LEARNING OBJECTIVES

Introduction

Periodic and rhythmic patterns are EEG findings that are unique to the critical care setting. While most studies to date have examined the incidence and significance of these patterns in adults, pediatric electroencephalographers also encounter these patterns and are asked to interpret their significance and offer recommendations on management. This chapter will review the approach to identification and classification of periodic and rhythmic patterns in the pediatric ICU, discuss associations of these patterns with etiology and outcome, and introduce the ictal-interictal continuum (IIC).

American Clinical Neurophysiology Society Standardized Critical Care EEG Nomenclature¹

To standardize ICU EEG interpretation, the American Clinical Neurophysiology Society (ACNS) standardized critical care EEG nomenclature in 2012, with an update expected soon. Key features of the standardized nomenclature are a transition away from designating patterns as epileptiform, given that it is often unclear whether these patterns are ictal or interictal, as well as a move away from naming patterns according to etiology. For example, periodic lateralized epileptiform discharges (PLEDS) are now called lateralized periodic discharges (LPDs). Likewise, triphasic waves, classically associated with hepatic encephalopathy, are now called generalized periodic discharges (GPDs) with a triphasic morphology. In making these changes, the nomenclature aimed to create standard definitions to allow for effective research into the significance of these patterns. The key to the critical care EEG nomenclature relies on understanding main terms 1 and 2.

Main Terms

MAIN TERM 1

When a periodic or rhythmic pattern is identified, the first step is to decide on its location as generalized (G), lateralized (L), bilateral independent (BI) or multifocal (MF). A generalized pattern is one that is seen equally in both hemispheres. Generalized patterns are bilateral, synchronous, and symmetric. A generalized pattern may have a restricted field. It can be bifrontally predominant, bioccipitally predominant, or midline predominant, in which case the amplitude in the bifrontal or bioccipital regions is equal (i.e., O1 and O2) and the amplitude in the bifrontal, bioccipital or midline regions is >50% of the amplitude of the other channels. Lateralized is used to describe a pattern that is confined to one region or hemisphere. Lateralized patterns can be unilateral or bilateral asymmetric, wherein the waveform is prominent in one region or hemisphere but has a synchronous field in the opposite hemisphere. Bilateral independent patterns are two independent or asynchronous lateralized patterns in opposite hemispheres. Multifocal refers to 3 or more lateralized, independent patterns with at least one in each hemisphere. Both bilateral independent (BI) and multifocal (MF) patterns can be prominent in one region or hemisphere, in which case the pattern is considered asymmetric. Figure 3.1 diagrams each main term 1 using schematic examples.

MAIN TERM 2

The second main term describes the morphology of the pattern as periodic discharges (PD), rhythmic delta activity (RDA), or spike/polyspike/sharp and wave (SW). Importantly, to use main term 2, the pattern must be present for 6 cycles (i.e., PDs/RDA/SW at 1 Hz for 6 seconds, 2 Hz for 3 seconds, 3 Hz for 2 seconds, etc.) (Figure 3.2). A periodic pattern is defined by a recurrence of consecutive waveforms separated by a nearly consistent interdischarge interval. The cycle length should vary by <50% in >50% of cycle pairs. The discharges should have no more than 3 phases (2 baseline crosses) and should be less than 0.5 seconds in duration. In contrast, the hallmark of a rhythmic pattern is the absence of an interval between recurring waveforms. The waveforms in a rhythmic pattern should be nearly regular with a consistent morphology and frequency ≤4 Hz (i.e., delta frequency). A nearly regular pattern is defined as the duration of one cycle varying by <50% when compared to the next cycle. This should hold true in >50% of cycle pairs. Finally, spike and wave or sharp and wave or polyspike and wave describes a pattern with an initial spike or poly-spike followed consistently by a slow wave. It is this consistent relationship that helps characterize this pattern when compared to others. There is no fixed interval between discharges (Figure 3.3). While the strict definitions are important, the points above may be better understood through visualization of each pattern. Figure 3.4 diagrams each main term 2 using schematic examples.

