The relation between quantitative MRI measures of hippocampal structure and the intracarotid amobarbital test

Selective correlation of hippocampal volumes with WADA memory scores in mesial temporal sclerosis patients

Objective

The WADA test is used to determine cerebral language dominance and assess the risk of postoperative amnesia following mesial temporal lobe resection. This study aims to explore the correlation between automated measures of hippocampal volume and WADA memory scores and to evaluate whether these volumetric measurements can reliably predict WADA memory scores.

Methods

This study included patients who underwent a comprehensive presurgical assessment along with bilateral WADA testing. Hippocampal volumes were measured from high-resolution brain MRIs using automated software (volBrain), which were harmonized and normalized to whole brain volume. These harmonized and normalized volumes were then correlated with ipsilateral WADA memory scores and stratified according to brain MRI findings. A similar analysis was conducted between hippocampal volume asymmetry and WADA memory score asymmetry (WMA). A Receiver Operating Characteristic (ROC) curve was generated to compare the sensitivity and specificity in predicting successful WADA outcomes based on ipsilateral harmonized normalized hippocampal volumes.

Results

In patients with mesial temporal sclerosis (MTS), significant positive correlations were found between harmonized normalized hippocampal volumes and ipsilateral WADA memory scores, as well as between harmonized hippocampal volume asymmetries and WMA. However, no significant correlations were found in patients with epileptogenic lesions other than MTS or those with normal brain MRIs. A harmonized normalized hippocampal volume threshold of ≥ 28.94 units was identified as a predictor of a WADA memory score exceeding 50% following contralateral carotid artery injection, with a sensitivity of 62.1% and a specificity of 100%.

Significance

This study indicates that hippocampal volumetry could potentially serve as an alternative to the WADA test in patients with MTS. Conversely, in individuals with normal MRI results or other types of epileptogenic lesions, hippocampal volumetry does not reliably predict memory deficits, necessitating the use of the WADA test or functional MRI for planning resections of mesial temporal structures in the dominant hemisphere.

Impact of white matter networks on risk for memory decline following resection versus ablation in temporal lobe epilepsy

BACKGROUND

With expanding neurosurgical options in epilepsy, it is important to characterise each options' risk for postoperative cognitive decline. Here, we characterise how patients' preoperative white matter (WM) networks relates to postoperative memory changes following different epilepsy surgeries.

METHODS

Eighty-nine patients with temporal lobe epilepsy with T1-weighted and diffusion-weighted imaging as well as preoperative and postoperative verbal memory scores (prose recall) underwent either anterior temporal lobectomy (ATL: n=38) or stereotactic laser amygdalohippocampotomy (SLAH; n=51). We computed laterality indices (ie, asymmetry) for volume of the hippocampus and fractional anisotropy (FA) of two deep WM tracts (uncinate fasciculus (UF) and inferior longitudinal fasciculus (ILF)).

RESULTS

Preoperatively, left-lateralised FA of the ILF was associated with higher prose recall (p<0.01). This pattern was not observed for the UF or hippocampus (ps>0.05). Postoperatively, right-lateralised FA of the UF was associated with less decline following left ATL (p<0.05) but not left SLAH (p>0.05), while right-lateralised hippocampal asymmetry was associated with less decline following both left ATL and SLAH (ps<0.05). After accounting for preoperative memory score, age of onset and hippocampal asymmetry, the association between UF and memory decline in left ATL remained significant (p<0.01).

CONCLUSIONS

Asymmetry of the hippocampus is an important predictor of risk for memory decline following both surgeries. However, asymmetry of UF integrity, which is only severed during ATL, is an important predictor of memory decline after ATL only. As surgical procedures and pre-surgical mapping evolve, understanding the role of frontal-temporal WM in memory networks could help to guide more targeted surgical approaches to mitigate cognitive decline.

Association Between Microstructural Asymmetry of Temporal Lobe White Matter and Memory Decline After Anterior Temporal Lobectomy

BACKGROUND AND OBJECTIVES

Risk for memory decline is a substantial concern in patients with temporal lobe epilepsy (TLE) undergoing anterior temporal lobectomy (ATL). Although prior studies have identified associations between memory and integrity of white matter (WM) networks within the medial temporal lobe (MTL) preoperatively, we contribute a study examining whether microstructural asymmetry of deep and superficial WM networks within the MTL predicts postoperative memory decline.

