Correlation of temporal lobe glucose metabolism with the Wada memory test

Glutamate metabolism in temporal lobe epilepsy as revealed by dynamic proton MRS following the infusion of [U13-C] glucose

Focal metabolic dysfunction commonly observed in temporal lobe epilepsy (TLE), and is associated with the development of medical intractability and neurocognitive deficits. It has not been established if this dysfunction is due to cell loss or biochemical dysfunction in metabolic pathways. To explore this question, dynamic ¹H MRS following an infusion of [U¹³- C] glucose was performed to measure glutamate (Glu) metabolism. Subjects (n=6) showed reduced Glu levels (p<0.01) in the ipsilateral mesial temporal lobe (MTL) compared with controls (n=4). However, the rate of ¹³C incorporation into Glu did not differ between those with epilepsy and controls (p=0.77). This suggests that reduced Glu concentrations in the region of the seizure focus are not due to disruptions in metabolic pathways, but may instead be due to neuronal loss or simplification.

The intracarotid amobarbital procedure: when is it worth repeating?

PURPOSE

Despite the reported diagnostic value of the intracarotid amobarbital procedure (IAP) or "Wada test" for determining hemispheric lateralization and memory functioning, it has never undergone formal reliability testing because a prospective test-retest study design is neither feasible nor ethical. However, some patients require repeat testing for clinical purposes, a circumstance that allows for exploration of issues related to reliability. The current investigation sought to: (1) evaluate the frequency of and reasons for repeated IAPs and (2) describe the test-retest reliability of repeated IAPs in a large tertiary epilepsy center.

METHODS

A 10-year review (2001-2011) of the New York University Langone Medical Center Comprehensive Epilepsy Center patient registry revealed 630 IAPs. Review of medical records identified 20 individuals who underwent two or more IAPs on separate days. Because IAPs repeated due to technical problems should be considered separate from IAPs repeated for other reasons because these IAPs likely included a change in the procedure (e.g., lower medication dose) in an attempt to ameliorate the complication, patients were grouped accordingly. Six patients underwent repeated IAPs due to technical complication and 14 patients underwent a repeated IAP due to other reasons (e.g., unexpected memory outcome, reconsideration of surgery years after a previous surgical work-up in which no surgery was performed, and/or consideration of a second surgery). Given that data obtained from injections ipsilateral to a seizure focus are sometimes considered in a manner clinically different from data obtained from injections contralateral to the seizure focus, memory outcome was classified relative to the side of identified seizure focus. The degree to which language and memory data were consistent across repeated IAPs was examined.

KEY FINDINGS

Language functioning was consistently lateralized across IAPs in all but one case. Among the six patients who experienced technical problems in the first IAP, three were fully participatory in the second procedure such that valid data were obtained. For the other three, the technical problem recurred with no change in outcome across procedures. Among the 14 patients with repeated IAPs due to other reasons, 79% of the available ipsilateral and 73% of the contralateral pass/fail outcomes were consistent across procedures. No difference between ipsilateral or contralateral injections was observed for the likelihood of a change in results (p = 0.57).

SIGNIFICANCE

Our data identified overall high reliability for both the ipsilateral and contralateral sides with repeated IAP testing. Results indicated that although patients for whom a correctable technical problem was identified during the IAP may benefit from a repeat study, there is little benefit to repeating the IAP in patients with discordant or unexpected results (i.e., results are not likely to change). These data support the overall reliability of both the language and memory data obtained from the IAP.

Metabolic evidence for episodic memory plasticity in the nonepileptic temporal lobe of patients with mesial temporal epilepsy

PURPOSE

Metabolic changes have been described in the nonepileptic temporal lobe of patients with unilateral mesiotemporal lobe epilepsy (MTLE) associated with hippocampal sclerosis (HS). To better understand the functional correlate of this metabolic finding, we have sought to characterize brain regions in patients with MTLE that show correlation between unilateral episodic memory performances, as assessed by intracarotid amobarbital test (IAT), and interictal regional cerebral metabolism measured by [(18) F]-fluorodeoxyglucose positron emission tomography (FDG-PET).

METHODS

Resting FDG-PET was performed interictally in 26 patients with unilateral MTLE caused by HS (16 female, mean age: 36 years; 16 left HS). Using statistical parametric mapping (SPM8), we performed a group comparison analysis comparing brain metabolism in the patients and in 54 adult controls (27 female, mean age: 32 years), with FDG-PET data of right HS patients being flipped. IAT scores of nonepileptic hemisphere functions (amobarbital injection ipsilateral to HS) were used as covariates of interest in a correlation analysis with regional brain metabolism.

KEY FINDINGS

The group comparison analysis revealed significant hypometabolic areas in a widespread temporofrontal network ipsilateral to HS. In addition, a significant increase in metabolism was found in mesial and lateral temporal regions contralateral to HS. Significant positive correlations were found between IAT scores of nonepileptic hemisphere functions and mesial temporal metabolism in this hemisphere.

