Abstracts

Rationale: A major limitation of Intracranial EEG(IEEG) is recording from a very confined region.This may falsely localize seizure onset if the distinction between ictal onset zone, proximity and spread is unclear or if the ictal rhythm is not clearly identified.We appraised the evidence to determine whether specific IEEG ictal patterns are associated with the seizure onset zone Methods: Using variations of the terms "Intracranial EEG", "Ictal patterns" we searched Pubmed and Medline upto 2014 for studies in English describing IEEG patterns.Abstracts were assessed by 2 independent reviewers.13 studies fulfilled eligibility criteria.Seizure freedom was considered the highest standard for identification of seizure onsets on IEEG.We assessed study quality and analysed data descriptively. Results: All 13 studies were retrospective.Reports focused on mesial temporal lobe epilepsy (mTLE) (n=4),nonlesional TLE (n=2),nonlesional extratemporal epilepsy (ETLE) (n=1),bitemporal epilepsy (n=1),neocortical TLE and ETLE (n=2) and various surgically treated patients (n=3).Two studies used the predominant pattern as the ictal onset,the remainder excluded seizures with multiple patterns.Five studies included clinical seizures,four included clinical and electrographic seizures and four did not specify.IEEG ictal onset zone was defined as the first unequivocal EEG change in all studies except 2 that pre-defined onset as sustained >2Hz rhythmic change.IEEG ictal onset patterns most consistently described were:Electrodecremental (ED) (n=5);Low voltage fast activity >13 Hz (LVFA) (n=8) ; < 2 Hz sharp/spike wave (n= 4);2-4 Hz sharp/spike wave (n=4) ;4-13 Hz rhythmic or sinusoidal pattern (n=5); > 13 Hz rhythmic spikes (n=5);Repetitive spikes (n= 6);Sinusoidal activity (n=4); Rhythmic delta (n=2);Delta brushes (n=1);Burst suppression (n=1).Of 8 studies assessing association ictal IEEG with surgical outcome, 3 reported better outcome with LVFA, 1 reported contradictory results, 3 found no association to surgical outcomes and 1 reported worse outcome with ED.Of 4 studies reporting association of ictal IEEG with lesions,one found that >8Hz activity occurred with mesial temporal onset.Another found LVFA and spikes associated with developmental lesions, and sinusoidal waves with "mature" lesions.One study reported <2 Hz spiking with mesial temporal atrophy and delta brush only in focal cortical dysplasia.One study on MTS reported rhythmic EEG onsets with mild gliosis as compared to repetitive spikes.Six studies correlated IEEG with seizure location. One found confined beta frequency in TLE and broader LVFA in ETLE.In contradiction,two studies found that LVFA implied less focal onsets. Two studies found focal rhythmic spikes in TLE and 1 found LVFA with neocortical onsets and rhythmic sharp theta with mesial temporal onsets. Conclusions: Studies are methodologically diverse and yield contradictory results.Seizures are counted instead of patients.No studies define ictal onsets in relation to postoperative seizure freedom (gold standard) or address multiple ictal patterns at onset, a challenging and common issue.A systematic approach is needed.