Periodic electroencephalographic patterns: usefulness of definition and classification

Abstract

F ourteen years after Cobb and Hill 1 first applied the term 'periodic' to characteristic electroencephalographic (EEG) changes, in 1950, Chatrian et al. 2 identified a periodic and lateralized pattern, which they described as periodic lateralized epileptiform discharges (PLEDs). The acronym encapsulates the essential characteristics of PLEDs, namely, wide distribution, polymorphism, one or more sharp components in a repetitive way (usually at the rate of one to two seconds), and localization over the hemisphere. Subsequent studies have described other variable periodic patterns as two independent PLED types in the same EEG (BIPLEDs), multifocal PLEDs, generalized periodic epileptiform discharges (GPEDs), and stimulus-induced rhythmic, periodic, or ictal discharges (SIRPIDs) [3][4][5][6] .PLEDs are usually observed in the context of acute, large, and destructive cortical processes, such as hemispheric stroke, supratentorial tumor, or encephalitis. Corresponding widespread lesions are thought to lead to a marginal zone of cortical hyperexcitability that serves as the 'pacemaker' for a spatially extended, synchronous EEG discharge 7 . Neuroimaging findings have demonstrated that while an acute cortical lesion is the most common structural substrate of PLEDs, subcortical lesions, chronic lesions, and nonlesional scans are not uncommon 7 . The spatially variable nature of brain lesions in PLEDs could point to their origin in variable segments of a cortical and subcortical ' system', assuming that it -whatever its precise components are -comprises a large-scale interconnected network, prone to synchronous oscillation springing from injury to any component 7 .Despite the frequent use of the term periodic, little attention has been paid to, or agreement reached on, how periodicity is defined or how the term should be used with respect to EEG phenomena. It has been suggested that the rate for a given patient should not vary more than 20% 8 . However, the method of measurement, the number of intervals that should be measured, and the putative clinical relevance of this value have not been discussed.Most of these studies were retrospective, with EEGs being performed at arbitrary and widely varying time points in the course of the disease, with detailed information about these variables being provided in only a few instances. The importance of EEG timing in the detection of epileptiform abnormalities has not been generally appreciated, despite the fact that at least one study has clearly demonstrated that the incidence of periodic EEG patterns increases when they are performed earlier in the course of the disease process 8,9 .Among the clinical manifestations, seizures are the most important and striking feature associated with PLEDs, occurring with a frequency from 58 to 100% 2,[8][9][10][11] . This wide range probably reflects the heterogeneity of patient selection and study design. Focal motor seizures are usually seen in this clinical setting, often presenting as status epilepticus (epilepsia partialis continua) or as ...

LPDs – «Linked to penumbra» discharges or EEG correlate of excitotoxicity: A review based hypothesis

“…Periodic EEG changes are usually classified as LPDs, bilateral independent periodic discharges (BIPDs), or generalized periodic discharges (GPDs), with further subclassification including LPDs-proper and LPDs-plus. (Andraus et al, 2012;Garzon, 2012).…”