Amplitude-Integrated EEG (aEEG)
Nathalie El Ters
Stephanie S. Lee
Amit M. Mathur
Conventional EEG (cEEG) in neonates remains the gold standard for EEG monitoring but requires skilled personnel to apply leads and interpret the EEG. Longitudinal studies can be challenging because of the high maintenance requirements of cEEG leads and the concern for skin breakdown in newborns. Amplitude-integrated EEG (aEEG) is a limited-channel modality that has the benefit of easy lead application and interpretation by clinicians. The EEG signal is recorded using one or two channels consisting of two or four electrodes, respectively, placed in the C3-P3, C4-P4 areas of the newborn’s head. The raw EEG signal is amplified, filtered by attenuating frequencies below 2 Hz and above 15 Hz to minimize muscle artifact, noise and electrical interference, compressed and displayed in a semi-logarithmic scale alongside the raw EEG (Fig. 14.1) (1).
INDICATIONS FOR aEEG MONITORING
A. Seizures
1. The true incidence of seizures in newborns is difficult to estimate due to the atypical presentation and subclinical nature of these events, especially in the preterm infants.
2. The reported incidence in term infants ranges between 1 and 5 per 1,000 newborns (2).
B. Population At-Risk
1. Hypoxic ischemic encephalopathy (HIE) (3)
2. Brain injury in acutely ill neonates (severe persistent pulmonary hypertension [PPHN], congenital heart defects requiring cardiopulmonary bypass, ECMO)
3. CNS infection
4. Intracranial bleeds, perinatal strokes, and venous sinus thrombosis
5. Inborn errors of metabolism, genetic syndromes involving the central nervous system
6. Preterm infants with high-grade intraventricular hemorrhage (IVH) or encephalopathy
C. Equipment
1. aEEG monitors: These are freestanding machines that are easy to set up and use.
a. Currently available aEEG comes with a seizure detection algorithm and an optional software for automated background classification.
b. The RecogniZe (Natus Medical Inc., San Carlos, CA) algorithm detects areas of regularity in an
EEG waveform, using at least five similar consecutive waves with wavelengths that are equivalent to a frequency of 14 Hz or less, peak-to-peak amplitude greater than 5 µV and at least 21 seconds of continuous detection, or 26 seconds of discontinuous detection, in 1 minute of EEG signal.
EEG waveform, using at least five similar consecutive waves with wavelengths that are equivalent to a frequency of 14 Hz or less, peak-to-peak amplitude greater than 5 µV and at least 21 seconds of continuous detection, or 26 seconds of discontinuous detection, in 1 minute of EEG signal.
 FIGURE 14.1 A: aEEG machine recording at the bedside of a preterm infant. B: Three hours of aEEG with 10 seconds of EEG from a preterm infant at term equivalent age, whose brain MRI at term was normal and his aEEG tracing shows normal continuous background with mature sleep-wake cycling. |
2. Types of electrodes
a. Subdermal needle electrodes
b. Gold cup electrodes
c. Hydrogel electrodes
D. Procedure
1. Preparation of the skin and application of the electrodes
a. Using a cotton swab, the skin is gently cleaned with Nuprep gel (Weaver and Company, Aurora, CO, USA)
(1) Nuprep is a mildly abrasive cleansing gel that effectively cleans the surface of the skin in order to achieve lower impedance.
(2) This step is used with subdermal needles, hydrogel, and gold cup electrodes.
b. Applying gold cup electrodes
(1) This method uses a conductive paste and a paper tape to fix the electrodes in place.
(2) Collodion is not used in this population due to its toxic properties, and it is usually used to glue down gold cup electrodes.
(3) This may affect the quality of the recordings and the stability of the electrodes, especially in a humidified and warm environment such as in an incubator.
c. Applying subdermal needles
(1) Clean the scalp around the insertion sites with an antiseptic solution appropriate for gestational age.
(2) Insert the needle electrodes at the insertion sites subdermally. The needles should be angled downward.
(3) Secure the electrodes with a tape over the needles.
d. Applying hydrogel electrodes
(1) Hydrogel electrodes are an alternative to gold cup and subdermal needles in aEEG recordings when used with adequate skin preparation (4), and they have replaced gold cup and subdermal needle electrodes in continuous aEEG monitoring in the neonatal intensive care unit.
(2) Hydrogel electrodes have the advantage of increased stickiness on the preterm skin especially in a humidified environment, and they have a flat surface that can decrease the pressure point on the skin when the infant’s head lies on them. In addition, hydrogel electrodes are sterile and disposable.
2. Placement of electrodes
a. Electrodes are placed in the P3-P4 positions for single-channel positions and C3-P3 and C4-P4 for two-channel.
(1) Electrode positions are based on the International 10-20 system for newborns, as shown in Figure 14.2 (5).
 FIGURE 14.2 The electrode positions are adopted according to the International 10-20 system for the newborn. For two-channel recordings, electrode positions are placed in F3-P3 and F4-P4 positions or C3-P3 and C4-P4 positions. |
(2) The electrode locations are determined by specific measurements between the landmarks of the skull.
(3) The numbers 10 and 20 refer to the measurements between the electrodes as 10% or 20% of the total front-back distance or total right-left distance of the head.
(4) The reference electrode is positioned in the midline on the forehead.
b. Most aEEG machines are accompanied by an electrode placement tape or measuring device that helps with electrode placement at C3-P3 and C4-P4 locations. The measuring device is placed between the tragus and the sagittal suture. Two arrows indicate the position of C3 and P3 on the left, C4 and P4 on the right.
3. Impedance
a. Continuous monitoring of electrode impedance is critical to all EEG recordings.
(1) The most important advancement in aEEG digital monitoring is the introduction of simultaneous display of raw EEG signal and continuous impedance monitoring.
(2) These two components allow clinicians and staff to detect artifact and alert them about improper electrode contact.
b. An impedance detector, incorporated within the device, alerts the staff about the quality of the tracings and about the location of the loose electrode.
(1) High impedance makes the EEG signal unreliable and will prevent triggering of the seizure detection algorithm. Ideally impedance should be <10 Ω although values of 10 to 20 Ω are acceptable. If poor impedance is detected, remove the electrode and perform a gentle cleaning of the underlying area before applying it again.
4. One-channel versus two-channel
a. Two-channel aEEG recordings allow clinicians to compare between the right and left hemispheres, detecting asymmetries or underlying focal abnormalities (6). This function cannot be achieved with single channel which detects the electrical activity across the cerebral hemispheres.
b. Using single-channel aEEG, without raw singlechannel EEG for confirmation, individual seizure detection is less than 50% (7).
(1) In a study comparing single-channel aEEG alone, two-channel aEEG alone, and two-channel aEEG with continuous raw EEG, the authors found a higher sensitivity and specificity for detecting seizures when aEEG is read simultaneously with raw EEG (8).
(2) Most new digital monitors display both the aEEG and the simultaneous raw EEG trace allowing a more specific interpretation of possible seizure events.
5. Duration of recordings
a. There is no definite guideline for the duration of aEEG.
b. Earlier analysis of aEEG in neonates relied on 3- to 4-hour recordings of aEEG as described in literature (1, 9, 10).
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