Functional MRI connectivity as a predictor of the surgical outcome of epilepsy

Abstract

Purpose: In planning epilepsy surgery, it is important to be able to assess the likelihood of success of surgery for each patient so that the possible risk and benefit can be properly considered. In this study, functional connectivity was investigated as a means for predicting surgical outcome from the preoperative functional magnetic resonance imaging (fMRI) of epilepsy patients.

Methods: Resting-state simultaneous electroencephalography (EEG)-fMRI data were collected from 18 patients with intractable epilepsy before surgery and from 14 healthy subjects. For each patient, EEG-spike correlated fMRI analysis was performed and an activation cluster that overlapped the most with the planned resection area for each patient was chosen as the seed for the functional connectivity analysis. After the functional connectivity maps were computed, laterality indices of functional connectivity were contrasted between patients who had seizures after surgeries (seizure-recurrence group) and those who did not have them for at least a year (seizure-free group).

Key findings: Patients in the seizure-recurrence group had less-lateralized functional connectivity than patients in the seizure-free group (t(16) = 2.3, after control subtracted and Fisher transformed, p < 0.05, two-tailed).

Significance: This study suggests the potential for using preoperative fMRI connectivity analysis as a predictive outcome measure. If confirmed by further research, a high laterality will be an important addition to the other predictors of better surgical outcome such as febrile seizures, mesial temporal sclerosis, tumors, abnormal MRI, and EEG/MRI concordance.

Figure 1

Examples of spike correlated fMRI-seeded functional connectivity maps with low (a), medium (b), and high (c) laterality indices. The laterality values displayed are before laterality values of the controls are subtracted. The distinct lateralization of the functional connectivity can be clearly recognized in (b) and (c).

Figure 2

Control-subtracted laterality index of the spike correlated fMRI seeded functional connectivity. Patients in the non-seizure-free group had significantly lower laterality indices than patients in the seizure-free group. The long horizontal bar shows the average, the rectangle show the range between the average plus and minus the standard deviation, and the “x” shows a data point.

Figure 3

Ipsilateral (blue solid lines) and contralateral functional connectivity (red dashed lines) used as covariates for predicting the surgical outcome. Neither control-subtracted contralateral connectivity nor control-subtracted ipsilateral connectivity alone was predictive of the outcome.

Figure 4

Comparison between the resection area-seeded functional connectivity analysis and the spike correlated fMRI seeded functional connectivity analysis based on 9 patients. The resection area-seeded functional connectivity analysis did not show a significant difference of laterality between seizure-recurrence and seizure-free groups, whereas the spike correlated fMRI seeded connectivity did. However, the lack of interaction indicates that there was not a significant difference between two seeding methods.