This chapter provides a brief introduction to fundamental aspects of electroencephalography (EEG), with a special emphasis on aspects of EEG that are particularly relevant to video-EEG monitoring (VEM). The sections below will summarize the biological basis of EEG, as well as its nomenclature, techniques for display, normal patterns, common artifacts, and relevant abnormalities. Most readers will already be familiar with common patterns found in the normal adult EEG, but several excellent references and atlases are available for more detailed discussions of these topics, as well as unusual variants and neonatal and pediatric EEG patterns.1–6 Although all components of routine EEG are applicable to VEM, several aspects of VEM introduce additional complexities in the acquisition, display, and accurate interpretation of the EEG. These aspects are discussed in more detail in subsequent chapters, with this chapter focusing on routine EEG interpretation for the beginning electroencephalographer.
Despite significant advancements in the design and manufacture of digital EEG and VEM systems, the active engagement of an experienced EEG technician remains a critical component for the reliable production and interpretation of EEG recordings. The introduction of digital media for both EEG and video has eliminated many routine maintenance tasks that historically fell under the supervision of EEG technicians, such as adjusting recording parameters and montages, ensuring the proper function of the ink writer, and accurately annotating the study with descriptions of behaviors or circumstances that may alter the EEG recording. Nevertheless, correct positioning and maintenance of electrodes; identifying, eliminating, or minimizing artifact in the modern acute care setting; and supervising the reliable and efficient storage of digital data have introduced new demands on the EEG technician (see Chapters 2 and 4).
Biological Sources of EEG Signals
Surface EEG represents a dynamic spatiotemporal summation of the extracellular current flow associated with excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) as they reach their synaptic targets along the soma, axon, or dendritic tree of the pyramidal neuron.7 Thus, an EPSP along an apical dendrite produces positive current flowing into the cell (a current “sink”) and a corresponding extracellular field that is negatively charged. Passive current flowing out of the soma (a current “source”) maintains the electrical neutrality of the system but generates an extracellular electrical field, or dipole, in a radial orientation, with the negative pole directed toward the cortical surface (Figure 1-1). These extracellular dipoles are more prolonged and more spatially synchronized than action potentials mediated by fast sodium channels. Thus, the spatiotemporal summation of extracellular dipoles is the most likely biological source of cortical electrical activity recorded in the EEG. Evidence for a prominent role for the thalamus in synchronizing both normal and pathologic EEG activity dates to the mid-20th century, when repetitive stimulation of the thalamus was shown to entrain rhythmic activity across a wide area of the cerebral cortex8 and later demonstrated to produce a ...