Potential Contribution of Bilateral Magnetic Source Imaging to the Evaluation of Epilepsy Surgery Candidates:

MEG predicts epileptic zone in lesional extrahippocampal epilepsy: 12 pediatric surgery cases

PURPOSE

To discover whether the spatial distribution of spike sources determined by magnetoencephalography (MEG) provides reliable information for planning surgery and predicting outcomes in pediatric patients with lesional extrahippocampal epilepsy.

METHODS

We retrospectively studied 12 children with extrahippocampal epilepsy secondary to cortical dysplasia (CD), tumor, or porencephalic cyst. We compared interictal MEG spike source locations and somatosensory evoked fields derived from equivalent-current dipole modeling with intraoperative or extraoperative electrocorticography (ECoG).

RESULTS

MEG spike sources were found in proximity to the lesion in all patients and extended from lesions in five patients with CD. Marginal spike sources were noted in three patients with tumors, one patient with a cyst, and one with CD, and extramarginal sources in three patients with tumors. Three patients with tumors underwent lesionectomy only; two had further cortical excisions. One patient with CD underwent lesionectomy only, three had lesionectomy and cortical excisions, and two had lesionectomy and multiple subpial transection. Asymmetric MEG spike sources correlated with ECoG findings in all patients. Residual epileptiform discharges on postexcisional ECoG corresponded to spike sources in three patients with tumors and one patient with a cyst. Eleven patients have been seizure free for 1-6 years (mean, 4 years). One patient had residual seizures after incomplete excision of right temporal CD.

CONCLUSIONS

MEG delineated asymmetric epileptogenicity surrounding lesions and the eloquent cortex. Complete tumor resection produced favorable outcomes despite residual postexcisional ECoG spikes and extramarginal MEG spike sources. CD characterized by clusters of MEG spike sources within and extending from lesions seen on magnetic resonance imaging (MRI) should be removed to prevent seizures.

Confirmation of two magnetoencephalographic epileptic foci by invasive monitoring from subdural electrodes in an adolescent with right frontocentral epilepsy

PURPOSE

To report our evaluation of interictal two epileptic spike fields on magnetoencephalography (MEG) by using invasive intracranial monitoring in a patient without lesion on magnetic resonance imaging (MRI).

METHODS

A 15-year-old left-handed boy with a 9-year history of refractory simple partial seizures, secondarily generalized, and a normal MRI, was studied with MEG to define magnetic spike sources, followed by invasive intracranial monitoring with subdural electrodes to delineate the epileptogenic zone and eloquent function pursuant to focal cortical excision.

RESULTS

MEG demonstrated two spike foci on the right middle frontal and inferior rolandic areas adjacent to the sensory area. Ictal recordings during prolonged invasive monitoring from subdural electrodes revealed two epileptogenic zones in the same locations as those defined by MEG. Focal cortical excision was performed of each epileptogenic zone. The patient has been seizure free for 24 months without neurologic deficit.

CONCLUSIONS

Magnetic source imaging is a valuable adjunct in the planning of subdural grid placement in epilepsy surgery, particularly in patients in whom conventional imaging fails to reveal a lesion.