METHODS

Patients with drug-resistant TLE were recruited from 2 epilepsy centers in a prospective longitudinal study. All patients completed preoperative T1 and diffusion-weighted MRI (DWI) as well as preoperative and postoperative neuropsychological testing. Preoperative fractional anisotropy (FA) of the WM directly beneath the neocortex (i.e., superficial WM [SWM]) and of deep WM tracts associated with memory were calculated. Asymmetry was calculated for hippocampal volume and FA of each WM tract or region and examined in linear and logistic regressions with preoperative to postoperative memory change as the primary outcome.

RESULTS

Data were analyzed from 42 patients with TLE (19 left TLE [LTLE], 23 right TLE [RTLE]) who underwent ATL. Leftward FA asymmetry of the entorhinal SWM was associated with decline on prose and associative recall in LTLE, whereas leftward FA asymmetry of the uncinate fasciculus (UNC) was associated with decline on prose recall only. After controlling for preoperative memory score and hippocampal volume, leftward FA asymmetry of the entorhinal SWM uniquely contributed to decline in both prose and associative recall (β = -0.46; SE 0.14 and β = -0.68; SE 0.22, respectively) and leftward FA asymmetry of the UNC uniquely contributed to decline in prose recall (β = -0.31; SE 0.14). A model combining asymmetry of hippocampal volume and entorhinal FA correctly classified memory outcomes in 79% of patients with LTLE for prose (area under the curve [AUC] 0.89; sensitivity 82%; specificity 75%) and 81% of patients for associative (AUC 0.79; sensitivity 83%; specificity 80%) recall. Entorhinal SWM asymmetry was the strongest predictor in both models.

DISCUSSION

Preoperative asymmetry of deep WM and SWM integrity within the MTL is a strong predictor of postoperative memory decline in TLE, suggesting that surgical decision-making may benefit from considering each patient's WM network adequacy and reserve in addition to hippocampal integrity.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that preoperative asymmetry of deep WM and SWM integrity within the MTL is a predictor of postoperative memory decline.

Memory outcome following left anterior temporal lobectomy in patients with a failed Wada test

PURPOSE

This study aimed to compare the memory outcome following left anterior temporal lobectomy (ATL) between patients with a failed Wada test and patients who passed the Wada test.

METHODS

From 1996 to 2002, we performed the Wada test on all patients with unilateral left mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS) and concordant electroclinical data before ATL. We used a 12-item recognition paradigm for memory testing and awarded a score of +1 for each correct response and -0.5 for each incorrect response. No patient was denied surgery on the basis of Wada scores. We assessed cognitive and memory functions using the Wechsler Adult Intelligence Scale and the Wechsler Memory Scale preoperatively and at one year after ATL. We compared the number of patients who showed decline in memory scores, as per the published reliable change indices, between the patients with a failed Wada test and the patients who passed the Wada test.

RESULTS

Out of the 116 eligible patients with left MTLE-HS, 88 underwent bilateral Wada test, while 28 underwent ipsilateral Wada test. None of them developed postoperative amnesia. Approximately, one-third of patients with a failed Wada memory test when the failure was defined as a contralateral score of <4, as an ipsilateral score of >8, and as an asymmetry score of <0. The patients with Wada memory failure had a longer pre-ATL duration of epilepsy (p<0.003). The memory and quality-of-life outcomes did not differ between the group with a failed Wada memory test and the group who passed the Wada memory test. The results remained the same when analyses were repeated at various other cutoff points.

CONCLUSION

The patients with left MTLE-HS with concordant electroclinical, MRI, and neuropsychological data should not be denied ATL solely on the basis of Wada memory test results.

Postoperative memory prediction in left temporal lobe epilepsy: the Wada test is of no added value to preoperative neuropsychological assessment and MRI

The contribution of the Wada test (intracarotid amytal procedure, IAP) to predicting postoperative memory outcome in left temporal lobe epilepsy (LTLE) is becoming increasingly controversial when preoperative neuropsychological evaluation and MRI findings are available. We retrospectively analyzed 59 patients with LTLE who underwent en bloc temporal lobe resection. All patients had valid bilateral IAP test results, complete pre- and postoperative neuropsychological evaluation, and MRI grading on a 5-point scale integrating T 2 signal increase and degree of atrophy. Thirty percent of patients showed postoperative memory decline. Multiple regression analysis revealed that significant predictors of decline [F(2.56)=22.71, P<0.001, r(2)=0.448] included preoperative memory learning score [t=-5.89, P<0.001] and MRI classification [t=3.10, P<0.003], but not IAP scores. The IAP is of no added value in the prediction of postoperative memory outcome in LTLE in the presence of comprehensive neuropsychological and MRI data.

