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The amplitude-integrated electroencephalogram (aEEG) is a bedside device used for assessing cerebral electrical activity. The aEEG recording represents a signal from an EEG channel that is transformed using signal amplification, passage through an asymmetric bandpass filter to attenuate signals < 2Hz and > 15Hz, semilogarithmic amplitude compression, rectification, and time compression. The original aEEG used a single cross-cerebral channel displayed on a semilogarithmic scale at a speed of 6 cm per hour, along with impedance.1 The result was a band of activity that could be characterized by the upper and lower voltage borders, leading to a description of patterns that non-electroencephalographers could recognize without any formal EEG training. Currently, the role of aEEG in neonatal intensive care units (NICUs) remains unclear; the spectrum ranges from a frequently used device that complements formal EEG for surveillance and monitoring of potential CNS abnormalities to consideration of aEEG as a research tool.
The areas of research most extensively investigated using aEEG include assessment of background activity in newborn encephalopathy and seizure detection. As the 1999 al Naqeeb and Toet articles (reviewed in this issue) discuss, aEEG performed shortly after birth in infants with a diagnosis of perinatal asphyxia or encephalopathy has an encouraging predictive profile for adverse outcomes of death or cerebral palsy assessed beyond 1 year of age. These results are consistent with others reviewed in a recent meta-analysis.2 The aEEG was used as the final step for inclusion criteria (after fulfilling clinical, biochemical, and neurologic assessments) in the Cool-Cap Trial, which evaluated therapeutic hypothermia for neonatal encephalopathy;3 the rationale for using aEEG was to improve the specificity of case selection. At 18 months of age, the rate of death or severe disability among noncooled, control infants in Cool-Cap was 66%. What remains puzzling is that in the National Institute of Child Health and Human Development (NICHD) Whole-Body Hypothermia Trial, which had similar inclusion criteria but did not use an aEEG, the rate of death or severe/moderate disability at 18 to 22 months of age was 62% in noncooled, control infants.4 Moderate disability occurred in only 1 control infant in the NICHD trial, which minimizes the differences in disability criteria between the 2 trials. These observations are difficult to reconcile with the published predictive value of aEEG, in addition to a high level of agreement for pattern classification among multiple observers. Considerations that may affect the use of aEEG include: 1) the expertise of individuals interpreting the aEEG under study and clinical conditions; 2) subjective assessments to assign a pattern, particularly if voltage straddles a cutoff value; 3) limitations of the aEEG in detecting seizures; and 4) use of short recording intervals (eg, 30 minutes) to accurately reflect cerebral activity. Artifacts on aEEG recordings have been noted (up to 12% of recording time) and may be secondary to electrical interference or muscle activity.5 Some investigators have questioned using aEEG as an inclusion criterion for therapeutic hypothermia, based on poor negative predictive value for short-term outcomes.6
Interest in the use of aEEG for detection of seizures is high among clinicians, given the diverse phenotypes of neonatal seizures, possible clinical-electrographic dissociation, and limitations in obtaining a conventional EEG on short notice in many centers. Based on the positioning of the electrodes, it is well recognized that seizures remote from the recording electrodes will not be detected. Some newer aEEG devices provide interhemispheric channels, in addition to the traditional cross-cerebral channels. Time compression facilitates inspection and monitoring of background activity, but limits detection of short seizures. Identification of electrographic seizures using aEEG has improved with the availability of the raw EEG on newer aEEG devices. As discussed in the Shah and Shellhaas articles, reviewed in this issue, availability of the raw EEG and the expertise of individuals interpreting the recording affect seizure detection, thus raising the issue of
whether nonelectroencephalographers can receive sufficient training to use aEEG as an adjunct to conventional EEG for seizure detection.
The aEEG is an attractive device since it provides potential information not readily available in most NICUs. Use of the aEEG has been improved with respect to proper electrode application and attainment of desired impedance, to facilitate implementation by providers without a background in EEG. Other areas currently being investigated will ultimately determine overall use of aEEG in the NICU. Characterization of background activity in a nonbiased, objective manner is essential; incorporation of software to analyze aEEG tracings may improve the value of aEEG in predicting neurodevelopmental outcome. The aEEG will probably be better suited as a screening tool for seizure detection when validated computerized seizure detection algorithms are incorporated into the software. Although the clinical importance of electrographic seizures remains uncertain, determination of whether abnormal movements/behaviors are seizure equivalents is an important issue for clinicians to research.
References
1. |
de Vries LS, Hellström-Westas L. Role of cerebral function monitoring in the newborn. Arch Dis Child Fetal Neonatal Ed. 2005;90(3):F201-F207.
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2. |
Spitzmiller RE, Phillips T, Meinzen-Derr J, Hoath SB. Amplitude-integrated EEG is useful in predicting neurodevelopmental outcome in full-term infants with hypoxic-ischemic encephalopathy: a meta-analysis. J Child Neurol. 2007;22(9):1069-1078. |
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3. |
Gluckman PD, Wyatt JS, Azzopardi D, Ballard R, Edwards AD, Ferriero DM, et al. Selective head cooling with mild systemic hypothermia after neonatal encephalopathy: multicentre randomised trial. Lancet. 2005;365(9460):663-670 |
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4. |
Shankaran S, Laptook AR, Ehrenkranz RA, Tyson JE, McDonald SA, Donovan EF, et al; National Institute of Child Health and Human Development Neonatal Research Network. Whole-body hypothermia for neonates with hypoxic-ischemic encephalopathy. N Engl J Med. 2005;353(15):1574-1584. |
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5. |
Hagmann CF, Robertson NJ, Azzopardi D. Artifacts on electroencephalograms may influence the amplitude-integrated EEG classification: a qualitative analysis in neonatal encephalopathy. Pediatrics. 2006;118(6):2552-2554. |
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6. |
Sarkar S, Barks JD, Donn SM. Should amplitude-integrated electroencephalography be used to identify infants suitable for hypothermic neuroprotection? J Perinatol. 2008;28(2):117-122. |
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