Abstracts

Rationale: Surgical treatment of mesial temporal lobe epilepsy (TLE) fails to provide a seizure-free outcome in 20 30% of patients. Several reasons have been identified to explain surgical failures after mesial temporal surgery. Insufficient resection of epileptogenic mesial temporal structures, relapse on the contralateral mesial temporal lobe, failure to resect the temporal neocortex and coexistence of mesial temporal sclerosis and a neocortical lesion are possible causes of seizure recurrence after mesial TLE surgery. Success of reoperation depends on accurate mapping of the residual or hidden epileptogenic zone. We studied the role of magnetoencephalography (MEG) in patients with recurrent or residual seizures after mesial temporal lobe resection. Methods: MEG data was reviewed in seven patients with recurrent or residual seizures following mesial temporal lobe surgery. MEG was performed as part of non-invasive work up after failed epilepsy surgery. MEG data was collected using a 275-channel whole head MEG system for a minimum duration of 60 minutes with simultaneous EEG recording. MEG data was analyzed using a single equivalent current dipole model applied to the earliest peak of each epileptic event as part of a standard clinical protocol.Results: Age of seizure onset ranged from 6 months to 40 years. Risk factors for epilepsy included previous head trauma in two patients, febrile seizures in two patients and no risk factors in the remaining three patients. MRI prior to the first surgery showed unilateral hippocampal atrophy in four patients, normal MRI in two patients and no previous MRI in one patient. Previous surgery consisted of anterior temporal lobectomy in four patients, hippocampectomy in one and selective amygdalohippocampectomy followed by anterior temporal lobectomy in two patients. Seizure recurrence occurred 2 weeks -7 months after surgery. No MEG spikes were detected in two patients. MEG dipole clusters were seen in the remaining five patients. Anterior insular dipole cluster was seen in three patients, over the residual superior temporal gyrus in one patient and over the posterior margin of the resection in one patient. One patient had reoperation and is seizure-free after insular resection.Conclusions: MEG provides useful localizing information after failed mesial temporal lobe surgery. Specific patterns of MEG localization can provide useful information explaining the reason for surgical failure and guiding future surgery. Missed insular focus should be considered in patients with failed mesial temporal resection.