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 the Intracarotid Amobarbital Procedure in Predicting Verbal Memory Decline after Temporal Lobe Resection

PURPOSE

The aim of this study was to compare the utility of baseline neuropsychological measures and scores from the intracarotid amobarbital procedure (IAP) in the prediction of postoperative memory decline in temporal lobe epilepsy surgery patients.

METHODS

Logistic regression analyses were used to determine the relation between demographic variables, baseline neuropsychological scores, and scores from the IAP (using mixed verbal and nonverbal stimuli) and postoperative deterioration in verbal learning and verbal recall in 91 patients (48 right, RTL; 43 left, LTL) who had undergone a standard anterior temporal lobe resection for the relief of medically intractable epilepsy and who had been followed up 1 year postoperatively.

RESULTS

In the RTL group, the IAP scores were not significant predictors of a postoperative decline in verbal learning or recall. In the LTL group, postoperative decline in verbal learning was associated with good preoperative baseline scores, an older age at the time of surgery, and an unexpected asymmetry on the IAP. Baseline neuropsychological scores and scores from the IAP were associated with a significant postoperative decline in verbal recall in the LTL group.

CONCLUSIONS

Scores from the IAP using mixed stimuli were not helpful in the prediction of postoperative verbal memory decline in RTL patients. The significance of IAP scores in predicting verbal memory deficits in LTL patients may be task specific.

MR-based in vivo hippocampal volumetrics: 2. Findings in neuropsychiatric disorders

Magnetic resonance imaging (MRI) has opened a new window to the brain. Measuring hippocampal volume with MRI has provided important information about several neuropsychiatric disorders. We reviewed the literature and selected all English-language, human subject, data-driven papers on hippocampal volumetry, yielding a database of 423 records. Smaller hippocampal volumes have been reported in epilepsy, Alzheimer's disease, dementia, mild cognitive impairment, the aged, traumatic brain injury, cardiac arrest, Parkinson's disease, Huntington's disease, Cushing's disease, herpes simplex encephalitis, Turner's syndrome, Down's syndrome, survivors of low birth weight, schizophrenia, major depression, posttraumatic stress disorder, chronic alcoholism, borderline personality disorder, obsessive-compulsive disorder, and antisocial personality disorder. Significantly larger hippocampal volumes have been correlated with autism and children with fragile X syndrome. Preservation of hippocampal volume has been reported in congenital hyperplasia, children with fetal alcohol syndrome, anorexia nervosa, attention-deficit and hyperactivity disorder, bipolar disorder, and panic disorder. Possible mechanisms of hippocampal volume loss in neuropsychiatric disorders are discussed.

Intracarotid Amytal memory test and hippocampal magnetic resonance imaging volumetry: validity of the Wada test as an indicator of hippocampal integrity among candidates for epilepsy surgery

OBJECT

Intracarotid Amytal testing (the Wada test) has been used to lateralize language and identify patients who may be at risk for memory impairment after temporal lobectomy. The goal of this study was to determine the validity of the Wada test in the assessment of pathological conditions of the hippocampus among candidates for epilepsy surgery. The authors examined the correlation between the functional integrity of the hippocampus, measured using the Wada test, and quantitative measures of hippocampal pathology, determined by obtaining volumetric measurements of the hippocampus with the aid of magnetic resonance (MR) imaging.

