A conventional electroencephalogram (EEG) is a useful ancillary and diagnostic test that detects the electrical activity of the brain. Electrodes are placed in the scalp and connected to the EEG recording instrument, with a standard set consisting of 21 recording electrodes and one ground (reference) electrode. Odd numbers refer to left-sided and even numbers to right-sided electrodes. The letter designations refer to the different cortical areas (F-frontal, C-central, T-temporal, O-occipital, Z-midline).
The neonatal EEG is useful for determining brain development, identify cortical hyperexcitability, epileptiform activity, or seizures. The analysis can be quite challenging and may require additional skills and training, given that benign variants exist within the different stages of development and must be differentiated from abnormal discharges. For a neonatal EEG, equipment modifications include eight scalp electrodes (FP1, C3, T3, O1, FP2, C4, T4, O2) and lower recording speed. Visual analysis is the gold standard for clinical interpretation of the neonatal EEG.
Individual variability is an essential factor when extracting clinically relevant features by visual analysis of the EEG signals. Although automated software can identify EEG signals, human extraction, and analysis of the clinical information by visual analysis is an essential part of the overall recording. The visual analysis will provide a complete description of the most salient features of the EEG to give an interpretation, a conclusion, and a clinical correlation. This article describes the visual analysis technique in neonates.
The electrical activity in the brain consists of postsynaptic potentials generated by cortical regions. Postsynaptic potentials can be either excitatory or inhibitory. The summation of potentials is mainly generated by pyramidal cells and recorded relative to the potentials between two brain locations plotted over time. Rhythmic bursting on the EEG is generated in the nucleus reticularis of the thalamus with extensive projections through the thalamocortical tracts.
Neonates allocate most of their time in the sleeping state. It is, therefore, critical to study the EEG patterns during this sleep state. Sleep can be divided into active sleep and quiet sleep. Active sleep corresponds to REM sleep, while quiet sleep corresponds to non-REM sleep. Significant changes have been documented in the visual analysis of neonatal EEG in the first 6 hours of life compared to 48-72 hours of life. These include discontinuous patterns, poor differentiated sleep-wake cycles, and transient sharp waves, with progressive normalization indicating neonatal adaptation in brain function. Similar immature patterns are found in low birth weight neonates when compared to normal birth weight neonates.
Abnormal facial or appendicular movements, hypotonia, poor feeding, abnormal low Apgar scores, altered level of consciousness, and unusual imaging could require an EEG if clinical seizures are thought to be the cause. EEG is indicated for determining brain development, identify cortical hyperexcitability, epileptiform activity, seizures, or assessing prognosis.
Neonatal EEG visual analysis helps to assess brain development and functionality as the baby matures week by week. Knowledge of gestation/postconceptional age, topography (skull and scalp thickness, abnormal cranial vault), interelectrode resistance, benign variants appropriate for age, artifacts, and sleep structures is essential for the proper interpretation. Postconceptional age is defined as gestational age (in weeks) plus the number of weeks since birth. Gestational age is the number of weeks/months the child was in the womb. Legal age is the age of the child since birth. Prematurity is defined as birth at a gestational age < 38 weeks; the full term is defined as birth at a gestational age of 38 to 42 weeks.
There are no contraindications except in those patients where the electrodes can not be attached to the head due to skin integrity problems.
EEG recording methods differ between adults and neonates due to smaller head circumference and a lack of significant activity in some cortical regions. Equipment modifications include the use of eight scalp electrodes (FP1, C3, T3, O1, FP2, C4, T4, O2), electrocardiogram (EKG), electromyography (EMG), eye movements, and respiration monitors. There is freedom for additional electrodes for electromagnetic field coverage in the midline (Fz, Pz, Cz). The neonatal montage provides coverage of the centrotemporal regions where most of the neonatal activity occurs.
Adequately trained EEG technicians are essential for proper electrode placement and appropriate recording. They should be familiar with artifactual background activity, prompt correction of misplaced electrodes, or adjust settings. Bias and artifactual background should be minimized. An interdisciplinary team that includes neonatal critical care nurses, primary care providers, consultants, technicians, and respiratory therapists must coordinate care and obtain the most accurate and precise data from the continuous video EEG.
