Memory and epilepsy: characteristics, course, and influence of drugs and surgery

Memory formation by refinement of neural representations: the inhibition hypothesis

There is no reasonable doubt that the hippocampus plays an important role in memory processing. A virtually uncountable number of studies in animals and humans have revealed changes in neural activity in this structure during memory formation [Squire LR. Memory and the hippocampus: a synthesis from findings with rats, monkeys, and humans. Psychol Rev 1992;99:195-231; Squire LR, Stark CE, Clark RE. The medial temporal lobe. Annu Rev Neurosci 2004;27:279-306], and hippocampal damage reliably leads to impairments in a large number of memory tests. However, while several correlates of successful memory formation have been found in the hippocampus, it is still an open question why specific neural processes support encoding of a particular item. An answer to this question would help to resolve current debates about which memory functions are actually supported by the hippocampus, and why activity in the neural networks of the hippocampus is involved in, or even necessary for, some memory processes but not for others. In this review, we first summarize findings on the electrophysiological activity within the hippocampus during different memory processes. We try to differentiate whether the hippocampus is merely involved in these processes, or whether the hippocampus appears to be necessary for them. Based on a distinction between a more general "encoding state" and the more specific process of "content-specific memory formation", we review data on neural representations within hippocampus and neocortex. We suggest that during memory formation, the hippocampus renders neural representations more sparse by providing an inhibitory signal to the neocortex.

Cognitive and magnetic resonance volumetric abnormalities in new-onset pediatric epilepsy

This paper addresses the issue of cognitive morbidity and abnormalities in quantitative MR volumetric in children with new and recent onset idiopathic epilepsy. The available literature suggests that mild diffuse cognitive problems are evident in children with new onset epilepsy in the context of intact whole brain and lobar volumetrics. Subsets of children can be identified with salient academic and volumetric abnormalities. These findings represent the baseline upon which any subsequent effects of chronic epilepsy may accrue.

Verbal cognition and attention deficits do not explain the verbal memory decline associated with pharmacoresistant partial epilepsy

The aim of this study was to explore whether change in verbal memory with time in patients with epilepsy is influenced by performance on tasks assessing verbal cognition or attention/processing speed. Thirty-six patients and twenty-five healthy controls were tested twice with median retest intervals of 4.8 and 3.1 years, respectively. Aspects of verbal memory, verbal cognition, and attention/processing speed were assessed. Decline in one verbal memory variable (Cronholm-Molander Memory Test Paired Associates -- Delayed Recall) was the strongest correlate of epilepsy. The second strongest correlate was a decrease in one attention/processing speed variable (Digit Symbol). The relationship between decline in verbal memory and epilepsy was not influenced by the decline in attention/processing speed, and the results did not support the notion that limited mental reserves as reflected in impaired verbal cognition or attention/processing speed can explain the relationship between verbal memory and epilepsy.

Cognitive decline in temporal lobe epilepsy due to unilateral hippocampal sclerosis

OBJECTIVE

We assessed the cognitive performance of patients with temporal lobe epilepsy (TLE) caused by unilateral hippocampal sclerosis (HS), in comparison with that of matched, healthy controls. We report the relationship between cognitive measures and duration of epilepsy, correlating with hippocampal volumes, and the impact of educational level on cognitive decline.

METHODS

This study involved 61 outpatients (40 with < or = 8 years and 21 with >8 years of formal education) with unilateral HS and 61 controls. Volumetric MRI was performed on all patients and 10 controls. The results (mean, SD) of the neuropsychological tests of healthy subjects and patients were compared using the Student t and Mann-Whitney tests.

RESULTS

Patients performed worse than controls in the neuropsychological evaluation. When adjusted z scores were used to calculate the impairment index, patients had a greater percentage of abnormal tests compared with controls. The cognitive decline, assessed through the impairment index, correlated with duration of epilepsy. Higher level of education did not protect against this decline, thus not supporting the hypothesis of cerebral reserve in this population. A significant correlation between hippocampal volumetric measures and duration of epilepsy was observed only in patients with left HS.

CONCLUSION

Patients with TLE caused by HS present with cognitive morbidity that extends beyond memory deficits. Cognitive decline is associated with duration of epilepsy, and in patients with left-sided HS, duration may correlate with volumetric hippocampal loss.

Is Basic Memory Structure Invariant Across Epilepsy Patient Subgroups?

