks gestation: In premature neonates, EEG changes show an inconsistent correlation with changes in the state that usually alternate between periods of activity and rest. Background activity is markedly tracé discontinu "discontinuous tracing"; short duration runs of monophasic or diphasic delta activity (0.3-1 Hz) with superimposed theta rhythms. Delta activity of high amplitude, up to 300 µV, may be regional in expression occurring over temporal, occipital, and central regions, which can be bilateral or unilateral, while frontal delta activity is less frequent. Theta bursts may predominate in temporal areas, bilateral in expression, and become more abundant with progression at 28 weeks gestation. 

• 28 to 31 weeks gestation: Behavioral states, including active wakefulness, quiet sleep, and active sleep with continuous or semi-continuous rapid eye movements, become better defined progressively on the EEG tracing. Background activity shows periods of continuity of up to 160 seconds, with interburst intervals of up to 30 seconds. Delta activity shows a reduction in amplitude at older gestational ages with superimposed theta or alpha rhythms. Delta waves are less diffuse, becoming more regionalized over occipital and central regions. Theta rhythms can occur in synchronized bursts or may undergo localization in the occipital and temporal areas. A few delta brushes, delta waves with superimposed alpha or beta rhythms, appear around this stage. By 31 weeks, we see delta activity 0.7 to 2 Hz with amplitudes of up to 200 µV, and the amplitude of theta activity also diminishes to about 20 µV. Delta brushes become prominent and diffuse. While synchronous delta activity is more common in AS, theta activity over temporal regions becomes more prominent in QS at this stage. We also see EEG reactivity to stimuli and an attenuation of background rhythms' amplitude.[17]

• 32 to 34 weeks gestation: At this stage, the sleep states become even more distinct; while the background activity is continuous in wakefulness and AS, QS is noticeably discontinuous with an increase in burst duration between 32 to 34 weeks gestation. A reduction in interburst intervals to <15 sec at 32 weeks and <10 sec at 34 weeks can also be noted. After 32 weeks, delta burst activity increases in frequency, approximately 1 to 2 Hz, but shows a reduction in amplitude, becoming exuberant or profuse by 34 weeks. Delta brush activity remains localized to the occipital at 34 weeks gestation. Theta activity seen in earlier gestational ages disappears in AS at 32 weeks. Additionally, immature, poorly defined frontal sharp transients may show up by 34 weeks gestation.[17]

• 35 to 36 weeks gestation: Sleep stages with increasing complexity are observed after 36 weeks gestation due to the development of cortical afferent connections, which include the 4 primary phases AW, QW, AS, and QS, as well as active sleep before the onset of QS (AS1), or first REM, and after QS (AS2), also known as second REM. The waking and sleep phases at this developmental stage show further differentiation and can be distinguished easily. In wakefulness and active sleep, background activity is continuous with mixed polyfrequency activity, or activité moyenne, and the AS1 stage precedes quiet sleep. The periods of high-voltage burst intervals during wakefulness and active sleep alternate with low-amplitude interburst intervals during quiet sleep, termed tracé alternant.[17] Here, the activity is continuous and high amplitude slow activity with bursts of monomorphic activity, seen on EEG tracings as delta activity 1 to 3 Hz and 50 to 100 µV in frontal regions, become apparent. This activity is termed anterior slow dysrhythmia. Delta brushes are more frequent in AS1. A transition from AS1 to QS is marked by discontinuous background activity and periods of relative attenuation lasting <10 sec. The second REM or AS2 phase is characterized by the appearance of continuous but lower amplitude waveforms in the background and a greater amount of theta waves. Background activity shows bi-synchronous activity in active sleep and is asynchronous in quiet sleep.[17]

• 37 weeks gestation and beyond: During this stage, waking and sleep states can be differentiated based on electrographic features. The background activity in waking shows activité moyenne, AS1 showing mixed frequencies and higher amplitude than in AS2, discontinuity, and trace alternant, which are features in QS. More specific features include localized expression of delta brushes in the occipital regions with progressive rarity beyond 40 weeks gestation. Frontal sharp transients and anterior slow dysrhythmia in AS1 become more apparent closer to term. Rolandic theta waves in AS1 and interhemispheric synchrony in QS are well-marked. By 44 weeks, other features replace the characteristic EEG findings of the different waking states in a term infant.[17]

Enhancing Healthcare Team Outcomes

Neonatal EEG recording can be challenging for inexperienced EEG technicians and untrained electroencephalographers because of its unique features, limitations, and technical data. The personnel involved with neonatal EEGs include nurses, EEG technologists, neonatologists, pediatricians, neurologists, and clinical neurophysiologists. EEG technicians and neonatal nurses are the primary personnel required to obtain an EEG, and pediatric neurologists and neurophysiologists assist with interpreting the results. The healthcare team should work together to achieve an adequate recording, including communicating relevant clinical information and limitations that are present.[18]