METHODS

The authors reviewed the relationship between memory scores on the Wada test and hippocampal volumes measured on preoperative MR images in 76 patients who underwent anteromedial temporal lobectomy and amygdalohippocampectomy for the treatment of medically refractory temporal lobe epilepsy. The data were analyzed with respect to their usefulness in lateralizing the seizure focus and predicting the long-term postoperative memory outcome. Right and left hippocampal volume measurements did not correlate with one another (p > 0.1). Similarly, following a left carotid artery injection of Amytal the patients' right hemisphere memory was not significantly related to their left hemisphere memory on the Wada test (p'> 0.1). On the other hand, the patients' right hemisphere memory significantly correlated with their right hippocampal volume (r = 0.51; p < 0.001) and their left hemisphere memory significantly correlated with their left hippocampal volume (r = 0.51; p < 0.001). Both right and left hemisphere memory scores correlated with the hippocampal volumetry ratio (r = 0.47 and r = 0.45, respectively; both p < 0.001). Lateralization of a seizure focus based on hippocampal volumetry results was significantly related to lateralization based on the results of the Wada test (r = 0.49; p < 0.01). The disparity between the Wada memory scores on ipsilateral and contralateral sides was significantly and inversely related to the change in verbal memory following temporal lobectomy (r = -0.28; p < 0.02). The preoperative hippocampal volumetry ratio also significantly and inversely correlated with the change in verbal memory after surgery (r = -0.31; p < 0.01).

CONCLUSIONS

The Wada memory test may be a valuable method of measuring the functional integrity of the hippocampus. The systematic study of MR imaging-acquired morphological data and Wada-acquired neuropsychological data may increase our understanding of the location of material-specific memory and the selection of eligible candidates for epilepsy surgery.

Bilateral hippocampal volume predicts verbal memory function in temporal lobe epilepsy

The present study used quantitative volume estimates of the hippocampus based on structural magnetic resonance imaging (MRI) to predict memory performance of individuals with epilepsy of temporal lobe origin (TLE). Twenty individuals with TLE completed standardized neuropsychological tests and a quality of life inventory, and participated in a brain MRI protocol designed to obtain high-resolution images of the hippocampus. The combined volume of the left and right hippocampi was found to be the best predictor of objective verbal memory performance. This finding is consistent with the functional adequacy model of hippocampal function. In contrast, the asymmetry between right and left hippocampal volume was the best predictor of subjective ratings of cognitive functioning, which is consistent with the functional reserve model. The collective and complementary functions of the left and right hippocampi merit further exploration in prospective studies of memory function and TLE.

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.

Inferring Function from Structure: Relationship of Magnetic Resonance Imaging-Detected Hippocampal Abnormality and Memory Function in Epilepsy

Although temporal lobectomy is an effective alternative treatment for many patients with medication-resistant epilepsy, the risk of cognitive morbidity is not inconsequential. The ability to predict cognitive outcome is increasingly dependent on convergent information from multiple sources, including direct (e.g., Wada test) and indirect (e.g., psychometric testing) functional assessments along with magnetic resonance imaging studies that detect structural abnormalities. This brief review summarizes the relationship between imaging and function at baseline and predicting cognitive outcome following temporal lobectomy.

Inferring Function from Structure: Relationship of Magnetic Resonance Imaging-Detected Hippocampal Abnormality and Memory Function in Epilepsy

Although temporal lobectomy is an effective alternative treatment for many patients with medication-resistant epilepsy, the risk of cognitive morbidity is not inconsequential. The ability to predict cognitive outcome is increasingly dependent on convergent information from multiple sources, including direct (e.g., Wada test) and indirect (e.g., psychometric testing) functional assessments along with magnetic resonance imaging studies that detect structural abnormalities. This brief review summarizes the relationship between imaging and function at baseline and predicting cognitive outcome following temporal lobectomy.

Memory for events and their spatial context: models and experiments

The computational role of the hippocampus in memory has been characterized as: (i) an index to disparate neocortical storage sites; (ii) a time-limited store supporting neocortical long-term memory; and (iii) a content-addressable associative memory. These ideas are reviewed and related to several general aspects of episodic memory, including the differences between episodic, recognition and semantic memory, and whether hippocampal lesions differentially affect recent or remote memories. Some outstanding questions remain, such as: what characterizes episodic retrieval as opposed to other forms of read-out from memory; what triggers the storage of an event memory; and what are the neural mechanisms involved? To address these questions a neural-level model of the medial temporal and parietal roles in retrieval of the spatial context of an event is presented. This model combines the idea that retrieval of the rich context of real-life events is a central characteristic of episodic memory, and the idea that medial temporal allocentric representations are used in long-term storage while parietal egocentric representations are used to imagine, manipulate and re-experience the products of retrieval. The model is consistent with the known neural representation of spatial information in the brain, and provides an explanation for the involvement of Papez's circuit in both the representation of heading direction and in the recollection of episodic information. Two experiments relating to the model are briefly described. A functional neuroimaging study of memory for the spatial context of life-like events in virtual reality provides support for the model's functional localization. A neuropsychological experiment suggests that the hippocampus does store an allocentric representation of spatial locations.

