Rhythmic and periodic patterns


5
Rhythmic and periodic patterns


Much time is spent identifying and classifying rhythmic and periodic patterns (RPPs). Many of these patterns are associated with seizures and can sometimes themselves be considered ictal. They also provide insight into the processes of the brain that may not be apparent with initial imaging. For example, in a patient who sustained a traumatic brain injury (TBI) and the CT head demonstrates a right frontal contusion, an EEG showing bilateral independent periodic discharges (BIPDs) provides information that the injury may in fact be bilateral and that either hemisphere is at risk of generating seizure.


Proper classification/terminology provides the ability to trend the burden of patterns over days to weeks (often across several different EEG readers). A higher pattern burden is one of the factors that is associated with seizures (and outcome) and gradual reduction of this (or changes in other modifiers) can predate clinical improvements. Classification also allows for robust multicenter research collaboration, and the associations and significance of many of these patterns have been determined over the past decade as a result of this.


Rhythmic and periodic patterns are described in accordance with the ACNS critical care EEG terminology 2021 by combining a localization descriptor from ‘Main term 1’ and a pattern type descriptor from ‘Main term 2’. These main terms combined provide a framework for a wide variety of patterns and these can be further refined by applying ‘major’ and ‘minor’ modifiers.


5.1 Main term 1 (location)


The first consideration is the location of a pattern (Figure 5.1). The location can be described as either:



  • generalized
  • lateralized
  • bilateral independent
  • unilateral independent, or
  • multifocal

The most commonly used terms are generalized and lateralized. A generalized pattern is bilaterally synchronous and symmetric, even if it has a restricted field (e.g., bifrontal), and even if it has a shifting predominance. A lateralized pattern is either strictly unilateral, or bilateral but clearly and consistently higher amplitude in one hemisphere (bilateral asymmetric), or bilateral but with a consistent lead-in from one side (bilateral asynchronous). The remainder of the location terms describe several patterns occurring simultaneously, i.e., overlapping in time rather than sequentially, with one starting after the other stops. These terms are bilateral independent (BI), during which there are two independent lateralized patterns, one in each hemisphere: unilateral independent (UI), with two independent lateralized patterns both in the same hemisphere; or multifocal (Mf), with at least three independent lateralized patterns, with at least one in each hemisphere (usually meaning two in one hemisphere and one in the other hemisphere).


It is intuitive that a higher number of RPPs signifies a more extensive process, greater association with seizures and worse clinical outcomes. This has been shown in patients with BIPDs that are associated with worse mental status (usually coma), and worse outcome, compared to patients with LPDs. Multifocal RPPs are rare but similarly occur in the setting of significant cerebral insult representing highly epileptic foci through multiple brain regions.


5.2 Main term 2 (pattern type)


Once the location of a RPP is determined, the second descriptor is the pattern type (Figure 5.2). The three descriptors for Main term 2 are rhythmic delta activity (RDA), periodic discharges (PDs), and spike-and-wave or sharp-and-wave (SW).


Rhythmic delta activity (RDA)


Rhythmic delta activity is delta activity (0.5 to ≤4 Hz) where the repetition of the waveforms has a relatively uniform morphology and duration, and without an interval between consecutive waveforms. The duration of one cycle (i.e., the period) of the rhythmic pattern should vary by <50% from the duration of the subsequent cycle for the majority (>50%) of cycle pairs to qualify as rhythmic.


The significance of RDA differs if it is generalized (GRDA) vs. lateralized (which includes any lateralized pattern [L, BI, UI and Mf]). GRDA is usually seen in the setting of diffuse dysfunction and encephalopathy, keeping in mind that a subset of the pattern is the previously described FIRDA (frontal intermittent rhythmic delta activity). Even if GRDA is sharply contoured (GRDA+S), it has not been shown to be associated with seizures, a finding that replicates the associations and teachings historically described of FIRDA. Any lateralized RDA, however, is highly associated with seizures. The best described/recognized historical example is temporal-predominant RDA (known as temporal intermittent rhythmic delta activity [TIRDA] in the epilepsy literature; a subset of LRDA), which is well associated with temporal lobe epilepsy. This association (of any lateralized RDA and seizures) has been robustly confirmed in the critical care setting, with multiple independent publications from numerous centers confirming the finding.