It is well known that epilepsy patients often exhibit material-specific deficits in memory for verbal versus nonverbal material. However, it is also apparent that such deficits are not always discernible, and that numerous factors may potentially moderate the degree to which fractionation between verbal and nonverbal material is evident. Using confirmatory factor analysis, we compared the relative fit of one-factor (general) and two-factor (material-specific) models of memory in data from 330 patients with intractable seizure disorder. Data from verbal and nonverbal selective reminding tests, as well as Logical Memory and Visual Reproduction (immediate and delayed recall indices), were used in analyses. The one-factor model fit the data poorly in the full sample, whereas the two-factor model fit the data significantly better. We then assessed the invariance of these two models using multiple-group modeling across subsamples of patients with left versus right sided seizure focus, earlier versus later age of seizure onset, lower versus higher Full Scale IQ, fewer versus more years of education, younger versus older patients, and male versus female patients. In all cases, the two-factor model comprised of verbal and nonverbal memory factors fit the data better than a one-factor (general) model. These findings indicate robust consistency of verbal and nonverbal memory constructs in the epilepsy population, which remain viable in this patient group despite considerable heterogeneity in other respects.

Cognitive phenotypes in temporal lobe epilepsy

The objective of this study is to determine if distinct cognitive phenotypes could be identified in temporal lobe epilepsy. Epilepsy patients (n = 96) and healthy controls (n = 82) underwent comprehensive neuropsychological assessment. Adjusted (age, gender, and education) test scores for epilepsy subjects were grouped into cognitive domains (intelligence, language, visuoperception, immediate and delayed memory, executive function, and cognitive/psychomotor speed). Cluster analysis revealed three distinct cognitive profiles types: (1) minimally impaired (47% of subjects); (2) memory impaired (24%); and (3) memory, executive, and speed impaired (29%). The three cluster groups exhibited different patterns of results on demographic, clinical epilepsy, brain volumetrics, and cognitive course over a 4-year interval. The specific profile characteristics of the identified cognitive phenotypes are presented and their implications for the investigation of the neurobehavioral complications of epilepsy are discussed.

Neuropsychological assessment and quality of life in patients with refractory temporal lobe epilepsy related to hippocampal sclerosis

INTRODUCTION: Epilepsy is a disorder that results in abnormal activity in a group of neurons that may have significant impact on the normal cognitive processes and behavior. Temporal lobe epilepsy (TLE) is the most frequent form of partial epilepsy in adults, and hippocampal sclerosis (HS) the most common neuropathologic finding in patients with medically refractory TLE. Patients with TLE often present cognitive difficulties that may be determined by the effects of epileptic discharges and side of the lesion. And its consequence is that patients have poor effects on quality of life (QOL). PURPOSE: We report the relationship between neuropsychological assessment and QOL under the hypotheses that patients with worst results in neuropsychological assessment have poorer QOL assessed by the QOLIE-31. RESULTS: Regarding seizure frequency, 23 (46%) had had 1-5, 20 (40%) 6-10 and 7 (14%) more than 10 seizures/month. In relation to seizure types, 5 (10%) had had auras, 37 (74%) complex partial seizures and 8 (16%) partial evolving to generalized tonic-clonic seizures (GTCS). Neuropsychological evaluation had a positive correlation with QOLIE-31 domains. QOL evaluations had the worst scores in QOLIE-31 were in Cognitive Function (45.0) and Social Function (46.0). The best was Overall QOL (62.0). CONCLUSION: People with epilepsy have great impact on their QOL not only because of daily seizures but because of the impact seizures cause in their cognitive functions. TLE is an example of how refractory epilepsy can exterminate any possibilities of work, study and live in a society that discriminates someone with epilepsy who also presents cognitive decline.

Dichotic listening with forced attention in patients with temporal lobe epilepsy: Significance of left hemisphere cognitive dysfunction

Fifty right-handed patients with focal temporal lobe epilepsy were administered a dichotic listening test with consonant-vowel syllables under non-forced, forced right and forced left attention conditions, and a neuropsychological test battery. Dichotic listening performance was compared in subgroups with and without left hemisphere cognitive dysfunction, measured by the test battery, and in subgroups with left and right temporal epileptic focus. Left hemisphere cognitive dysfunction led to more correct responses to left ear stimuli in all three attention conditions, and fewer correct responses to right ear stimuli in the non-forced attention condition. This was probably caused by basic left hemisphere perceptual dysfunction. Dichotic listening was less affected by a left-sided epileptic focus than by left hemisphere cognitive dysfunction. General cognitive functioning influenced dichotic listening performance stronger in forced than in non-forced attention conditions. Larger cerebral networks were probably involved in the forced attention conditions due to the emphasis on conscious effort.

