title: Interictal spikes and epileptic seizures: their relationship and underlying rhythmicity.
abstract: We report on a quantitative analysis of electrocorticography data from a study that acquired continuous ambulatory recordings in humans over extended periods of time. The objectives were to examine patterns of seizures and spontaneous interictal spikes, their relationship to each other, and the nature of periodic variation. The recorded data were originally acquired for the purpose of seizure prediction, and were subsequently analysed in further detail. A detection algorithm identified potential seizure activity and a template matched filter was used to locate spikes. Seizure events were confirmed manually and classified as either clinically correlated, electroencephalographically identical but not clinically correlated, or subclinical. We found that spike rate was significantly altered prior to seizure in 9 out of 15 subjects. Increased pre-ictal spike rate was linked to improved predictability; however, spike rate was also shown to decrease before seizure (in 6 out of the 9 subjects). The probability distribution of spikes and seizures were notably similar, i.e. at times of high seizure likelihood the probability of epileptic spiking also increased. Both spikes and seizures showed clear evidence of circadian regulation and, for some subjects, there were also longer term patterns visible over weeks to months. Patterns of spike and seizure occurrence were highly subject-specific. The pre-ictal decrease in spike rate is not consistent with spikes promoting seizures. However, the fact that spikes and seizures demonstrate similar probability distributions suggests they are not wholly independent processes. It is possible spikes actively inhibit seizures, or that a decreased spike rate is a secondary symptom of the brain approaching seizure. If spike rate is modulated by common regulatory factors as seizures then spikes may be useful biomarkers of cortical excitability.
authors.affiliation: Department of Medicine, The University of Melbourne, St. Vincent's Hospital, Fitzroy VIC 3065, Australia NeuroEngineering Research Laboratory, Department of Electrical and Electronic Engineering, The University of Melbourne, Parkville, VIC, 3010, Australia pkaroly@student.unimelb.edu.au.
authors.forename: Philippa J
authors.initials: PJ
authors.lastname: Karoly
authors.affiliation: Department of Medicine, The University of Melbourne, St. Vincent's Hospital, Fitzroy VIC 3065, Australia NeuroEngineering Research Laboratory, Department of Electrical and Electronic Engineering, The University of Melbourne, Parkville, VIC, 3010, Australia.
authors.forename: Dean R
authors.initials: DR
authors.lastname: Freestone
authors.affiliation: Department of Medicine, The University of Melbourne, St. Vincent's Hospital, Fitzroy VIC 3065, Australia.
authors.forename: Ray
authors.initials: R
authors.lastname: Boston
authors.affiliation: Department of Medicine, The University of Melbourne, St. Vincent's Hospital, Fitzroy VIC 3065, Australia NeuroEngineering Research Laboratory, Department of Electrical and Electronic Engineering, The University of Melbourne, Parkville, VIC, 3010, Australia Centre for Neural Engineering, The University of Melbourne, Parkville, VIC, 3010, Australia.
authors.forename: David B
authors.initials: DB
authors.lastname: Grayden
authors.affiliation: NeuroVista Corporation, Seattle, WA 98109 USA.
authors.forename: David
authors.initials: D
authors.lastname: Himes
authors.affiliation: NeuroVista Corporation, Seattle, WA 98109 USA.
authors.forename: Kent
authors.initials: K
authors.lastname: Leyde
authors.affiliation: Department of Medicine, The University of Melbourne, St. Vincent's Hospital, Fitzroy VIC 3065, Australia.
authors.forename: Udaya
authors.initials: U
authors.lastname: Seneviratne
authors.affiliation: Department of Medicine, The University of Melbourne, Austin and Repatriation Medical Centre, Heidelberg VIC 3084, Australia.
authors.forename: Samuel
authors.initials: S
authors.lastname: Berkovic
authors.affiliation: Department of Medicine, The University of Melbourne, Royal Melbourne Hospital, Parkville VIC 3010, Australia.
authors.forename: Terence
authors.initials: T
authors.lastname: O'Brien
authors.affiliation: Department of Medicine, The University of Melbourne, St. Vincent's Hospital, Fitzroy VIC 3065, Australia.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Cook
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000328:Adult; D001921:Brain / Q000503:physiopathology; D002940:Circadian Rhythm / Q000502:physiology*; D004569:Electroencephalography / Q000379:methods; D004827:Epilepsy / Q000175:diagnosis* / Q000503:physiopathology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010507:Periodicity; D055815:Young Adult
keywords: circadian rhythms; epilepsy; interictal spikes; seizures