A temporoparietal and prefrontal network for retrieving the spatial context of lifelike events

Virtual reality (VR) and event-related functional magnetic resonance imaging were used to study memory for the spatial context of controlled but lifelike events. Subjects received a set of objects from two different people in two different places within a VR environment. Memory for the objects, and for where and from whom they were received was tested by putting the subject back into a place in the company of a person and giving a paired forced choice of objects. In four conditions objects had to be chosen according to different criteria: which was received in that place, which was received from that person, which object was recognized, and which object was widest. Event-related functional magnetic resonance imaging was performed during testing to identify areas involved in retrieval of the spatial context of an event. A network of areas was identified consisting of a temporoparietal pathway running between the precuneus and parahippocampi via retrosplenial cortex and the parieto-occipital sulcus, left hippocampus, bilateral posterior parietal, dorsolateral, ventrolateral and anterior prefrontal cortices, and the anterior cingulate. Of these areas the parahippocampal, right posterior parietal, and posteriodorsal medial parietal areas were specifically involved in retrieval of spatial context compared to retrieval of nonspatial context. The posterior activations are consistent with a model of long-term storage of allocentric representations in medial temporal regions with translation to body-centered and head-centered representations computed in right posterior parietal cortex and buffered in the temporoparietal pathway so as to provide an imageable representation in the precuneus. Prefrontal activations are consistent with strategic retrieval processes, including those required to overcome the interference between the highly similar events.

Quantitative magnetic resonance imaging in consecutive patients evaluated for surgical treatment of temporal lobe epilepsy

We present the results of quantitative Magnetic Resonance Imaging (MRI) in 55 consecutively referred patients with clinical evidence of temporal lobe epilepsy (TLE). The Cavalieri method was used in combination with point counting to provide unbiased estimates of the volume of the left and right hippocampus, amygdala, temporal lobe, lateral ventricles and cerebral hemisphere, and pixel by pixel maps of the T2 relaxation time were computed for both central and anterior sections of the hippocampus. The 99th centiles of hippocampal volume, hippocampal volume asymmetry and T2 relaxation times in 20 control subjects provided limits which identified the presence of MTS. The results of the quantitative MRI were compared with the results of conventional diagnostic MRI, foramen ovale (FO) recording and the WADA test. Thirty-one patients were found to have unilateral MTS (17 left and 14 right) and 7 bilateral MTS. No evidence of MTS was detected in 16 patients. Of the 31 patients diagnosed with unilateral MTS on the basis of hippocampal volume and T2 measurement, 74% and 77% would respectively have received the same diagnosis on the basis of hippocampal volume and T2 measurements alone. In comparison to FO recording, quantitative MRI has a sensitivity of 55% and a specificity of 86%, while conventional diagnostic MRI has a sensitivity of 42% and a specificity of 80% for detection of MTS. Unilateral abnormalities were detected by FO recording in 30% cent of patients who appeared normal on quantitative MRI. WADA test results were available for 40 patients. The findings were consistent with quantitative MRI showing reduced memory function ipsilateral to unilateral MTS in 18 patients, but reduced memory function contralateral to unilateral MTS in two patients, and reduced memory function without MR abnormality in seven patients. WADA testing revealed unilateral memory impairments where MRI found bilateral pathology in 4 patients and in 4 patients in whom quantitative MRI detected unilateral MTS there was no evidence of reduced memory during WADA testing of the corresponding cerebral hemisphere. In the patients with unilateral right MTS a highly significant negative correlation (p = 0.0003) was observed between age of onset and the volume of the contralateral temporal lobe. Quantitative MR imaging of the hippocampus (i.e. volume and T2 measurement) is preferable to conventional radiological reporting for providing objective evidence of the presence of MTS on which to base the referral of patients for surgery, and since it has associated morbidity FO recording is now only being used in selected patients. Furthermore, stereology provides a convenient method for estimating the volume of other brain structures, which is relevant to obtaining a better understanding of the effects of laterality and age of onset of TLE.

