ICU EEG-Background

ICU EEG—Background

Written by Trent Tollefson, MD and Sarah Durica, MD

Both routine and continuous EEG are commonly performed in critically ill patients in the ICU. EEG can be used to evaluate for seizures, characterize paroxysmal events, provide data for prognostication, and monitor for cerebral ischemia.

Background Voltages and Continuity:

When describing the ICU EEG background, it is important to describe background continuity. To assess continuity, the definitions of suppression and attenuation must be understood. The American Clinical Neurophysiology Society’s Standardized Critical Care EEG Terminology: 2021 Version defines suppression as voltages less than 10 microvolts and attenuation as voltage greater than or equal to 10 microvolts but less than 50% of background voltage.

A background is considered continuous when periods of suppression or attenuation make up less than 1% of the recording (Figure 1). Nearly continuous backgrounds contain 1–9% suppression or attenuation. Discontinuous backgrounds include 10–49% such periods (Figure 2). Burst-suppression or burst-attenuation patterns are present when suppression or attenuation occupies 50–99% of the EEG. Finally, a background is classified as suppressed when more than 99% of the tracing meets criteria for suppression.

Figure 1: Continuous background. Image Credit: Trent Tollefson, MD

Figure 2: Discontinuous background. Image Credit: Zulfi Haneef, MD

Figure 3: Burst suppression. Image Credit: Trent Tollefson, MD

Reactivity

Another important feature of the ICU EEG is background reactivity. A reactive background will show demonstrable change (e.g. increase in faster frequencies or attenuation) in the EEG waveforms in response to external stimuli such as verbal, tactile, or noxious stimulation (Figure 4). The appearance of muscle activity or eye-blink alone without other changes should not be interpreted as reactivity. The presence or absence of spontaneous state changes should also be assessed. Some patients may not have discernable waking or sleep states but may have distinct alternating patterns known as Cyclic Alternating Pattern of Encephalopathy (CAPE). The clinical significance of CAPE is still being studied.

Other features of the ICU EEG background that can be described include the symmetry, the voltage, the predominant frequency band(s), and the presence of elements of organization such as the posterior dominant rhythm and anterior-posterior voltage and frequency gradients.

Figure 4: Reactivity is present in this EEG. Note the increased prominence of theta frequencies in the last 4-5 seconds of this recording in response to stimulation. Image Credit: Trent Tollefson, MD