Nonlinear Trends in Hippocampal Metabolic Function and Verbal Memory: Evidence of Cognitive Reserve in Temporal Lobe Epilepsy?

Cognitive reserve moderates the relationship between neuropsychological performance and white matter fiber bundle length in healthy older adults

Recent work using novel neuroimaging methods has revealed shorter white matter fiber bundle length (FBL) in older compared to younger adults. Shorter FBL also corresponds to poorer performance on cognitive measures sensitive to advanced age. However, it is unclear if individual factors such as cognitive reserve (CR) effectively moderate the relationship between FBL and cognitive performance. This study examined CR as a potential moderator of cognitive performance and brain integrity as defined by FBL. Sixty-three healthy adults underwent neuropsychological evaluation and 3T brain magnetic resonance imaging. Cognitive performance was measured using the Repeatable Battery of Assessment of Neuropsychological Status (RBANS). FBL was quantified from tractography tracings of white matter fiber bundles, derived from the diffusion tensor imaging. CR was determined by estimated premorbid IQ. Analyses revealed that lower scores on the RBANS were associated with shorter whole brain FBL (p = 0.04) and lower CR (p = 0.01) CR moderated the relationship between whole brain FBL and RBANS score (p < 0.01). Tract-specific analyses revealed that CR also moderated the association between FBL in the hippocampal segment of the cingulum and RBANS performance (p = 0.03). These results demonstrate that lower cognitive performance on the RBANS is more common with low CR and short FBL. On the contrary, when individuals have high CR, the relationship between FBL and cognitive performance is attenuated. Overall, CR protects older adults against lower cognitive performance despite age-associated reductions in FBL.

Correlation between memory, proton magnetic resonance spectroscopy, and interictal epileptiform discharges in temporal lobe epilepsy related to mesial temporal sclerosis

OBJECTIVE

The aim of the study described here was to examine the relationship between memory function, proton magnetic resonance spectroscopy ((1)H-MRS) abnormalities, and interictal epileptiform discharge (IED) lateralization in patients with temporal lobe epilepsy (TLE) related to unilateral mesial temporal sclerosis.

METHODS

We assessed performance on tests of memory function and intelligence quotient (IQ) in 29 right-handed outpatients and 24 controls. IEDs were assessed on 30-minute-awake and 30-minute-sleep EEG samples. Patients had (1)H-MRS at 1.5 T.

RESULTS

There was a negative correlation between IQ (P=0.031) and Rey Auditory Verbal Learning Test results (P=0.022) and epilepsy duration; between(1)H-MRS findings and epilepsy duration (P=0.027); and between N-acetylaspartate (NAA) levels and IEDs (P=0.006) in contralateral mesial temporal structures in the left MTS group. (1)H-MRS findings, IEDs, and verbal function were correlated.

CONCLUSIONS

These findings suggest that IEDs and NAA/(Cho+Cr) ratios reflecting neural metabolism are closely related to verbal memory function in mesial temporal sclerosis. Higher interictal activity on the EEG was associated with a decline in total NAA in contralateral mesial temporal structures.

The use of neuroimaging to study behavior in patients with epilepsy

Structural and functional neuroimaging continues to play an increasing role in the presurgical evaluation of patients with epilepsy. In addition to its value in localizing the epileptogenic zone and eloquent cortex, neuroimaging is contributing to our understanding of mood comorbidity in epilepsy. Although the vast majority of research has focused on patients with temporal lobe epilepsy (TLE), neuroimaging studies of patients with extratemporal epilepsy and primary generalized epilepsy are increasing in number. In this review, structural and functional imaging modalities that have received considerable research attention in recent years are reviewed, and their strengths and limitations for understanding behavior in epilepsy are assessed. In addition, advances in multimodal imaging are discussed along with their potential application to the presurgical evaluation of patients with seizure disorders.

Cognitive reserve, age, and neuropsychological performance in healthy participants

The first aim of this study was to explore the relation between cognitive reserve, age, and neuropsychological functioning in a healthy sample; and second, to determine the risk of showing cognitive deficits as a function of cognitive reserve. One hundred forty-six healthy participants between the ages of 20 and 79 were submitted to neuropsychological assessment, focusing on attention, memory, visuo-construction, conceptualization and reasoning. Premorbid IQ as measured with the Wechsler Adult Intelligence Scale Vocabulary subtest was used as a proxy of cognitive reserve. Multivariate regression analysis with age and premorbid IQ as explanatory factors revealed a significant effect in all neuropsychological tests. Logistic regression revealed that participants with low cognitive reserve were more likely to obtain deficient scores (< or =1.5 SD below the mean) in the cognitive domains of attention (odds ratio [OR], 3.13; 95% confidence interval [CI], 1.059.29), memory (OR, 6.17; 95% CI, 1.69-22.61) and global functioning (OR, 6.44; 95% CI, 2.56-16.22) than participants with high cognitive reserve. Results suggest that cognitive reserve acts as a protective factor against the expression of cognitive decline related to age in healthy individuals.

