22 Subcortical vascular dementia is more likely to cause a loss of fast activity, which helps to differentiate it on QEEG from Alzheimer dementia, which causes more slowing of the posterior dominant alpha mean frequency. 19–21 Degree of QEEG impairment corresponded to the stroke volume, and preserved QEEG cortical function often corresponded to subcortical lacunes as the pathology. 8-3).ĭegree of slowing or loss of fast activity corresponded to National Institutes of Health Stroke Scale (NIHSS) outcome and disability at 30 days and 6 months.
17, 18 The kinds of EEG changes seen were similar despite obvious differences in the pathology, leading the investigators to conclude that these tests may be sensitive but are nonspecific ( Fig. QEEG does not differentiate among types of focal cerebral pathology, e.g., ischemic infarction, intracranial hemorrhages, brain tumors, and head trauma. Several groups have found correlation coefficients of 0.67 to 0.76 relating such EEG features to metabolic parameters. Relationships are particularly good for relative delta or alpha activity and the ratio of slow to fast rhythms. QEEG changes correspond well to regional cerebral blood flow, regional oxygen extraction, and metabolism. However, QEEG did not do well in precise localization. Routine EEG was abnormal in only one-half of these patients, and often was just diffusely abnormal or poorly localizing. In three further studies of 15 to 20 patients each, QEEG was abnormal in 85 percent of patients, but no abnormal results were found in control subjects. This included QEEG abnormalities in 84 percent of patients whose EEGs had been read as normal in routine visual EEG assessment. 14 The classification was abnormal in 90 percent of the patients, compared with only 3 percent false-positive among the normals. In one study, QEEG prospectively classified 64 normal control subjects and 94 patients (54 with stroke and 40 whose symptoms cleared within a week). Routine EEGs are abnormal in about 40 to 70 percent of patients with cerebrovascular accidents (CVAs). Nevertheless, it is inexpensive, noninvasive, reproducible, and sensitive, and it can be done in a variety of settings (e.g., the intensive care unit or operating room), even on a continuous monitoring basis. EEG, whether quantified or routine, can detect abnormalities but cannot differentiate between kinds of pathology nor does it have the exquisite localizing ability of CT or MRI. QEEG is more sensitive than routine EEG for detecting cerebral hemispheric abnormalities related to cerebrovascular disease. Nuwer, Pedro Coutin-Churchman, in Aminoff's Electrodiagnosis in Clinical Neurology (Sixth Edition), 2012 Cerebrovascular Disease Many types of artifact may also resemble seizures on QEEG, leading to “false positives.” Therefore QEEG trending displays should always be interpreted in conjunction with careful review of the accompanying raw EEG tracing. Seizures of low amplitude or shorter duration are more challenging to identify by QEEG ( Stewart et al., 2010). The sensitivity of QEEG displays for seizure identification can reach as high as 80% however, sensitivity varies by seizure type. Frequency-specific EEG power is depicted on the y-axis, with varying degrees of EEG power (power = amplitude 2) depicted using a color-coded scale. CDSA is a technique that applies fast-Fourier transformation (FFT) to convert raw EEG signals into a time-compressed and color-coded display, also termed a color spectrogram. The top and bottom margins of the aEEG tracing reflect the maximum and minimum EEG amplitudes at a given time. aEEG is a technique that displays time-compressed and rectified EEG amplitude on a semilogarithmic scale. 35.23, A and B illustrate the typical appearance of seizures on amplitude-integrated EEG (aEEG) and color density spectral array (CDSA) displays, respectively. One of the most appealing applications of QEEG displays is their potential use as a screening tool for seizures. Table 35.1 lists QEEG display tools commonly available from various manufacturers and their primary clinical applications. However, it is important to emphasize that QEEG tools should not replace careful review of the underlying raw EEG. To address this challenge and facilitate interpretation of prolonged EEG recordings, several quantitative EEG (QEEG) display tools have been developed to provide insight into trends in the EEG over time and to highlight significant electrographic events. Increasing awareness and concern about NCSs has led to a growing demand for continuous EEG monitoring in ICUs, generating large volumes of data that can be overwhelming to interpret using conventional reviewing techniques that display 10–20 seconds of raw EEG data per screen.
Joseph Jankovic MD, in Bradley and Daroff's Neurology in Clinical Practice, 2022 Quantitative Electroencephalogram