SIGNIFICANCE

This study demonstrates the existence of significant increase in relative regional cerebral glucose metabolism in mesial and lateral temporal regions contralateral to the epileptic focus in patients with unilateral MTLE associated with HS. The positive correlation in these brain regions between IAT scores and metabolism supports the role of disease-induced plasticity mechanisms contralateral to HS in the preservation of episodic memory processes.

Performance in recognition memory is correlated with entorhinal/perirhinal interictal metabolism in temporal lobe epilepsy

In addition to the hippocampus, the entorhinal/perirhinal cortices are often involved in temporal lobe epilepsy (TLE). It has been proposed that these anterior parahippocampal structures play a key role in recognition memory. We studied the voxel-based PET correlation between number of correctly recognized targets in a new recognition memory paradigm and interictal cerebral metabolic rate for glucose, in 15 patients with TLE with hippocampal sclerosis. In comparison to healthy subjects, patients had decreased recognition of targets (P<0.001) and ipsilateral hypometabolism (relative to side of hippocampal sclerosis) of the hippocampus, entorhinal/perirhinal cortices, medial temporal pole, and middle temporal gyrus (P<0.05, corrected by false discovery rate method). Performance correlated with interictal metabolism of ipsilateral entorhinal/perirhinal cortices (P<0.005, Spearman's rank test), but this relationship was not significant in the hippocampus itself (P>0.18, Spearman's rank test). These findings highlight the preferential involvement of entorhinal/perirhinal cortices in recognition memory in patients with TLE, and suggest that recognition memory paradigms may be useful in assessing anterior parahippocampal functional status in TLE.

Hemisphere-specific Episodic Memory Networks in the Human Brain: A Correlation Study between Intracarotid Amobarbital Test and [18F]FDG-PET

The purpose of the present study was to explore the brain regions involved in human episodic memory by correlating unilateral memory performance estimated by the intracarotid amobarbital test (IAT) and interictal cerebral metabolism measured by [18F]fluorodeoxyglucose positron emission tomography ([18F]FDG-PET). Using this method, regional alterations of cerebral metabolism associated with epilepsy pathophysiology are used to predict hemisphere-specific episodic memory function, hence, investigate the differential distribution of memory in each hemisphere. Sixty-two patients with unilateral temporal lobe epilepsy (35 left and 27 right) were studied using [18F]FDG-PET with complementary voxel-based statistical parametric mapping (SPM) and region-of-interest (ROI) methods of analysis. Positive regression was analyzed in SPM with a series of different thresholds (p = .001, .01 or .05) with a correction to 100 voxels. IAT memory performance in which left hemisphere was tested by right-sided injection of amobarbital correlated with [18F]FDG uptake in left lateral and medial temporal regions, and in the left ventrolateral frontal cortex. Right IAT memory performance correlated with [18F]FDG uptake in the right inferior parietal lobule, right dorsolateral frontal cortex, right precentral gyrus, and caudal portion of the right anterior cingulate cortex. ROI analysis corroborated these results. Analyses carried out separately in patients with left (n = 50) and nonleft (n = 12) dominance for language showed that in the nonleft dominant group, right IAT scores correlated with right fronto-temporal regions, whereas left total memory scores correlated with left lateral and medial temporal regions. The findings indicate that (i) episodic memory is subserved by more widespread cortical regions beyond the core mesiotemporal lobe memory structures; (ii) there are different networks functional in the two hemispheres; and (iii) areas involved in memory may be different between patients with left and nonleft dominance for language, particularly in the right hemisphere.

The role of positron emission tomography with [18F]fluorodeoxyglucose in the evaluation of the epilepsies

Cerebral glucose metabolic mapping using positron emission tomography (PET) and 2-[18F]fluoro-2-deoxyglucose (FDG) has been extensively studied in the epilepsies. Regions of interictal glucose hypometabolism are highly associated with cerebral sites of seizure generation-propagation in focal epilepsies. The volume of reduced glucose metabolism is often widespread and even bilateral in focal epilepsies, although ictal onset zones typically are located at the sites of most severe hypometabolism within a larger volume of hypometabolism.

Presurgical assessment of memory-related brain structures: the Wada test and functional neuroimaging

Medial temporal lobe structures are known to play a major role in memory processing. Recent work has revealed that extratemporal structures (e.g. the frontal lobe and thalamus) may also be important in memory function. In candidates for epilepsy surgery, particularly in those with temporal lobe seizures, presurgical evaluation of memory function is essential, since seizures may originate in the neural substrate that is critical for memory. In this article, we review the tools used for presurgical evaluation and their contribution to the understanding of memory function, focusing on the Wada test, [18F]fluorodeoxy-glucose positron emission tomography ([18F]FDG-PET) and functional magnetic resonance imaging (fMRI). We also explore perspectives on future studies that may elucidate the role of the temporal and extratemporal structures in memory function and the mechanisms of cerebral plasticity.