Abstracts

Rationale: Scalp video EEG is the most important diagnostic modality for localizing seizure focus in drug refractory epilepsy (DRE) patients who undergo presurgical work up. However, in some cases the EEG lacks any ictal pattern or is non-lateralizing, often requiring invasive monitoring. The purpose of our study is to understand the ictal onset pattern on stereo-EEG when there is a no ictal pattern or in non-localizing/non-lateralizing ictal pattern on scalp EEG. Methods: We retrospectively analyzed 60 patients (patients age 18-70,mean age 33) with DRE that underwent extensive scalp video EEG recordings, MRI, PET and stereotactic EEG (sEEG) for surgical evaluation at University of Alabama at Birmingham (UAB). Of the 60, 10 patients (17%) were identified in whom the ictal scalp EEG onset was lacking or had non-lateralizing findings (such as diffuse delta or theta slowing or bisynchronous ictal pattern). We studied the sEEG ictal onset patterns for these 10 patients to understand which are the common intracranial EEG patterns. Results: Of the 10 patients with DRE, 6 were males and 4 females, mean age was 33 (range 26-51). Analyses of over 100 electro-clinical habitual seizures were performed. 6 patients had normal MRI (pt # 1,4,6,7,8,10), pt. #2 had extensive cortical malformation, pt.# 3 and 9 had bilateral hippocampal sclerosis, pt. #5 had extensive polymicrogyria. The ictal onset pattern was herald spike followed by electrodecrement followed by low amplitude fast activity for 5 seconds before organizing into a theta pattern (pt. # 1 and 2) (n=28 ) (Fig 1a). Pt. # 3 and 4 had low amplitude fast activity >20 Hz before organization into a moderate voltage theta pattern (Fig. 1b) (n=17). Pt.#5 had a diffuse spike with electrodecremental response followed by overriding fast in few isolated electrodes and then evolution in the mesial parietal electrodes (pathology showed polymicrogyria) (Fig. 1c) (n=24). Pt # 7 and 10 had seizure pattern which was low amplitude fast activity for the entire seizure duration, originating from right insula (pt. #7) (n=10) and another from the non-sclerotic hippocampus (pt. #10) (n=9) (Fig. 2).One patient with severe hippocampal sclerosis (pt. # 9) had repetitive spike followed by decremental and overriding fast activity (aka hypersynchronous pattern) (n=30) (Fig. 3). Pt. # 8 had moderate to high voltage spike burst (>20 Hz) followed by a decrement and LAFA in multiple electrodes at several contacts (brief seizure lasting less than 10 sec). Scalp pattern for this patient was diffuse delta activity, and the ultimate conclusion was this is generalized epilepsy and not focal. For pt.# 6, we were not able to localize the seizure onset zone by sEEG. Overall the pattern that seemed most common was electrodecrement with overriding fast or sustained low amplitude fast activity. The plausible explanation for this pattern to be lacking or nonlateralizing on scalp EEG, is that they are of such fast frequencies that they do not appear as a particular waveform on limited filters of scalp EEG and that the deeper focus makes it difficult to propagate as clear waveforms on scalp EEG. Most of the patients that had a non-localizable/lateralized pattern had a deep focus (anterior cingulate- Pt. # 3, 4, mesial parietal -pt. #5 and insular-pt # 7).  Conclusions: In DRE the SEEG ictal onset pattern that is most common in cases of absent or non-lateralizing scalp EEG is electrodecrement with overriding fast or low amplitude fast activity from a deeper focus (as cingulate, insular, mesial parietal). Further studies with bigger sample size is needed.  Funding: No funding