The conceptual dichotomy between generalized and lateralized RDA makes inherent sense. Under normal physiologic conditions, cortical generators of EEG are largely asynchronous and have a relatively small projection to surface recording electrodes. When a sufficient proportion of this cortex becomes ‘dysfunctional’, then large volumes of the cortex can be synchronized by thalamic mediators, resulting in high-amplitude, symmetric and synchronous rhythmic delta waves (GRDA). Conversely, the pathophysiology of TIRDA has been shown (using intracranial EEG) to be commonly associated with repetitive epileptiform discharges (including bursts of polyspikes) from the temporal lobe on intracranial recordings that are poorly projected to the surface. Limited intracranial EEG in critically ill patients has confirmed the intracranial correlate of LRDA as periodic epileptiform discharges (or even electrographic seizures), hence making sense that the pattern is similarly highly associated with seizures. In fact, the pathophysiology of LRDA does not differ from LPDs (although the surface projections differ), which makes it of no surprise that the clinical, imaging and seizure associations are very similar between the two patterns, if not identical.


Periodic discharges (PDs)


‘Discharges’ are waveforms that clearly stand out from the background that last <0.5 seconds, or ≥0.5 seconds but must have no more than 3 phases (as opposed to ‘bursts’ that are ≥0.5 seconds and have at least 4 phases). Periodic discharges (PDs) are the repetition of waveforms with relatively uniform morphology and duration with a clearly discernible inter-discharge interval between consecutive waveforms and recurrence of the waveform at nearly regular intervals. ‘Nearly regular intervals’ is defined as the cycle length (i.e., period) should vary by <50% from one cycle to the next in most (>50%) cycle pairs.


The dichotomy between generalized and lateralized patterns that exists for RDA does not apply to PDs, i.e., both GPDs and LPDs are highly associated with seizures (though LPDs more so). GPDs (like GRDA) are commonly seen in the context of encephalopathy; however, more than one third of patients will have definite seizures during their acute illness. GPDs can be seen in a variety of settings including in postanoxic coma (when the background is often flat between the discharges), after convulsive status epilepticus, with metabolic disorders, in Creutzfeldt–Jacob disease, during Hashimoto encephalopathy, from medication toxicity (e.g. baclofen, lithium, ifosfamide and cefepime) and in end-stage Alzheimer disease.


Generalized periodic discharges can have triphasic morphology (also known as triphasic waves [TW]). They were initially described in hepatic encephalopathy but as the volume and variety of patients monitored with cEEG has increased, they are commonly seen across a variety of presenting etiologies. One study even demonstrated that patients with TWs were less likely to have a toxic-metabolic encephalopathy compared to a cohort with GPDs without triphasic morphology. When GPDs have a triphasic morphology they tend to recur at 1–2 per second and wax and wane throughout a recording, partly dependent on level of alertness. Nonconvulsive status epilepticus can appear quite similar. Although some have suggested specific features which are more common in patients with metabolic encephalopathy rather than seizures, almost all of these studies suffer from lack of a gold standard for making the final diagnosis, and many have circular logic. It is clear that in a given individual, EEG alone often cannot distinguish between triphasic waves of metabolic encephalopathy (if they exist) and nonconvulsive seizures. Unfortunately, both resolve with benzodiazepines as well. Thus, only EEG and clinical improvement with IV anti-seizure medication can prove the presence of nonconvulsive status epilepticus. It is almost impossible to disprove it. Both epileptiform patterns (including seizures) and TWs commonly increase or appear with alerting stimuli in the critically ill; thus, that cannot be used to differentiate them.


Lateralized periodic discharges (LPDs) consist of lateralized complexes usually recurring every 0.5–2 s. The complexes often (but not always) consist of sharp waves or spikes that may be followed by a slow wave. The clinical picture associated with LPDs is usually obtundation, focal seizures, and focal neurological signs. Many patients with LPDs (more than half) will also have seizures during the acute stage of illness. For the majority of cases, LPDs are considered an interictal pattern. There are fewer cases where LPDs are clearly an ictal pattern, mainly when associated with concordant time-locked jerking or focal neurological deficits that resolve when the pattern is treated.


Acute stroke (hemorrhagic and ischemic) is the most common etiology of LPDs, although any acute brain injury that results in focal cortical hyperexcitability can manifest as LPDs. Most patients with herpes simplex encephalitis develop LPDs, maximal in the temporal region(s) and often consisting of prolonged complexes (∼0.5 s), often not even sharply contoured, recurring every 1–4 s; however, this pattern is certainly not specific for this diagnosis. Regardless of etiology, LPDs are usually a transient phenomenon. With time (days to weeks), the discharges tend to decrease in amplitude, the repetition rate decreases and ultimately the discharges cease.


