Periodic Epileptiform Discharges



Periodic Epileptiform Discharges







Description

Periodic epileptiform discharges (PEDs) classically are triphasic with a sharply contoured wave followed by a slow wave, and this is the basis for including “epileptiform” in the name. This also makes them morphologically similar to both interictal epileptiform discharges (IEDs) and the triphasic pattern, but their clinical significance is distinct from these other patterns despite the similar waveform. As such, they are considered a separate pattern. PEDs are uncommon in routine EEGs with an overall incidence between 0.4% and 1%, but they are relatively more common in EEGs recorded as continuous monitoring of patients receiving intensive care (Pohlmann-Eden et al., 1996). When present, they most often occur with a singular focus, and the term periodic lateralized epileptiform discharges (PLEDs) indicates this focality. The focus may be at any scalp electrode location, and, contrary to the name, it may be at the midline. The term PEDs at the midline has been proposed to avoid the misleading terminology, but it is not commonly used (Westmoreland et al., 1997).

Although a triphasic waveform is most characteristic, PLEDs may have one to several phases (Schear, 1984). The total complex duration usually is between 100 and 300 milliseconds, and the amplitude usually is greater than the background and in the range of 100 to 300 μV (Kaplan, 2007; Markand, 1990; Niedermeyer, 1999; Striano et al., 1986). Beyond these waveform features, the essential characteristic of PLEDs is a stereotyped recurrence, which is an essential combination of unchanging waveform and interdischarge timing throughout the routine EEG recording or a similar time of continuous EEG monitoring (Hirsch et al., 2005; Kuroiwa and Celesia, 1980). The term periodic in the name conveys the regularity of recurrence, but similarity across recurrences is necessary and tacit within the use of periodic for this pattern. This similarity across recurrences may manifest as identical waveforms that are separated by an interval that is almost constant, but similarity alternatively may be recurrences of waves that have strong resemblances to each other and a mostly regular interval, that is, a close to constant interdischarge interval. Often, either the waveform or the interdischarge interval is fixed and the other is more variable. Regardless, the type of similarity for the PLEDs is the same throughout the EEG recording. Recurrence frequencies for PLEDs commonly fall in the range of one transient every 0.5 to 4 seconds. Usually, the transients recur at least every 2 seconds, and the activity between them usually is low-amplitude slowing (Fisch, 1999).

PLEDs have been qualified by several terms. The term pseudoperiodic has been used with two definitions with regard to PLEDs. Traditionally, it refers to greater variation in wave appearance across recurrences, but it currently is more commonly used to indicate greater variation in recurrence regularity (Cobb and Hill, 1950). Stimulus-induced PLEDs (SIRPIDs) are consistently triggered by a stimulus that would typically alert a patient even if clinical alerting is not evident. Spontaneous PLEDs associated with alerting also may be considered stimulus induced despite the absence of an evident stimulus (Hirsch et al., 2005). The suffix plus, as presented PLEDs+, indicates superimposition of fast or rhythmic activity upon the PLED complex to produce a waveform that appears more like what would be expected to accompany a clinical seizure (Hirsch et al., 2005). As such, the use of plus is subjective but it also communicates of a clinical impression.



Distinguishing Features


• Compared to Electrocardiographic Artifact

PLEDs and electrocardiographic (ECG) artifact share a diphasic or triphasic waveform and a periodic recurrence. Differentiating these patterns is straightforward when comparison to an ECG channel is possible. Without an ECG channel, the regularity of the intervals between the transients is the key distinguishing feature. PLEDs usually are not nearly as regular in their interval as ECG artifact. This is especially true because the conditions for obtaining an EEG do not produce significant changes in heart rate. Other distinguishing features are distribution and frequency. Although ECG artifact may be unilateral, it often is bilateral and PLEDs are rarely bilaterally synchronous. Bilateral periodic epileptiform discharges (BiPEDs) are bilaterally synchronous, but they have large, bifrontal fields, and ECG artifact usually is maximal in the temporal regions. Frequency is a less reliable means for differentiation. Most ECG artifact will be at 1 Hz or faster because a heart rate slower than 60 beats per minute is atypical. In contrast, PLEDs occur with intervals both greater than and less than 1 second. Overall, the possible intervals between PLEDs vary more than ECG.


• Compared to Environmental Device Artifact

When an environmental device causes intermittent artifact, it often has a regular interval and may be similar to PLEDs by being periodic. However, this type of artifact rarely has the diphasic or triphasic waveform of PLEDs and usually has a distribution that is highly unusual for PLEDs. Environmental device artifact often does not have a distribution consistent with cerebrally generated waves. Instead, it may be generalized or occur at nonadjacent electrodes.