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Modifiers

Once main terms 1 and 2 are determined, “modifiers” are used to describe the pattern in greater detail. For this chapter we have divided the modifiers into basic modifiers and advanced modifiers. Note that the ACNS does not make this distinction, but for the purposes of this chapter we find this subdivision to be useful. Basic modifiers are important to the description of any EEG feature. They include location, prevalence, duration, frequency, number of phases, sharpness, amplitude, and polarity. In contrast, advanced modifiers are relevant primarily to ICU patterns and describe the presence of evolution, fluctuation, “plus” features, and reproducible induction by a stimulus.

86BASIC MODIFIERS

Basic modifiers to include when describing a periodic or rhythmic pattern include location, frequency, and morphology. The location of a pattern is described anatomically as frontal (Fp1, F3, Fp2, F4), occipital (O1, O2), temporal (F7, T7[T3)], P7[T5], F8, T8[T4], P8[T6]), parietal (P3, P4), and/or midline (Fz, Cz, Pz). Temporal location should be defined more precisely as anterior temporal (F7, F8), midtemporal (T7[T3], T8[T4]), or posterior temporal (P7[T5], P8[T6]). The frequency of the waveform should be documented as this can be important in determining which patterns have a high association with seizures. Finally, for periodic patterns, the sharpness of the waveform should be described as spikey (<70 ms), sharp (70–200 ms), sharply contoured (>200 μV), or blunt (Figure 3.5). The definitions of the remaining basic modifiers are presented in Table 3.1.

ADVANCED MODIFIERS

Evolution and Fluctuation

It is important to understand the definitions of evolution and fluctuation as they are particularly relevant to the IIC. Both terms rely on distinct changes in frequency, morphology, and/or location.

Evolution is defined as at least 2 sequential changes in frequency, morphology, or location as defined below. An evolving pattern cannot remain unchanged for 5 or more minutes at any stage of evolution. Evolution in frequency: at least 2 consecutive changes in frequency by at least 0.5/second. Each change must persist for a minimum of 3 cycles (1 Hz for 3 seconds, 1.5 Hz for 2 seconds, 3 Hz for 1 second, etc.). Evolution in morphology: 2 consecutive changes to a different morphology for a minimum of 3 cycles each. Evolution in location: sequential spread into or out of at least two standard electrode locations for at least 3 cycles each.

Fluctuation is defined as 3 or more changes in frequency, morphology or location. The minimum frequency change is 0.5 Hz, and the minimum location change is one standard interelectrode distance. The three or more changes cannot be more than 1 minute apart, and the pattern cannot meet criteria for evolution. These definitions can often be better understood pictorially as shown in Figures 3.6A–C and 3.7A–C.

Plus Modifiers

Plus modifiers are features of the waveforms of periodic and rhythmic patterns that are thought to make the pattern more “ictal.” Plus modifiers apply 88to periodic discharges and rhythmic delta activity but not to the spike wave pattern, and specific combinations of plus modifiers do not apply equally to periodic and rhythmic patterns. There are three main plus modifiers and two combinations of the three. The main plus modifiers are +F (fast activity), +S (sharp, spikey, sharply contoured activity), and +R (rhythmic activity). Combinations include +FR (overriding fast with a rhythmic component) for periodic patterns and +FS (overriding fast and a spike, sharp wave, or sharply contoured component) for rhythmic delta patterns. The modifier +R does not apply to rhythmic delta and +S does not apply to periodic discharges. This is intuitive as rhythmic delta is a rhythmic pattern and therefore would not need the modifier +R, and periodic patterns are generally spikes, sharp waves, or sharply contoured waves and so would not benefit from the +S modifier. If the +F or +S component is seen over only one hemisphere of a bilateral pattern, the plus modifier is still applied to the pattern as a whole. The fast activity (+F) and sharp/spike/sharply contoured (+S) should only be present when the pattern is present and should not be part of the EEG background. Table 3.2 defines and illustrates the plus modifiers.

