Continuous brain-function monitoring: State of the art in clinical practice

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Summary

Continuous electroencephalographic (EEG) monitoring gives direct information on brain function in newborn infants needing intensive care. To improve the possibilities of long-term monitoring, the EEG is time-compressed and recorded with a reduced number of electrodes. A trend measure of the EEG, the amplitude-integrated EEG (aEEG), has proved capable of giving relevant information in newborn infants of differing levels of maturity. The electrocortical background activity gives information on the level of brain activity, which is associated with outcome in both term asphyxiated infants and in preterm infants. However, the background activity is also affected by several medications, and this must be considered when interpreting the aEEG trace. The aEEG also reveals subclinical epileptic seizure activity, and can be used for evaluation of anti-epileptic treatment. The aEEG should be used as a complement to the standard EEG, and close collaboration between neonatologists and clinical neurophysiologists is necessary for optimal performance of EEG monitoring.

Introduction

Continuous brain-function monitoring with amplitude-integrated electroencephalography (aEEG) is now a daily part of clinical surveillance of sick newborn infants in many neonatal intensive care units (NICUs). During the last decade the aEEG method has become increasingly acknowledged as a method for continuous evaluation of brain function in neonates. One reason for this was the finding that the very early background pattern is sensitive for predicting outcome in asphyxiated full-term infants even during the first postnatal hours.1, 2

However, the aEEG is not a new method for monitoring cerebral function. The cerebral function monitor (CFM) was created in the late 1960s by Maynard et al.3 The original CFM was used to monitor brain function in adult patients with status epilepticus, after cardiac arrest, and during surgery. The first neonatal CFM recordings were performed in the 1970s and 1980s, and included normal tracings from term and moderately preterm infants, as well as reports on cerebral recovery after severe hypoxic–ischaemic insults, and of detection of clinically silent seizures.4, 5, 6, 7, 8 The original CFM concept has been developed further, and the method is often called amplitude-integrated EEG (aEEG) to denote a method rather than a specific monitor. This chapter will review previous findings and current use of aEEG monitoring in the NICU. In the future, the aEEG and other EEG trends will probably constitute an integrated part of standard clinical monitoring in newborn infants needing intensive-care treatment. The aEEG has mainly been used for the following purposes in newborn infants, which are further discussed below.

  • 1.

    Evaluation of cerebral recovery after a hypoxic–ischaemic insult.

  • 2.

    Detection of epileptic seizure activity and evaluation of anti-epileptic treatment.

  • 3.

    Cerebral monitor as part of the clinical monitoring.

Section snippets

Amplitude-integrated EEG method

The aEEG method is based on a time-compressed semi-logarithmic (linear 0–10 μV, logarithmic 10–100 μV) display of the peak-to-peak amplitude values of a filtered and rectified EEG. The EEG is passed through an asymmetric band pass filter that strongly enhances intermediate EEG frequencies. Most EEG activity below 2 Hz and above 15 Hz is suppressed in order to minimize artefacts from sweating, movements, muscle activity and electrical interference. The bandwidth of the aEEG trace reflects variations

Normal aEEG tracings in term and preterm infants

Knowledge from neonatal EEG studies can be used when evaluating aEEG. This is relevant especially for evaluation of background patterns and inter-burst intervals (IBIs) and for interpretation and verification of epileptic seizure activity.9, 10 Several studies have described normal aEEG development in term and preterm infants. The aEEGs have been described from various aspects: amplitude (minimum and maximum voltage), pattern and cyclicity corresponding to sleep–wake cycling (SWC). Three early

Suggested new classification of aEEG tracings

Classification and description of aEEG tracings differ between studies. Some classifications are mainly for term infants, and some fit only the normal aEEG recordings. In order to have a unifying nomenclature and definition that suits both term and preterm infants, as well as normal and abnormal circumstances, we recently suggested a new classification of aEEG tracings (Table 1; Figure 2, Figure 3).25

Evaluation of cerebral recovery after hypoxic–ischaemic insult

Several studies have shown that the aEEG is very accurate for prediction of outcome in term asphyxiated infants between 3 and 48 h postnatal age.1, 2, 26, 27 Around 80–90% of asphyxiated infants are correctly predicted by the aEEG background pattern at 3–6 h postnatal age. The sensitivity of the aEEG is highest between 6 and 24 h, and is lost after 48 h.28 The specificity for predicting abnormal outcome is increased when the early aEEG is combined with a clinical evaluation.29 Infants with a

Effects of medications

Several medications – among them phenobarbital, diazepam, midazolam, morphine, lidocaine, surfactant – affect the electrocortical background,47, 48, 49, 50 usually by inducing transient or more sustained depression of electrocortical background activity. The aEEG background of preterm infants may be transiently but markedly depressed by medications such as diazepam, midazolam and surfactant. Administration of phenobarbital or morphine to term or preterm infants is often associated with a

Conclusion

The aEEG concept has proved to be of clinical value in sick neonates and aEEG monitoring is now increasingly used in neonatal intensive care units. The aEEG method does not replace the standard EEG, but should be used as a complement to EEG in high-risk infants. In the future other EEG trends will probably be evaluated (Fig. 5), and cost–benefit analyses will hopefully be performed. The results from an ongoing Dutch trial, evaluating outcome after use of aEEG and treatment of subclinical

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