Characterization of Seizure Generating and Propagating Regions in Human Focal Epilepsy with Resting State Functional Connectivity MRI

Abstract

Focal seizures represent aberrant electrical rhythms that originate in a specific cortical region, the seizure onset zone (SOZ), and propagate in a network of anatomically connected regions. MRI has emerged as a leading diagnostic methodology in the field of epilepsy over the last 20 years and has significantly impacted the surgical evaluation of patients with medically refractory focal epilepsy. However, thus far clinical MRI has largely been used as a structural tool, and there is an interest in developing MRI-based measures of seizure propagation and inter-regional connectivity. Resting state functional connectivity MRI (rsfMRI) allows identification of macroscopic brain networks based on functional connectivity, or co-activation, within regions (nodes) comprising the network. Functional connectivity (FC) describes the temporal correlation between spatially remote neurophysiologic events, and has been measured using different modalities including rsfMRI. Given the network characteristics of epilepsy, there has been an interest in applying rsfMRI to examine the seizure-generating regions of the brain and their functional interactions with other parts of the brain. In this review, we have focused on focal epilepsy, one of the two subtypes of epilepsy by physiology. Most studies of focal epilepsy, with notable exceptions, reported reduced functional connectivity of epilepsy producing and propagating regions, as well as associated reduction in functional connectivity of networks performing cognitive functions including memory, attention and language. We also reviewed reports of early applications of rsfMRI to lateralize/localize epileptic activity. Overall, resting state functional connectivity MRI (rsfMRI) has already contributed to a better understanding of epilepsy-related networks and promises to add to the non-invasive evaluation of patients with a non-lesional clinical MRI.