During a neonatal EEG recording, the following steps are essential:
The neonatal EEG total number of electrodes is reduced, given the small head circumference. Usually, a longitudinal or average montage is used, with a standard 10-20 electrode system. Electrodes are connected in frontotemporal, frontocentral, temporal-occipital, and central-occipital measurements. The interelectrode distance is 40% from the diameter of the head for neonatal recordings. Physiological leads include an EMG, EKG, and respiratory monitors; they can distinguish artifactual from epileptiform activity.
The eye electrodes are placed 0.5 cm from the outer canthus. Calibration of montage is necessary to eliminate artifact with a recording speed of 15 mm/s (half from adults), a time constant of 0.3 seconds, 70 Hz low pass filter, and sensitivity of 7 µV/mm. EKG, EMG, and eye electrodes are set at sensitivities of 50 µV/mm, 50 µV/mm, and 7 µV/mm, respectively. These settings may be adjusted as needed and are meant to be guidance. Abnormal sharp spike and slow waves should disrupt the background and be sharply contoured. A recording of at least 2 to 3 hours is recommended to capture waves in the wake and the sleep stages.
The neonatal EEG voltage can be decreased by cooling therapy (head or body cooling) and some medications, including benzodiazepines, barbiturates, morphine, and antiepileptic drugs. Eye movements and respirations help to differentiate the sleep stages. Artifacts in the EEG can be produced by EKG, pulse, nursing care, parental care, respirations, ventilator, incubator, twitches, loud noise, and feeding.
Complications are common during extensive monitoring and include the following:
The use of moisturizers, gel, and topical antibiotics can preserve skin and avoid infections.
Introduction and Obstetric Definitions
Neonatal seizures or convulsions are epileptic events occurring from birth to the end of the neonatal period, which is about 28 days. The neonatal period is the most vulnerable period for the development of seizures, with an incidence of 80% in the first 24 to 48 hours of life. The corrected age should be calculated to interpret a neonatal EEG. Benign variants will be reported according to the corrected age and not the chronological age of the neonate. The first trimester runs from conception to about 12 weeks, the second trimester 13-26 weeks, and the third trimester 27 to 42 weeks.
Visual analysis inspects the background rhythm for continuity/discontinuity, symmetry, synchrony, amplitude, reactivity, and morphology. A continuity pattern is constant and with a similar amplitude, while the discontinuity pattern has high amplitude burst mixed with low amplitude interburst. Background varies with age becoming more continuous, synchronous, and reactive to stimuli with advancing gestational age. Active sleep corresponds to REM sleep, while quiet sleep corresponds to non-REM sleep. Symmetry compares homologous brain regions in both hemispheres for amplitude changes. Synchrony refers to bursts occurring in the right and left hemisphere separated no more than 1.5 seconds. During 31 and 36 weeks, the bursts are 70% synchronous compared to the periods before 31 weeks or after 36 weeks when it is 100% synchronous. The amplitude decreases from 24 weeks until term.
A neonatal epilepsy disorder can point towards a more ominous sign of sustained and permanent brain damage. Early detection, management, and aggressive treatment of seizures and status epilepticus can prevent long term neurological sequela. About 3/1000 neonates will have a seizure disorder in the first two days of life in term babies and up to 132/1000 in pre-term infants. Neonatal seizures can be classified as subtle, tonic, clonic, generalized tonic-clonic, myoclonic, with behavioral features that can be paroxysmal, non-paroxysmal, repetitive, and stereotypical.
EEG patterns can vary depending on the gestational and corrected age. Features to identify in the EEG include changes in posterior dominant rhythm, frequency, wave amplitude, synchrony of burst of activity, sleep wave activity, and percentage of non-REM/REM stages.
2. EEG patterns per gestational age
An interdisciplinary neonatal team of pediatric neurologists, clinical pediatric neurophysiologists, pediatric critical care specialists, neonatal nurses, and respiratory therapists should work together to achieve an optimal video EEG recording. Documentation of clinical symptoms, current medications, topography, gestational age, corrected age, and state of the infant, among other clinical data, is essential and should be conveyed to the EEG reader for accurate study interpretation. Protocols should be in place to assist and facilitate coordination of care. [Level 1]
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