A New Method for Quantification and Assessment of Epileptiform Activity in EEG with Special Reference to Focal Nocturnal Epileptiform Activity

Quantification of epileptiform activity in EEG has been applied for decades. This has mainly been done by visual inspection of the recorded EEG. There have been many attempts using computers to quantify the activity, usually with moderate success. In a row of contexts, including Landau-Kleffner syndrome and the syndrome of epilepsy with continuous spike wave during slow sleep, the spike index (SI) has been applied to quantify ‹interictal nocturnal focal epileptiform activity’, which is suggested as a general term for the epileptiform activity enhanced by sleep. However, the SI has been implemented differently by different authors and has usually not been well described and never properly defined. This study suggests a definition of SI that gives a semiautomatic and relatively robust algorithm for assessment. The method employs spike detection by means of template matching of the current source density estimate. The percentage of time within an epoch with interspike interval (ISI) below a given limit, usually 3 s, is returned as the SI. This is calculated during daytime and in non-REM sleep. The standard epoch length is 10 min. The parameter selection is discussed in the context of the influence of spikes and bursts on cognition. The described method gives reproducible results in routine use, gives clinical valuable information, and is easily implemented in a clinical setting. There is only a minor added workload for the electroencephalographer.