Epileptic activity influences the lateralization of mesiotemporal fMRI activity

Objective: To identify clinical factors contributing to the lateralization of mesiotemporal memory functions in epilepsy by using memory-activated fMRI. Methods: Sixty patients aged 16 to 63 years with mesial temporal lobe epilepsy (MTLE) and 20 patients aged 16 to 60 years with extratemporal epilepsy (ETE) due to circumscribed epileptogenic lesions who consecutively underwent presurgical evaluation including continuous video-EEG monitoring and structural MRI examinations were examined. During memory fMRI, the activation condition consisted of retrieval from long-term memory induced by self-paced performance of an imaginative walk through the patient's hometown. On the basis of a previous study, memory lateralization was defined as typical if larger fMRI activation was in the mesiotemporal structures contralateral to the epileptic focus. Results: There were 45 patients with MTLE who had typical memory lateralization (75%), whereas only 9 patients (45%) with ETE exhibited typical memory lateralization (p=0.013). In MTLE patients, bilateral independent epileptiform discharges occurred more often in the atypical group than in patients with typical memory lateralization (p=0.015). Conclusions: The fMRI lateralization of mesiotemporal visuospatial memory functions in patients with mesiotemporal lobe epilepsy (MTLE) is asymmetric: The larger activation usually appears contralateral to the side of the epileptogenic region. These findings occur more often in MTLE; in patients with extratemporal epilepsy, such type of asymmetry is not characteristic. In MTLE patients with bilateral independent epileptiform discharges, this type of asymmetry is also less frequent.