Abstract
Individuals with medial temporal lobe epilepsy (mTLE) often show material-specific memory impairment (verbal for left, visuospatial for right hemisphere), which can be exacerbated following surgery aimed at the epileptogenic regions of medial and anterolateral temporal cortex. There is a growing body of evidence suggesting that characterization of structural and functional integrity of these regions using MRI can aid in prediction of post-surgical risk of further memory decline. We investigated the nature of the relationship between structural and functional indices of hippocampal integrity with pre-operative memory performance in a group of 26 patients with unilateral mTLE. Structural integrity was assessed using hippocampal volumes, while functional integrity was assessed using hippocampal activation during the encoding of novel scenes. We quantified structural and functional integrity in terms of asymmetry, calculated as (L − R)/(L + R). Factor scores for verbal and visual memory were calculated from a clinical database and an asymmetry score (verbal − visual) was used to characterize memory performance. We found, as expected, a significant difference between left and right mTLE (RTLE) groups for hippocampal volume asymmetry, with each group showing an asymmetry favoring the unaffected temporal lobe. Encoding activation asymmetry showed a similar pattern, with left mTLE patients showing activation preferential to the right hemisphere and RTLE patients showing the reverse. Finally, we demonstrated that functional integrity mediated the relationship between structural integrity and memory performance for memory asymmetry, suggesting that even if structural changes are evident, ultimately it is the functional integrity of the tissue that most closely explains behavioral performance.
Highlights
Medial temporal lobe epilepsy is characterized by recurrent seizures generated in temporal lobe structures, the hippocampus
The MTLs are known to be highly involved in episodic memory [1, 2] and, the disruption of neural circuitry seen in Medial temporal lobe epilepsy (mTLE) is accompanied by material-specific memory deficits [3]
Healthy controls had significantly more years of education than patients with right mTLE (RTLE), t(25) = 2.52, p < 0.05 and patients with left mTLE (LTLE), t(23) = 2.09, p < 0.05, but there was no difference between the two patient groups, t(25) = 0.68, p = 0.51
Summary
Medial temporal lobe epilepsy (mTLE) is characterized by recurrent seizures generated in temporal lobe structures, the hippocampus. Correlates of memory in TLE resection of the anterior hippocampus, amygdala, and anterior temporal neocortex in the affected hemisphere to relieve seizures. This resection can result in a de novo material-specific memory decline or an exacerbation of pre-surgical weaknesses [7,8,9]. Individuals who have mesial temporal sclerosis (MTS) tend to show smaller declines in memory postoperatively compared to those without MTS These individuals tend to have worse memory presurgically, which may be attributed to their lesioned hippocampus and the atrophy in the surrounding tissue [4, 13, 14]. Few studies have looked at all three of these predictors concurrently and, among those that have, there is disagreement regarding which predictor performs most effectively [15, 16]
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