• Compared to Interictal Epileptiform Discharges

Since PLEDs may have waveform features of IEDs, differentiation depends on the presence of regular recurrence. IEDs also may recur but most commonly do so sporadically or without a set regularity. Recurring discharges within an EEG that are separated by intervals that range from less than 1 second to greater than several minutes are most likely to be IEDs. Although PLEDs may be absent for portions of an EEG, they typically are not absent for more than a few minutes and typically recur for at least a few seconds and usually for more than 10 seconds when they are present.


Co-occurring Patterns

The clinical states that produce PLEDs usually also cause background slowing, which may be either disorganized or synchronous. Either type of slowing is abnormal and contrasts with more regular and faster normal background activity. Because of close temporal associations with myoclonus, muscle and movement artifact may be time-locked with PLEDs (Markand, 1990).


Clinical Significance

PLEDs are a sign that usually indicates a focal pathology that is acute or subacute, almost always affecting cerebral cortex, and more likely to occur in the context of coexisting metabolic abnormality (Neufeld et al., 1997; Niedermeyer, 1999; Raroque et al., 1993a; Raroque and Purdy, 1995). As exceptional cases, PLEDs have been reported as due to caudate or thalamic pathology or purely metabolic abnormality (Gross et al., 1998; Janati et al., 1986; Wheless et al., 1991). In such situations, PLEDs tend to have more stereotyped waveform and a slightly shorter complex duration (Kalamangalam et al., 2007). Overall, the significance of PLEDs is the same in children as in adults when considered in the context of their underlying cause (Chen et al., 2003; Raroque et al., 1993b). PLEDs usually last days to weeks and rarely may last years (Fisch, 1999; Niedermeyer, 1999; Westmoreland et al., 1986).

PLEDs are accompanied by co-localized focal deficit in about 80% of instances (Daly, 1990). Cortical stroke is responsible for almost half of the incidence of PLEDs, and tumors and infections are each responsible for almost another 20% (Fischer-Williams and Dike, 1999; Garcia-Morales et al., 2002). As such, these three etiologies may account for more than 80% of occurrences. Among strokes, embolism and watershed distributions are more likely to produce PLEDs than thrombosis of single vessel territories. Watershed distribution strokes are more likely to produce PLEDs at the parasagittal midline. Infections causing PLEDs are most commonly viral, but can be bacterial, fungal, and parasitic (Carrera et al., 2008; Noone et al., 2007). Other causes include prion diseases, epilepsy, and intracerebral and extra-axial hematomas (Claassen et al., 2007; Garcia-Morales et al., 2002; Mehryar and McIntyre, 1975; Westmoreland, 2001; Westmoreland et al., 1986). Less commonly, PLEDs are caused by Alzheimer’s disease, mitochondrial disease, multiple sclerosis, and intoxication with medications including baclofen, lithium, levodopa, and ifosfamide (Brenner, 1999; Brick et al., 1991; Chabolla et al., 1996; Funakawa et al., 1997). Trauma without subsequent hemorrhage is a rare cause for PLEDs (Rumpl, 1999).

Among infectious diseases, herpes simplex encephalitis (HSE) is the most likely to produce PLEDs, and most occurrences of HSE demonstrate PLEDs at some point along the clinical course (Brenner and Schaul, 1990). This is almost always within a week of the onset of symptoms, and the PLEDs often disappear within 2 weeks after the onset (Lai and Gragasin, 1988). PLEDs due to HSE almost always are centered over one or both temporal lobes. When they are bilateral, they
usually are synchronous or time-locked. Their recurrence interval commonly is 1.5 to 2.5 seconds, and they are almost always within the interdischarge interval range of 1 to 5 seconds (Lai and Gragasin, 1988; Niedermeyer, 1999). The interdischarge interval of PLEDs due to viral encephalitides varies significantly less than for other PLED etiologies, but considerable overlap with other etiologies limits the specificity and usefulness of this feature (Gross et al., 1999). Influenza B and LaCrosse virus are other causes for encephalitis that produce temporal PLEDs (Kurita et al., 2001; Sokol et al., 2001).

Among prion diseases, Creutzfeldt–Jakob disease (CJD) more commonly produces PLEDs, and PLEDs are a useful sign to help distinguish CJD from other dementias or progressive encephalopathies even if PLEDs are not fully specific for CJD (Au et al., 1980). Between 67% and 100% of patients with CJD demonstrate PLEDs for a self-limited period of time during the disease course; however, the PLEDs may not manifest until several months after the clinical onset. PLEDs may replace frontal intermittent rhythmic delta activity (FIRDA), which sometimes is CJD’s initial EEG abnormality (Aguglia et al., 1997; Brenner, 1999

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May 26, 2016 | Posted by in NEUROLOGY | Comments Off on Periodic Epileptiform Discharges

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