ICU-Rhythmic and Periodic Patterns

ICU-Rhythmic and Periodic Patterns

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.

Rhythmic and Periodic Patterns:

Rhythmic and periodic patterns refer to typically interictal patterns that signify increased risk for seizure. Definitions and descriptions of these patterns can be reviewed in detail by reading The American Clinical Neurophysiology Society’s Standardized Critical Care EEG Terminology: 2021 Version. Briefly, these patterns can be described based on their spatial distribution (Main Term 1) and their morphology (Main Term 2).

Main Term 1: Localization

Patterns can be described as generalized, lateralized, bilateral independent, unilateral independent, and multifocal. Generalized (G) patterns are expressed bilaterally and synchronously throughout the record (Figures 1 and 4), while lateralized (L) patterns occur within one focus in a single hemisphere (Figure 2). When more than one rhythmic or periodic pattern is present, they are described as bilateral independent (BI) when occurring asynchronously within each hemisphere (Figure 3) or unilateral independent (UI) when there are 2 foci within the same hemisphere. If three or more foci are present with at least one focus occurring in each hemisphere, then the pattern is described as multifocal (Mf).

Main Term 2: Morphology

Morphologically, patterns are described as periodic discharges, rhythmic delta activity, or spike/sharp-and-wave activity. Periodic discharges (PDs) look similar each time, are of similar duration, have a clear interdischarge interval, and recur at regular intervals for at least 6 cycles (Figures 1-3). Rhythmic delta activity (RDA) is repetition of a waveform with consistent shape and duration without any interdischarge interval, occurring for at least 6 cycles (Figure 4). Spike-and-wave or sharp-and-wave (SW) refers to spike, polyspike, or sharp waves that is consistently followed by a slow wave forming a repeating spike-wave pattern for at least 6 consecutive cycles with a stable relationship between the spike and the wave and no interdischarge interval between consecutive spike-wave complexes.

Figure 1: Generalized Periodic Discharges (GPDs). Image Credit: Zulfi Haneef, MD

 

Figure 2: Lateralized Periodic Discharges (LPDs). Image Credit: Lynn Liu, MD

 

Figure 3: Bilateral Independent Periodic Discharges (BIPDs). Image Credit: Jay Gavvala, MD

 

Figure 4: Generalized Rhythmic Delta Activity (GRDA). Image Credit: Zulfi Haneef, MD

 

Plus (+) Features

If rhythmic or periodic patterns include intermixed fast activity, they can be described with a +F modifier (e.g. LPD+F, Figure 5). In addition, periodic discharges with superimposed rhythmic activity can be described with a +R modifier. Rhythmic Delta Activity with superimposed sharp waves or spikes can be described with a +S modifier. Use of plus modifiers generally indicates a higher potential for seizures and suggests a pattern is on the Ictal-Interictal Continuum (IIC). Rhythmic or periodic patterns between 1-2.5 Hz may also be on the IIC.

Figure 5: Lateralized Periodic Discharges + Fast Activity (LPD+F). Image Credit: Zulfi Haneef, MD