Neurophysiologie Clinique/Clinical Neurophysiology
Review Mise au pointPathological features of neonatal EEG in preterm babies born before 30 weeks of gestationnal ageLes aspects pathologiques de l’EEG du nouveau-né prématuré avant 30 semaines d’âge postmenstruel
Introduction
Preterm birth is associated with neurodevelopmental impairment. The most severe adverse outcomes can be diagnosed before the age of two when motor impairments or severe cognitive delay may be detected. Premature birth is also related to learning disabilities that become apparent in later childhood (at school age) [37], [40]. The origins of these sequelae are multifactorial; some are clearly linked to neonatal brain insults [21] but antenatal pathological conditions may play a role as well as events in early childhood. Social and educational environment is also a key factor.
Most brain injury occurring in the antenatal and neonatal period that leads to neurodevelopmental impairment tends to be clinically silent. Routine neurological assessment by cerebral imaging and neurophysiological recordings is required. Long-term follow-up until school age is mandatory in this population in order to assess the effects of specific EEG and cerebral imaging abnormalities on outcomes. Routine neonatal neurological assessment is unable to detect non-neurological injuries such as sensorial impairment and will not detect events that occur after the neonatal period. Therefore the relationship between adverse outcome and neonatal events is important to keep in mind when the prognostic value of EEG or cerebral imaging is considered.
Cerebral imaging allows diagnosis of brain injury as it becomes established. Bedside cranial ultrasound is useful for detecting severe white-matter lesions such as periventricular leucomalacia; diffuse or cystic (see clinical case report of Julien and Rose below), unilateral or bilateral intraventricular hemorrhage, hemorrhagic parenchymal infarction and ventricular dilation. MRI gives more precise information on diffuse and punctuate white-matter lesion, cerebellar hemorrhage, and basal ganglia and thalamic abnormalities [16], [19], [47], [68].
Neonatal EEG allows a precise study of brain maturation during a phase of rapid evolution – equivalent to the third trimester of gestation ex-utero. This maturation process follows a precise schema, which can be assessed using successive EEGs that show progressive changes over short periods, closely linked to the post-menstrual age (PMA) [3]. EEG as a neurophysiological tool has an excellent temporal resolution but a low spatial resolution. This is more evident in the newborn as the smaller size of head allows fewer electrodes. Hence, precise information on location of potential brain lesions, which may be the cause of the abnormal waveforms, is not obtained. Conversely, EEG allows a comprehensive approach to the study of evolution of ongoing cerebral injury bringing additional information to cerebral imaging [23], [30]. As a result, the accuracy of EEG data depends greatly on the timing of the record and serial recordings are recommended [33]. This early diagnosis of presymptomatic or subclinical ongoing brain insult may allow a window for neuroprotection before the constituted brain lesions become established.
Analysis of abnormal EEG is commenced with the description of significant changes of background activity and of pathological features such as sharp waves or ictal events. The combination of these abnormalities constitutes a selection of EEG patterns of various severities. Some of these patterns may herald the onset of severe brain lesion, such as PVL. Precise information regarding the medication the infant is receiving at the time of each EEG is required as most sedative drugs can affect EEG patterns. Several clinical cases are presented to illustrate the practical use of EEG in NICU.
Section snippets
Background abnormalities
Numerous parameters contribute to the constitution of a normal background EEG pattern; continuity, amplitude, spatial and temporal organization, spontaneous or provoked lability, synchrony and symmetry, sleep state cycle. Between 24 and 30 weeks, PMA the most important parameter is continuity, significantly correlated to the degree of global cortical folding [7], but the presence of sleep state changes remains an excellent index of normality.
Assessment of continuity should include the
Interpretation of abnormal EEGs patterns
Background abnormalities combined with ictal or non-ictal transients constitute abnormal patterns of different diagnostic and prognostic values. Various classifications have been proposed. The first studies made a distinction between moderate or marked abnormalities [59]. More recent studies by Watanabe et al. [67], introduced information regarding the time course of EEG abnormalities relating to the suspected brain injury and distinguished between “acute-stage” and “chronic-stage”
Diagnostic and prognostic value of abnormal EEG patterns
Specific significance of EEGs have been studied in comparison with either cerebral ultrasound findings, clinical outcome or both. Comparing EEG and cranial sonograms allows evaluation of the diagnostic accuracy of EEG for morphologic brain lesions. But babies with normal head ultrasound scans still share a risk of neurodevelopmental delay. Hence, it is important to assess the prognostic value of EEG abnormalities independently from cerebral imaging.
Neurotropic drugs effects’ on preterm EEGs
In neonates including the very pretem infants, anticonvulsive, analgesic and sedative drugs are presently of current use in neonatal intensive care units. The purpose of these treatments is to reduce stress and pain and to facilitate mechanical ventilation in order to improve survival and decrease morbidity in the neonates [1], [2].
Several kinds of drugs are administrated to neonates: anticonvulsive drugs as phenobarbitone, analgesics as morphine and synthetic opioids, sedative drugs as
Conclusion
This co-authored study emphazises the fundamental contribution of neonatal EEG in assessing normal and pathological brain maturation. This technique remains very sensitive to artifacts and require a reliable technique for each EEG and respect of a precise schedule of serial recordings.
Recent technical improvement now allows easy long-term recordings, routine EEG video recordings and on-line or off-line signal analysis which may bring additiontional information to visual analysis. Conventional
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