Spatial distribution of intracranially recorded spikes in medial and lateral temporal epilepsies

Although seizures and interictal spikes are not always colocalized, there may be valuable localizing information in the spatial distribution of spikes. To test this hypothesis, we studied the spatial distribution of intracranially recorded interictal spikes in patients with medial temporal (MT) and lateral temporal (LT) neocortical seizure onset.

A total of 21 patients (MT n = 12, LT n = 9) who completed intracranial monitoring were selected for this study. Two 4-h intracranial electroencephalography (icEEG) epochs were analyzed, one during wake and one during sleep, both at least 6 h removed from seizures. Spikes detected automatically in medial temporal structures [hippocampal formation (H) and entorhinal cortex (EC)], and in five cortical areas (occipital, frontal, parietal, lateral temporal, and inferior temporal) were tabulated.

Interictal spikes occurred broadly over medial temporal structures and cortical areas in MT and LT patients. The patients differed with a greater number of spikes in medial temporal structures in the MT group (p < 0.05 for H and p < 0.001 for EC) and a greater number of spikes in parietal (p < 0.01) and frontal (p < 0.001) areas in the LT group. There were sleep-related increases in spike rates in inferomedial temporal structures in both groups. The two groups could be separated with a classifier based on medial temporal and parietal and frontal spikes (p < 0.0001).

MT and LT patients have different spatial distributions of interictal spikes and can be distinguished by the relative spike rates in medial temporal and extratemporal areas during sleep and wake.