Quantitative EEG: basics, seizure detection and avoiding pitfalls
9 Quantitative EEG: basics, seizure detection and avoiding pitfalls
Due to technical advances and increased awareness of the high prevalence of nonconvulsive seizures, prolonged EEG recording in the critically ill is standard of care in many centers. It is clear that the majority of seizures in the intensive care unit are nonconvulsive and can only be recognized with EEG. It is also clear that it is labor-intensive to review these studies. Quantitative EEG techniques have helped immensely in speeding this up. Although many are intimidated by these techniques, they are actually quite simple in principle and easy to learn. As software programs have improved, QEEG has become more and more useful. Not only can seizures be detected, but other acute brain events as well, such as ischemia, hydrocephalus, hemorrhage and so on. Several existing software programs will enable one to set alarms and, theoretically, to do true real-time monitoring once the infrastructure has been put in place and someone is available to respond to the alarms and interpret the study (currently only possible in a small percent of institutions). In the not-too-distant future, this type of real-time monitoring, or ‘neurotelemetry’, will be available in many centers, akin to cardiac telemetry today. This chapter covers the initial basics of quantitative EEG with some of the special indications and uses outlined in a subsequent chapter.
Perhaps the simplest way to consider QEEG is to think of it as an additional montage. As introduced in the very first chapter of this book, selecting a different montage does not change the content of the EEG, but rather how this content is displayed. QEEG merely has the advantage of being able to display hours and even days of EEG on a single page. As previously introduced, different montages (or ‘trends’, the term commonly used with QEEG) have strengths and weaknesses, and this concept also applies to the use of QEEG. The QEEG can be thought of as a Swiss army knife. As a whole it is immensely versatile. However, the ability to apply it to every situation depends on the understanding of the range of unique tools that it contains. This chapter aims to introduce the abilities of QEEG, demonstrate certain limitations, and most importantly demonstrate how these limitations can often be overcome by knowledge-based selection of a trend best suited to the task.
Quantitative EEG is excellent for detecting seizures, rhythmic and periodic patterns, asymmetries, and for following long-term trends, as discussed in this chapter. The use of QEEG can reduce review times by up to 80% compared to manual screening of the raw EEG, while maintaining good (but not perfect) sensitivity. This can be achieved with appropriate training at a resident level and is easy to incorporate into regular EEG review. Automated seizure detection algorithms have also improved over time and are now at the stage of demonstrating non-inferiority compared to expert review in some studies. The gold standard remains review of the raw EEG; however, QEEG can drastically reduce the time this takes, can highlight subtle features such as asymmetries that can be hard to appreciate, and can easily demonstrate long-term trends, which are often hard to register if looking at the EEG page by page. In addition, it allows detailed, focused review of times of interest (especially times of changing EEG patterns or automated detections) rather than superficial review of all times (as occurs with rapid review of the entire raw EEG). QEEG is particularly useful in monitoring for ischemia; such special applications will be covered in Chapter 10.
9.1 Spectrogram basics
Figure 9.1 Spectrogram basics: state changes and alpha rhythm.
Figure 9.2 Spectrogram basics: mechanical artifact – bed oscillator.
Figure 9.3 Spectrogram basics: drug-induced beta and asymmetry.
Figure 9.32 QEEG pitfalls and solutions: artifact mimicking seizures on amplitude-integrated EEG (aEEG).
Figure 9.33 QEEG pitfalls and solutions: multiple seizures and identical-appearing false positives on amplitude-integrated EEG (aEEG).
Figure 9.34 QEEG pitfalls and solutions: brief right and left hemispheric seizures in the context of GPDs and muscle artifact.
Figure 9.35 QEEG pitfalls and solutions: very focal seizures within breach effect.
Figure 9.36 QEEG pitfalls and solutions: very focal unilateral independent seizures and regional rhythmicity.
Figure 9.37 QEEG pitfalls and solutions: bilateral independent BIRDs and seizures.
EEGs throughout this atlas have been shown with the following standard recording filters unless otherwise specified: LFF 1 Hz, HFF 70 Hz, notch filter off.
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