Mesial temporal sclerosis and temporal lobe epilepsy: MR imaging deformation-based segmentation of the hippocampus in five patients

In five patients with mesial temporal sclerosis, the authors verified the precision and reproducibility of hippocampal segmentations with deformation-based magnetic resonance (MR) imaging. The overall percentage overlap between automated segmentations was 92.8% (SD, 3.5%), between manual segmentations was 73.1% (SD, 9.5%), and between automated and manual segmentations was 74.8% (SD, 10.3%). Deformation-based hippocampal segmentations provided a precise method of hippocampal volume measurement in this patient population.

Magnetic resonance imaging follow-up of progressive hippocampal changes in a mouse model of mesial temporal lobe epilepsy

PURPOSE

Hippocampal sclerosis (HS) is the most frequent lesion found in mesial temporal lobe epilepsy (mTLE). MR imaging is considered to be the most sensitive and specific method to detect HS. Despite extensive studies performed on humans and except in a recent study, the morphologic pattern of HS is usually analyzed when the disease has already fully developed, thus not allowing any insight into the mapping of the progressive morphologic changes inducing the development of mTLE. We have recently characterized a model of mTLE that reproduces the unilateral pattern of HS, induced by intrahippocampal injection of low doses of kainate (KA) in mice.

METHODS

In this study, we monitored the temporal evolution of the development of HS in this model of mTLE by using T2-weighted sequence, T2-relaxation time measurements, and T1-weighted spin-echo technique after injection of gadolinium, from 1 h to 120 days after KA injection.

RESULTS

HS induced by intrahippocampal KA injection occurred in two phases. First, we observed a transient hyperintense T2-weighted signal in the cortex above the injected hippocampus, most likely indicative of vasogenic edema partly due to the neurotoxic effect of KA. The concomitant increase in the T2 signal in the injected hippocampus and ipsilateral amygdala likely reflects the phase of cytotoxic edema occurring probably in relation to the excitotoxic consequences of both KA and seizure activity. Second, from 15 days on, a persistent unilateral increased T2 signal was detected in the hippocampus, which most probably reflects gliosis.

CONCLUSIONS

Our findings indicate that longitudinal follow-up would permit a better understanding of the mechanisms underlying the constitution of HS in humans and eventually development of prevention strategies.

Combined measurements of hippocampal N-acetyl-aspartate and T2 relaxation time in the evaluation of mesial temporal lobe epilepsy: correlation with clinical severity and memory performances

PURPOSE

In this study we tried to find a correlation between the clinical severity and memory performances, by comparing proton magnetic resonance (MR) spectroscopy and T2 relaxation time measurements in the hippocampi, in a homogeneous group of 27 patients with unilateral mesial temporal lobe epilepsy with ipsilateral hippocampal sclerosis on MR imaging, with a view to answer the following questions: (a) how sensitive is this approach for the assessment of the apparently normal contralateral hippocampus, (b) do the results relate to the clinical severity, and (c) does it allow evaluation of the degree of hippocampal dysfunction.

METHODS

Volume-selective proton MR spectroscopy of the head of both hippocampi was performed at 3 T, by using the PRESS sequence, with an echo time of 135 ms, to estimate NAA/(Cho + Cr) ratios. The relaxation times were measured at 0.28 T, by using a conventional Carr-Purcell-Meiboom-Gill sequence, with a repetition time of 2,000 ms, an echo time of 15 ms, and 48 echoes.

RESULTS

The combination of NAA/(Cho + Cr) ratio and T2 relaxation time values was allowed to classify contralateral hippocampus abnormalities in two groups: first, decreased NAA/(Cho + Cr) ratio with strongly increased T2 relaxation time values corresponding to abnormalities observed in sclerotic ipsilateral hippocampi; and second, decreased NAA/(Cho + Cr) ratio with normal or slightly increased T2 relaxation time values. Whereas the NAA/(Cho + Cr) ratio or T2 relaxation time value alone was not correlated with memory performances, their association shows that left hippocampal injury evaluated both by NAA and T2 relaxation time measurements was clearly correlated with verbal memory scores, and right hippocampal injury, with visual memory scores. On the other hand, the maximal seizure frequency reported by the patients was correlated with ipsilateral NAA/(Cho + Cr) ratio and T2 relaxation time values but not with contralateral results.

CONCLUSIONS

We showed that the combination of NAA and T2 relaxation time measurements can be used to examine the degree of ipsi- and contralateral hippocampal dysfunction or injuries and their relations with memory performances in the presurgical evaluation of patients.