‘Spike-and-wave’ and ‘sharp-and-wave’ (SW)


SW is defined as a spike, polyspike or sharp wave consistently followed by a slow wave in a regularly repeating and alternating pattern (i.e., spike, then wave, then spike, then wave, etc. for at least six cycles). Technically, this should be called ‘spike-and-slow wave’, ‘sharp wave-and-slow wave’ or ‘polyspike-and-slow wave’, but they are all abbreviated SW for convenience. This specific pattern is much less common than RDA and PDs. The largest difference between SW and PDs, is that in SW there is no inter-discharge interval i.e., the beginning of a subsequent sharp-and-wave complex follows immediately after the end of the sharp-and-wave complex preceding it. The clinical significance, radiographic associations, and association with seizures is probably similar to patients with comparative PDs (i.e., GSW has similar connotations to GPDs, and LSW has similar implications to LPDs), though there are few publications on SW in the critically ill.


5.3 Major and minor modifiers (including plus modifiers)


All RPPs can be classified as a combination of main term 1 and 2 above. However, there are multiple features of a pattern that make it more (or less) epileptiform and these are accommodated for in the modifier sections. Major modifiers provide the bulk of description for RPPs, such as how often they occur (prevalence), how long they last (duration), the typical frequency (e.g., 1.5 Hz), or their sharpness.


Plus (+) modifiers describe activity superimposed on RPPs that result in them appearing more ‘ictal’. Plus modifiers include admixed sharp waves (+S), added fast activity (+F) or superimposed rhythmic activity (+R) (Figure 5.3). RDA cannot have a +R modifier as the pattern is by definition already rhythmic. PDs cannot have a ‘+S’ since they already have a sharpness modifier that has four categories (spiky, sharp, sharply contoured and blunt). Occasional patterns can have both plus modifiers, for example LPDs+FR. The emergence, or the increase in, plus modifiers can herald the onset of seizures. For example, LPDs changing to LPDs+F can signify that the excitability of underlying cortex has increased, from which transition into an electrographic seizure is probable. Extreme delta brushes (EDB) is an even more specific subset of +, and given its association with NMDA encephalitis has already been presented in Chapter 3 on encephalopathy and coma.


Although there are many descriptors that reside under the modifiers umbrella, it should be noted that their association with seizures is not uniform. Multivariable analysis of nearly 5000 patients revealed that increasing typical frequency (especially ≥2 Hz), and plus modifiers were significantly associated with a higher risk of seizures, whereas modifiers such as voltage, stimulus-induced or not, or triphasic morphology had little independent bearing on the risk of acute seizures.


5.4 Stimulus-induced rhythmic, periodic or ictal discharges (SIRPIDs)


With the advent of continuous video-EEG recordings in the ICU, it became apparent that alerting stimuli (suction, exam, noise, pain) in encephalopathic patients commonly elicit highly epileptiform patterns made up of any of the RPPs described above, or unequivocal evolving electrographic and even electroclinical seizures. These can be focal or generalized. The phenomena of stimulus-induced patterns and seizures is encapsulated within the term SIRPIDs; in the terminology, any pattern can have an ‘SI-’ prefix, indicating that it can be induced by stimulation (even if it also occurs spontaneously). Note: that the term SIRPIDs no longer features in the terminology except as a specific form of reactivity in which the only change in the EEG is ‘SIRPIDs-only’. As patterns and seizures have been further studied, it has become apparent that it is the content of the pattern (i.e., GRDA vs. GPD vs. ESz, etc.) that determines the chance of further seizures, rather than whether or not the pattern is stimulus-induced. The duration and prominence of the pattern often correlate with the duration and degree of stimulation, and the pattern can usually be reproduced with further stimulation (after allowing return to the non-stimulated background). This is usually a purely electrographic finding with no obvious clinical accompaniment, although some patients will have ECSz as well; these are typically focal motor, as other types would be very difficult to detect.


5.5 Brief potentially Ictal Rhythmic Discharges (BIRDs)


Brief potentially Ictal Rhythmic Discharges (BIRDs) represents a group of highly epileptic patterns with a strong association with subsequent seizures. BIRDs are defined as focal or generalized rhythmic activity >4 Hz (at least six waves at a regular rate) lasting ≥0.5 to <10 s, not consistent with a known normal pattern or benign variant, not part of burst suppression or burst attenuation, without definite clinical correlate, and that has at least one of the following features (Figure 5.4):



  1. evolution (‘evolving BIRDs,’ a form of definite BIRDs)
  2. similar morphology and location to interictal epileptiform discharges or seizures in the same patient (definite BIRDs)
  3. sharply contoured but without (1) or (2) (possible BIRDs).