Verbal memory outcome in patients with normal preoperative verbal memory and left mesial temporal sclerosis

PURPOSE

Previous studies have shown that structural integrity (i.e., presence/absence of mesial temporal sclerosis (MTS)) of the left mesial temporal lobe is associated with verbal memory outcome following left anterior temporal lobectomy (ATL). However, the functional integrity of the left temporal lobe, as exemplified by preoperative verbal memory performance, has also been associated with verbal memory outcome following surgery. We investigated the risk of verbal memory loss in patients with known structural abnormality (i.e., left mesial temporal sclerosis by MRI) and normal preoperative verbal memory performance who undergo left ATL.

METHODS

Seventeen patients with left temporal lobe epilepsy, MRI-based exclusive left MTS, and normal preoperative verbal memory were identified. Normal verbal memory was defined as performance on both Acquisition (learning across trials 1-5) and Retrieval (long delayed free recall) portions of the California Verbal Learning Test (CVLT) above a T score of 40 (>16%ile). Postoperative verbal memory outcome was established by incorporating standardized regression-based (SRB) change scores.

RESULTS

Postoperative declines across both CVLT Retrieval T scores and Acquisition T scores (average 20% and average 15% declines from baseline scores, respectively) were measured for the group. The average CVLT Retrieval SRB change score was -2.5, and the average CVLT Acquisition SRB change score was -1.0. A larger proportion of patients demonstrated postoperative declines on Retrieval scores than Acquisition scores (64.7% vs 17.6%, respectively).

CONCLUSIONS

Even in the presence of left MTS, patients exhibiting normal presurgical verbal memory are at risk for verbal memory declines following ATL. These results suggest that the functional integrity of the left mesial temporal lobe may play an important role in the verbal memory outcome in this patient group.

Anterior perirhinal cortex kindling produces long-lasting effects on anxiety and object recognition memory

Temporal lobe epilepsy (TLE) is frequently accompanied by memory impairments and, although their bases are unknown, most research has focused on the hippocampus. The present study investigated the importance of another medial temporal lobe structure, the perirhinal cortex (Prh), in changes in memory in TLE using kindling as a model. Rats were kindled twice daily with anterior Prh stimulation until three fully generalized seizures were evoked. Beginning 7 days later and on successive days, rats were tested in an elevated plus maze, a large circular open field, an open field object exploration task and a delayed-match-to-place task in a water maze in order to assess anxiety-related and exploratory behaviour, object recognition memory and spatial cognition. Kindling increased anxiety-related behaviour in both the elevated plus and open field mazes and disrupted spontaneous object recognition but spared all other behaviours tested. These results are consistent with other findings indicating a greater role for the Prh in object memory and emotional behaviour than in spatial memory and contrast with the selective disruption of spatial memory produced by dorsal hippocampal kindling. The site-selectivity of the behavioural disruptions produced by kindling indicates that such effects are probably mediated by changes particular to the site of seizure initiation rather than to changes in the characteristic circuitry activated by limbic seizure generalization. Further investigation of the behavioural effects of Prh kindling may be useful for studying the mechanisms of mnemonic and affective dysfunction associated with TLE and offer insights into bases for variability in such dysfunction across patients.

Relationships among hippocampal volumetry, proton magnetic resonance spectroscopy, and verbal memory in temporal lobe epilepsy

PURPOSE

To examine the relationship between hippocampal volumes, 1H magnetic resonance spectroscopy (MRS)-identified hippocampal metabolic function, and verbal memory in patients with unilateral mesial temporal lobe epilepsy (MTLE).

METHODS

Hippocampal volumes, 1H MRS-derived hippocampal creatine to N-acetylaspartate (Cr/NAA), and verbal memory assessment were obtained preoperatively in 22 patients (six right, 16 left) with EEG-defined unilateral MTLE.

RESULTS

Left hippocampal volume correlated significantly with left hippocampal Cr/NAA (r=-0.549, p < 0.01), whereas right volume correlated significantly with right Cr/NAA (r=-0.478, p < 0.05). Verbal memory correlated significantly with left hippocampal Cr/NAA (r=-0.594, p < 0.01), but not with left hippocampal volume or right hippocampal measures.

CONCLUSIONS

Hippocampal volumes and 1H MRS-derived metabolite ratios are statistically related, but share only a small percentage of variance, suggesting separate but related pathophysiologic processes. Left hippocampal Cr/NAA appears to be more sensitive to verbal memory function than volumes.