Functional and structural mri connectivity associated to autobiographic memory in temporal lobe epilpepsy with uni and bilateral hippocampal sclerosis.

Resumen

Structural damage associated with hippocampal sclerosis (HS) is the most common cause of temporal lobe epilepsy (TLE). TLE patients with persistent paroxysmal activity during seizures have structural and functional brain connectivity disturbances with abnormalities in the wider cortico-subcortical network. The majority of lesion studies report that mesial temporal lobe (MTL) is associated with impairment of autobiographic memory (AM), namely the memory for experiences that occurred in a specific time and place, probably because the human hippocampus known to be a crucial node in the neural network supporting AM. In addition, there are many other factors that may influence the pattern and extent of retrograde memory loss after a temporal lobe lesion, such as the impact of lesion side, or the method of assessing and/or scoring memory performance. We extensively reviewed the literature about the neurofunctional changes elicited by TLE, bilateral mesial TLE (BMTLE), and surgical treatment on AM, and we realized that this memory function has scarcely been examined. Furthermore, there is also a lack of information regarding white-matter (WM) interhemispheric seizure propagation pathways and commissures. Addressing these weaknesses, we examined patients with mesial TLE (MTLE; unilateral, bilateral or with TLE surgery) in two projects with specific aims: (i) To examine the AM neurofunctional correlates in TLE patients and (ii) to identify the WM microstructural connectivity in TLE and bitemporal HS (TLE+BHS) patients. First, to study AM-functional related activity in TLE patients we designed a new fMRI AM paradigm. We selected, for the first time to our knowledge, a group of 19 TLE patients with anterior temporal lobe resection (ATLR) or BMTLE, as well as 18 subjects for the control group. We observed that, while the control and right MTLE group showed a left-lateralized AM network, left MTLE and patients with BMTLE also showed strong activation in the right temporal and frontal lobe regions. These results support the idea of the use of alternate brain areas involved in retrieval of specific AM memories and encourage the idea of neuroplastic changes in damaged structures. It is likely that the successful retrieval of AMs in these patients, although impaired, is supported by increased activation of neighboring or contralateral regions. Secondly, we studied for the first time microstructural connectivity in a unique patient group with TLE+BHS. We performed a multimodal structural MRI [high resolution T1-weighted and diffusion tensor imaging (DTI)] analysis of seven TLE+BHS patients compared to unilateral TLE and healthy control groups. Our results showed WM derangement in commissural pathways in TLE+BHS patients when compared to unilateral TLE patients and the control group. These findings support the idea that commissures might play a contributory role in interhemispheric TLE seizure propagation in TLE+BHS. In both projects we studied patients with bilateral mesial temporal lobe epilepsy (BMTLE or TLE+BHS). This is a singular and uncommon TLE patient group which offered the opportunity to study memory and microstructural connectivity in a sample of patients in whom contralateral reliance is difficult and interhemispheric seizure transmission plays a notable role. The differences observed between healthy controls and TLE patients, in both structural and functional studies, demonstrated and strengthened the idea of induced brain changes due to epilepsy activity. The functional changes observed during AM retrieval and the structural WM abnormalities, are important and might provide a more detailed evaluation of AM in TLE patients by using a different perspective, such as indicators of the recruitment of different functional regions or of the functionality of remnant tissue in a damaged structure. These results also may be of use in patient therapeutic management and presurgical performance guidance in the study of disease severity and tracking progression, as well as presurgical planning in patients with epilepsy, enabling avoidance of functional zones and interruption of seizure propagation pathways.