BIRDs often represent the beginnings of ESzs and their strong association with seizures is not surprising. If a >4 Hz RPP, or a pattern with evolution, lasts for ≥10 s, these would qualify as ESzs. Whether to keep or remove the historical and somewhat arbitrary cut off for ESz to having to be ≥10 s has been discussed at length, but there has not been sufficient consensus by experts to remove this barrier, and currently there is not sufficient evidence in the literature to have discussion of what this cut off should realistically be. Roughly three quarters (75%) of patients with BIRDs will go on to develop definite ESz, and nearly 100% of critically ill patients with evolving BIRDs will have definite seizures during their acute illness. In non-critically ill patients, BIRDs are a sign of poorly controlled epilepsy, and are very useful for localizing the seizure onset zone. In both critically ill and non-critically ill patients, when seizures are fully controlled, BIRDs resolve as well.


Figure list



  • Figure 5.1 Main term 1 (localization).
  • Figure 5.2 Main term 2 (pattern type).
  • Figure 5.3 Plus (+) modifiers.
  • Figure 5.4 Brief potentially Ictal Rhythmic Discharges (BIRDs).
  • Figure 5.5 Generalized rhythmic delta activity (GRDA).
  • Figure 5.6 Generalized rhythmic delta activity (GRDA).
  • Figure 5.3 Generalized rhythmic delta activity (GRDA).
  • Figure 5.7 Lateralized rhythmic delta activity (LRDA).
  • Figure 5.8 Lateralized rhythmic delta activity (LRDA), unilateral.
  • Figure 5.9 Lateralized rhythmic delta activity (LRDA), unilateral.
  • Figure 5.10 Lateralized rhythmic delta activity (LRDA), unilateral.
  • Figure 5.11 Focal irregular (non-rhythmic) slowing.
  • Figure 5.12 Lateralized rhythmic delta activity (LRDA), bilateral asymmetric.
  • Figure 5.13 Lateralized rhythmic delta activity (LRDA), bilateral asynchronous.
  • Figure 5.14 Generalized periodic discharges (GPDs).
  • Figure 5.15 Generalized periodic discharges (GPDs).
  • Figure 5.16 Generalized periodic discharges (GPDs).
  • Figure 5.17 GPDs with triphasic morphology.
  • Figure 5.18 GPDs with triphasic morphology.
  • Figure 5.19 Lateralized periodic discharges (LPDs).
  • Figure 5.20 Lateralized periodic discharges (LPDs).
  • Figure 5.21 Lateralized periodic discharges (LPDs), bilateral asymmetric.
  • Figure 5.22 Lateralized periodic discharges (LPDs).
  • Figure 5.23 Generalized Sharp-and-Wave (GSW).
  • Figure 5.24 Generalized spike-wave vs. EKG artifact.
  • Figure 5.25 Lateralized Spike-and-Wave (LSW).
  • Figure 5.26 Lateralized Sharp-and-Wave (LSW).
  • Figure 5.27 Unilateral independent rhythmic delta activity (UIRDA).
  • Figure 5.28 Bilateral independent rhythmic delta activity (BIRDA).
  • Figure 5.29 Multifocal rhythmic delta activity (MfRDA).
  • Figure 5.30 Unilateral independent periodic discharges (UIPDs).
  • Figure 5.31 Unilateral independent periodic discharges (UIPDs).
  • Figure 5.32 Bilateral independent periodic discharges (BIPDs).
  • Figure 5.33 Bilateral independent periodic discharges (BIPDs).
  • Figure 5.34 Bilateral independent periodic discharges with polyspikes.
  • Figure 5.35 Multifocal periodic discharges (MfPDs).
  • Figure 5.36 Generalized rhythmic delta activity plus S (GRDA+S).
  • Figure 5.37 Generalized rhythmic delta activity plus S (GRDA+S).
  • Figure 5.38 Lateralized rhythmic delta activity plus S (LRDA+S).
  • Figure 5.39 Lateralized rhythmic delta activity (LRDA) with evolution versus seizure.
  • Figure 5.40 Symptomatic LPDs mimicking stroke.
  • Figure 5.41 Stimulus-induced rhythmic, periodic or ictal discharges (SIRPIDs) and SI-GPDs.
  • Figure 5.42 SIRPIDs: GPDs vs. seizure.
  • Figure 5.43 SIRPIDs: SI-GSW vs. SI-seizure.
  • Figure 5.44 Brief potentially Ictal Rhythmic Discharges (BIRDs), with evolution.
  • Figure 5.45 Brief potentially Ictal Rhythmic Discharges (BIRDs), generalized.
  • Figure 5.46 Brief potentially Ictal Rhythmic Discharges (BIRDs), evolving BIRDs with similar appearance as the beginning of a seizure.
  • Figure 5.47 Brief potentially Ictal Rhythmic Discharges (BIRDs), LPDs transitioning into BIRDs.
  • Figure 5.48 Brief potentially Ictal Rhythmic Discharges (BIRDs), evolving BIRDs, some seizures with similar appearance as BIRDs and others with independent seizure focus.

