Introduction: Anterior temporal lobe resection (ATLR) conduces to remission in 80% of patients with refractory temporal lobe epilepsy (TLE); however, in 30% of cases there is also significant verbal memory loss after speech dominant hemisphere ATLR. The assessment of structures responsible for memory formation through fMRI may help balancing the risk-benefit of ATLR. 
 
 Method: We reviewed pre-surgical clinical data and imaging results of 26 patients with refractory epilepsy. For fMRI, a verbal memory encoding paradigm was applied, consisting of words visualization with out-of-scanner recognition assessment. An event-related analysis located brain activations for words remembered (WR) followed by calculation of laterality index (LI) within a mask for fronto-temporal region. 
 
 Result: Structural lesion was bilateral on 1 patient, left in 14 patients and right in 11 patients. These right TLE patients were referred to memory fMRI based on EEG incongruences and/or left temporal dysfunction in neuropsychological assessment - 5 of them showing temporal dysfunction contralateral to the side of lesion. In 7 patients, there was no activation of mesial structures. Frontal structures were the main contributors to LI in 14 patients, with greater relevance of temporal mesial structures in 6 patients. There was no difference of LI value between gender, age at onset of epilepsy or type of lesion, but we found a correlation between LI and duration of epilepsy (-0.628, p=0.001). A stronger lateralization (>|0.5|) was associated with better performance, with a mean WR of 82% vs. 65% for poorer lateralization, and with significant correlation (p=0.006). 
 
 Discussion: Our results reflect neural plasticity consequential to disruption from temporal lesions over time. This is especially relevant for left side lesions, with compensatory network reorganization, including an anticipated lateralization of verbal memory function to the right hemisphere and/or a quite unexpected impact from left frontal structures. In either, better performance was achieved with stronger lateralization. Bilaterality was seen in patients with left lesions and long disease duration, with globally poor activations and worse performance, suggesting a widespread neural dysfunction besides the primary lesion.
 
 Conclusion: Laterality index in our sample was highly dependent of left frontal structures, which seem to be involved in verbal memory compensation in pre-surgical TLE. This aspect may influence the perspective of fMRI as a prognostic marker of post-surgical deficit in ATLR. Further studies are warranted to clarify the exact significance of this finding and how it may be influenced by technical issues of memory fMRI implementation in each centre.

43rd ESNR Annual Meeting