Stimulus-Induced

Rhythmic and periodic patterns and seizures can be induced by a stimulus and, if present, should be documented as such. The stimulus can be tactile or auditory and performed as part of standard reactivity testing or as part of routine bedside cares. Formerly called SIRPIDs (stimulus-induced rhythmic, periodic, or ictal discharges), the nomenclature suggests that the patterns be described using the modifier stimulus-induced (SI) in conjunction with main terms 1 and 2 (e.g., SI-LPD, SI-GRDA, etc.).

Other Advanced Modifiers

Quasi may be used to described the periodicity of periodic patterns or the rhythmicity of rhythmic delta. Quasi refers to a pattern that varies by 25% to 50% from one cycle to the next in >50% of cycle pairs. This term should only be used when determined by computer analysis.

Sudden or gradual onset is defined as a pattern that progresses from absent to established within 3 seconds.

89Triphasic morphology describes a waveform that includes 2 or 3 phases and the positive phase must be the most prominent phase (Figure 3.8).

Anterior to posterior lag or posterior to anterior lag is present when a waveform appears with a delay of >100 ms from the anterior to posterior region within a hemisphere. This finding should be confirmed on both a referential and bipolar montage.

Figure 3.9 outlines a systematic approach to the description of periodic and rhythmic patterns.

Interrater Reliability

Interrater agreement for the critical care terminology introduced above varies depending on the pattern. Main terms 1 and 2, +S modifier, sharpness, amplitude, frequency, and number of phases had almost perfect agreement among 49 readers who reviewed a web-based assessment focused on key components of the terminology (kappa values were used to assess agreement). Other plus modifiers, specifically +F and +R, had substantial agreement. Moderate agreement was seen for identification of triphasic morphology, while agreement was fair for evolution and combinations of plus modifiers. Poor agreement was found when assessing for the presence of “any” plus modifier.² When introducing the terminology to adult neurology residents, there were similar findings with substantial agreement (based on kappa values) for main terms 1 and 2, +S modifier, sharpness, and frequency.³

Significance of Periodic and Rhythmic Patterns in Children

Periodic and rhythmic patterns are typically a sign of acute cerebral pathology. They can provide insight into associated etiologies such as specific infections (i.e., HSV) or structural etiologies (i.e., stroke), prognosis, and seizure risk. The incidence of these patterns in critically ill children is not well-defined. Two studies focusing on large cohorts of critically ill children report an incidence that ranges from 2% to 13%;^4,5 however, these studies predated the use of standardized terminology to describe these patterns. Other studies are small and focused primarily on lateralized periodic discharges (LPDs).^6–12 This is in contrast to the adult literature, which in recent years has expanded to include multicenter collaborations focusing on the role of rhythmic and periodic patterns in seizure prediction and their associations with etiology and short- and long-term outcome.^13–21

Early comparisons of periodic discharges in adults and children yielded mixed results. One of the first studies examining lateralized periodic discharges in children by PeBenito et al. found that these occurred in the absence 90of altered consciousness and represented chronic diffuse CNS lesions.⁶ This was in contrast to adult studies demonstrating an association between lateralized periodic discharges, acute lesions, and altered consciousness.^22,23 Follow-up pediatric studies by Raroque et al. and Garg et al. found similar associations in adults and children; in both of these studies, LPDs (formerly PLEDs) were associated with acute structural lesions, metabolic derangements, and acute encephalopathy in both children and adults.^7,11 Similarities in adults and children have yet to be determined in the pediatric critical care setting.

Related posts:

Templates and Treatment Algorithms Cases Basics of EEG Monitoring in Pediatric Critical Care Questions and Answers by Chapter Seizures in Critically Ill Children Convulsive and Nonconvulsive Status Epilepticus in Critically Ill Children

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