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.


Additional RPPs can be found in other chapters as follows:



Suggested reading



  1. Alzawahmah M, Fong MWK, Gilmore EJ, Hirsch LJ. Neuroimaging Correlates of Lateralized Rhythmic Delta Activity, Lateralized Periodic Discharges, and Generalized Rhythmic Delta Activity on EEG in Critically Ill Patients. J Clin Neurophysiol. 2022; 39(3):228–234.
  2. Chatrian GE, Shaw CM, Leffman H. The significance of periodic lateralized epileptiform discharges in EEG: an electrographic, clinical, and pathological study. Electroencephalogr Clin Neurophysiol 1964; 17:177–193.
  3. Chong DJ, Hirsch LJ. Which EEG patterns warrant treatment in the critically ill? Reviewing the evidence for treatment of periodic epileptiform discharges and related patterns. J Clin Neurophysiol 2005; 22:79–91.
  4. Cockerell OC, Rothwell J, Thompson PD, Marsden CD, Shorvon SD. Clinical and physiological features of epilepsia partialis continua. Cases ascertained in the UK. Brain. 1996; 119(Pt 2):393–407.
  5. de la Paz D, Brenner RP. Bilateral independent periodic lateralized epileptiform discharges. Clinical significance. Arch Neurol 1981; 38:713–715.
  6. Fong MWK, Jadav R, Alzawahmah M, Hussein OM, Gilmore EJ, Hirsch LJ. The Significance of LRDA With Bilateral Involvement Compared With GRDA on EEG in Critically Ill Patients. J Clin Neurophysiol. 2021;Publish Ahead of Print.
  7. Foreman B, Mahulikar A, Tadi P, et al. Generalized periodic discharges and ‘triphasic waves’: A blinded evaluation of inter-rater agreement and clinical significance. Clin Neurophysiol. 2016; 127(2):1073–1080.
  8. Garcia-Morales I, Garcia MT, Galan-Davila L, Gomez-Escalonilla C, et al. Periodic lateralized epileptiform discharges: etiology, clinical aspects, seizures, and evolution in 130 patients. J Clin Neurophysiol 2002; 19:172–177.
  9. Garzon E, Fernandes RM, Sakamoto AC. Serial EEG during human status epilepticus: evidence for PLED as an ictal pattern. Neurology 2001; 57:1175–1183.
  10. Gaspard N, Manganas L, Rampal N, Petroff OA, Hirsch LJ. Similarity of lateralized rhythmic delta activity to periodic lateralized epileptiform discharges in critically ill patients. JAMA Neurol. 2013; 70(10):1288–1295.
  11. Hirsch LJ, Claassen J, Mayer SA, Emerson RG. Stimulus-induced rhythmic, periodic, or ictal discharges (SIRPIDs): a common EEG phenomenon in the critically ill. Epilepsia 2004; 45:109–123.
  12. Hirsch LJ, Fong MWK, Leitinger M, et al. American Clinical Neurophysiology Society’s Standardized Critical Care EEG Terminology: 2021 Version. J Clin Neurophysiol. 2021; 38(1):1–29.
  13. Hirsch LJ, Pang T, Claassen J, Chang C, Abou Khaled K, Wittman J, Emerson RG. Focal motor seizures induced by alerting stimuli in critically ill patients. Epilepsia 2008; 49:968–973.
  14. Kaplan PW, Schlattman DK. Comparison of triphasic waves and epileptic discharges in one patient with genetic epilepsy. J Clin Neurophysiol. 2012; 29

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May 12, 2023 | Posted by in Uncategorized | Comments Off on Rhythmic and periodic patterns

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