Abstracts

Rationale: HFOs are discrete electroencephalographic events described as ripples (80-250Hz) or fast ripples (FRs, 250-500Hz) and suspected to play a role in epileptogenesis. They were first described in rats and epileptic patients with intracerebral microwires (contact surface 0.0013 mm²), but can also be recorded using clinical depth electrodes with macrocontacts. In recent studies, microcontacts have been shown to better detect HFOs than adjacent macrocontacts. The use of microcontacts clinically is limited by the availability of sufficiently powerful amplifiers and by reduced spatial coverage. Current clinical depth electrode contacts range from 1 to 9 mm². Smaller macroelectrodes may record HFOs better if they are near a generator but larger macroelectrodes may provide better spatial sampling. The purpose of this study was to assess detection rates and durations of HFOs from macroelectrode contacts of different sizes in a chronic epileptic rat model. Methods: Depth electrodes comprising of one large (0.85 mm²), one medium (0.051 mm²) and one small (0.018 mm²) contiguous contacts were implanted in the limbic structures of 8 Sprague-Dawley rats with complex partial seizures obtained using the intraperitoneal pilocarpine chronic epileptic rat model. Five minute artefact-free segments of slow wave sleep were selected from chronic recordings sampled at 2000 Hz. Spikes, ripples and FRs were marked manually in separate review sessions, with the reviewer blind to contact size and to previously marked events. Rates and durations were compared between contiguous contacts of the three sizes using Kruskal-Wallis non-parametric tests. Results: 2370 spikes, 10966 ripples and 1475 FRs were marked in recordings from 30 contacts (10 three-contact electrodes). There were no significant differences in spike, ripple nor FR rates between the different size contacts. HFO durations were similar to previous publications (∼80 msec for ripples and ∼30 msec for FRs). Small but significant differences in duration were observed between contact sizes: ripples were minimally but significantly shorter in the small contact compared with the medium and large contacts (p < 0.0001) and FRs were minimally but significantly longer in the medium contact compared to the large and small contacts (p < 0.0001). Conclusions: These results indicate that 0.85 mm² macrocontacts detect HFOs as well as contacts almost 50 times smaller (0.018 mm²). Consistent but minimal differences in duration of events between the different size contacts are unlikely to be clinically relevant. These findings are consistent with a previous report in humans where a small but significant increase in ripple detection was seen between adjacent large and small macrocontacts but FR detection was not significantly different. Supported by CIHR grant MOP-10189 and a CECR Montreal Neurological Institute grant.