The intracranial correlate of the 14&6/sec positive spikes normal scalp EEG variant

To investigate the intracranial correlate of the 14&6/sec positive spikes normal variant of scalp EEG.

Out of 35 adult refractory focal epilepsy patients who underwent intracranial electrode implantation with simultaneous scalp EEG electrodes, the 14&6/sec positive spikes variant was found in 4. We used three methods to identify and quantify intracranial correlates to the variant: visual inspection, time-referenced waveform averaging and 3D brain volume spectrum-based statistical parametric mapping (SPM).

We discovered a novel and robust relationship between the scalp variant and an atypical hippocampal discharge. This intracranial correlate is an ipsilateral hippocampal burst of highly synchronized high-amplitude paroxysmal-like spikes of negative polarity, with a ramping up amplitude profile, which often ramps down and is accompanied by an underlying sequence of low-amplitude negative slow waves. The 14/sec positive spikes of the variant are time-locked to the negative peak of the hippocampal spikes, while the 6/sec positive spikes are time-locked to the negative spikes overlying the low-amplitude slow waves.

The 14&6/sec positive spikes variant correlates with bursts of negative polarity spikes in the ipsilateral hippocampus.

The identification of the hippocampal correlate of the 14&6/sec positive spikes variant fills a gap in our knowledge of normal intracranial variants. In clinical practice, this knowledge should reduce the chance that this electrophysiological signature is misinterpreted as epileptiform activity, which could inappropriately influence the interpretation of the intracranial study and subsequent surgical recommendation.