Approach to Visual Analysis and Interpretation
The basic principles of visual analysis and interpretation of the electroencephalogram (EEG) that apply to older patients (Kellaway, 2003) also generally apply to neonates, although with some additional special considerations. For example, because of the rapid rate of cerebral development in the neonatal period, the age-dependent features of the EEG become critically important. The interpretation of the neonatal EEG requires the recognition of EEG changes from conceptional ages (CAs) less than 28 weeks through 44 weeks. In addition, types of abnormalities that are age-dependent must also be identified. Further, because of the special clinical problems of neonates it is critical to understand how specific etiological factors may affect cerebral function and, in turn, the neonatal EEG.
Nearly 50 years ago it was pointed out that the characteristics of the EEG known to be normal and, to a lesser degree, abnormal in neonates had not been established (Kellaway and Crawley, 1964); in 2015, the situation has only modestly improved. One problem is that an assumption of normality cannot be made in a newborn with the same degree of confidence as it can in older children. This is related to the brevity of the period of life available for study, the limitations of the neurological examination, and the limitations of longitudinal studies that would consider the impact of intercurrent disease on long-term development. Traditionally, in studies of neonatal EEG, infants have been considered normal if they had no abnormal neurological signs at birth and were clinically normal at the time of discharge from the hospital. Some manifestations of cerebral dysfunction may not become clinically evident until a certain level of brain maturation has been achieved. In addition, to date, systematic, long-term serial studies that correlate neonatal EEG with neurological, psychological, and behavioral development from birth to adolescence have not been carried out. As a consequence of these limitations, the significance of certain specific features of the EEG has not been established.
In current practice, it is generally assumed that, without intercurrent injury, the infant will demonstrate developmental EEG features based upon either intrauterine or extrauterine determined CA (Selton et al., 2010). Recent data suggest that preterm infants, when reaching term, may have less mature EEG patterns than matched controls (Nunes et al., 2014). In addition, more detailed longitudinal studies have begun to reexamine and verify the normal features of the neonatal EEG (Crippa et al., 2007; Dos Santos et al., 2014; Korotchikova et al., 2009; Vecchierini et al., 2003).
Despite these limitations, the imperatives of clinical practice require a clear presentation of the current knowledge of neonatal EEG. The data presented in this atlas reflect our experience in the study of normal and abnormal neonates and, importantly, the work of other investigators (Blume and Dreyfus, 1982; Clancy et al., 1993; Dreyfus-Brisac, 1957, 1959, 1962, 1964, 1968, 1970, 1978; Dreyfus-Brisac and Blanc, 1956; Dreyfus-Brisac and Monod, 1970; Dreyfus-Brisac et al., 1957, 1961, 1962; Ellingson, 1958, 1979; Engel and Butler, 1963; Lombroso, 1979, 1982, 1985; Monod and Pajot, 1965; Monod et al., 1960, 1972; Sainte-Anne-Dargassies et al., 1953; Tharp et al., 1981; Torres and Anderson, 1985; Torres and Blaw, 1968; Watanabe and Iwase, 1972; Watanabe et al., 1974). In this regard, we owe a large debt to the pioneering French group led by Dreyfus-Brisac. The group’s early studies of the EEGs of premature infants were conducted in a rich institutional environment that was virtually unique in those days. They provided the basic insights that facilitated consistent progress in the development of knowledge and rational interpretation of the EEG of the newborn.
TERMINOLOGY RELATED TO AGE-DEPENDENT DEVELOPMENTAL FEATURES
Traditionally, age-dependent developmental features of the neonatal EEG have been defined in terms of CA. In 2004, the American Academy of Pediatrics (AAP) issued a policy statement, developed by its Committee of Fetus and Newborn, recommending standardization of terminology to describe age during the perinatal period. The AAP recommended the use of the post-menstrual age (PMA) rather than CA (Committee on Fetus and Newborn, 2004). The PMA is calculated as gestational age measured from the time of the last menstrual period plus the chronological age. The Committee recommended against the use of the term CA. The Committee also recommended that in publications reporting fetal and neonatal outcomes, methods should be clearly defined to determine gestational age. The use of the term CA is well-engrained in the historical and current literature related to neonatal EEG and we will continue to use this terminology in this atlas. Thus, gestational age is in the calculation of the use of CA, the last menstrual period occurs approximately 2 weeks before conception.
THE PROCESS OF VISUAL ANALYSIS
In the interpretation of the EEG of children and adults, we believe that the only data that should be considered before visual analysis is initiated are the age and the state of consciousness of the patient (Kellaway, 2003). When interpreting EEGs of neonates, the first of these should not be considered or known, because the determination of the EEG CA is a critical part of the analysis and assessment of the recording. It is recognized that the determination of the state of consciousness of the neonate may be difficult to determine, but as will be shown later, it may have impact on the eventual interpretation of the recording.
Analysis of the neonatal EEG is initially directed toward the determination of CA based upon detection and the recognition of the various developmental features occurring in the record (see Chapter 4). If the EEG lacks recognized developmental features that would permit the determination of CA, then this, in itself, may be evidence of brain dysfunction. Developmental EEG features may suggest a specific CA, but there may be discrepancies between the clinically determined CA and the EEG-derived CA, referred to as external dyschronism. This may simply arise from a miscalculation of the clinically determined age, and is most likely if the EEG is normal in all other respects. The developmental characteristics of the EEG in deep non-rapid eye movement (NREM) sleep may be more immature than those of the EEG awake and in light sleep; referred to as internal dyschronism. In this instance, the most immature features of the deep NREM sleep findings reflect the age at or before which a cerebral insult may have occurred.
The final step in the process of analysis is the detection and characterization of any abnormal features that may be present. These features may also be age-dependent, are described in Chapters 5 to 7, and include characterization of background activity and focal features.
As in older children and adults, visual analysis of the EEG of newborns should be an orderly process, involving a series of logical steps that result in the technical analysis and upon which interpretation is based. Only then should a correlation be made with the clinical history and findings to derive a clinical impression. Figure 1.1 summarizes the steps of this intellectual process.
THE CLINICAL IMPRESSION
The neonatal EEG can be a powerful tool when applied to specific clinical questions (Table 1.1). However, the usefulness of the EEG in these situations will depend upon the scope and quality of the information provided by the referring physician and also upon the clinical neurophysiologist’s understanding of the neurological disorders of newborns.
What Is the CA?
The CA-dependent features of the neonatal EEG are the character of the background activity, the presence of wake–sleep stages, specific waveforms and patterns (collectively referred to as graphoelements) and reactivity (see Chapter 4). Recognition of these features allows the clinical neurophysiologist to determine the CA of an infant usually within a 2-week epoch. There may be clinical circumstances in which the CA of the infant is unknown, indeterminate, or based upon inconsistent data from maternal history, infant physical examination, or prenatal head ultrasound. However, determination of an accurate EEG-derived CA may add to the understanding of ongoing clinical problems and assessment of the potential risk of future concerns.
Is There Evidence of Focal Brain Dysfunction?
As in older children and adults, the EEG in neonates may demonstrate the presence of a consistent focal brain abnormality. Findings such as persistent voltage asymmetries, focal slow activity, and recurrent and persistent sharp waves—either in isolation or in combination—may indicate focal intracranial abnormalities such as subdural fluid effusion, subarachnoid hemorrhage, intracranial hemorrhage, cystic or atrophic lesions, cortical infarction, cerebral malformation, or other focal processes.