Frontoparietal activation during delayed visuospatial recall in patients with epilepsy due to hippocampal sclerosis

We hypothesized that brain activation during encoding and retrieval of visual material differed between epilepsy patients with hippocampal sclerosis (HS) and healthy controls. Eleven patients with epilepsy and HS and nine age- and education-matched control subjects were tested during functional MRI recording. A three-block design for visuospatial memory encoding and retrieval and an interference interval longer than 1 minute without memory tasks were used. All subjects revealed parietal, occipital, and prefrontal activation patterns during encoding. Interference revealed parietal more than occipital activation, whereas retrieval revealed asymmetrical frontal and parietal activation. Patients demonstrated a relative increase in occipitoparietal versus frontal cortical activation as compared with controls. Memory performance did not differ between patients and controls. The increased activation in occipitoparietal versus frontal areas in the patients suggests cortical reorganization of visuospatial recognition memory in epilepsy patients with HS. The study is limited by other factors that may contribute to the results, for example, antiepileptic drugs, effects of greater cognitive effort allocated in patients than controls, and possibly subclinical epileptic activity. However, normal visuospatial memory performance in our patients with HS suggests successful network plasticity.

Autobiographical amnesia and accelerated forgetting in transient epileptic amnesia

BACKGROUND

Recurrent brief isolated episodes of amnesia associated with epileptiform discharges on EEG recordings have been interpreted as a distinct entity termed transient epileptic amnesia (TEA). Patients with TEA often complain of autobiographical amnesia for recent and remote events, but show normal anterograde memory.

OBJECTIVE

To investigate (a) accelerated long term forgetting and (b) autobiographical memory in a group of patients with TEA.

METHODS

Seven patients with TEA and seven age matched controls were evaluated on a range of anterograde memory tasks in two sessions separated by 6 weeks and by the Galton-Crovitz test of cued autobiographical memory.

RESULTS

Patients with TEA showed abnormal long term forgetting of verbal material, with virtually no recall after 6 weeks. In addition, there was impaired recall of autobiographical memories from the time periods 1985-89 and 1990-94 but not from 1995-1999.

CONCLUSIONS

TEA is associated with accelerated loss of new information and impaired remote autobiographical memory. There are a number of possible explanations including ongoing subclinical ictal activity, medial temporal lobe damage as a result of seizure, or subtle ischaemic pathology. Future analyses should seek to clarify the relationship between aetiology, seizure frequency, and degree of memory impairment.

ApoE epsilon4 allele and disease duration affect verbal learning in mild temporal lobe epilepsy

PURPOSE

To clarify the possible role of other factors including the ApoE epsilon4 allele for memory decline in temporal lobe epilepsy (TLE).

METHODS

We conducted a neuropsychological and molecular study in 138 consecutive patients (78 female patients; mean age, 50.2 years, SD +/- 17.9; range, 14 to 87 years) with mild nonlesional TLE, who rarely or never had seizures at long-term follow-up. The mean age at seizure onset was 33.0 years (SD, +/-21.7), and the mean duration of epilepsy was 17.1 years (SD, +/-15.7).

RESULTS

Thirty-four (25%) of 138 patients had test scores indicating verbal learning deficit (VLD). The presence of an ApoE epsilon4 allele was associated with an increased risk of VLD (OR, 4.18; 95% CI, 1.66-10.55). The effect of the ApoE genotype was independent of both the age at epilepsy onset and disease duration as well as of a low educational level, which were separately associated with VLD (p values = 0.045, 0.001, and 0.001, respectively). A significant linear trend (p = 0.005) was seen in the relation between disease duration and cognitive impairment, with the highest risk being in patients with an epilepsy duration > or =25.5 years (OR, 7.06; 95% CI, 1.67-29.85), especially if they carried the epsilon4 allele (OR, 32.29; 95% CI, 5.23-195.72).

CONCLUSIONS

These results provide evidence for an alteration in cognitive performance as a function of the presence of the ApoE epsilon4 allele and point to the critical role of disease duration itself for cognitive impairment in TLE.

Depressed mood and memory impairment in temporal lobe epilepsy as a function of focus lateralization and localization

OBJECTIVE

Memory disorders and depressed mood are prominent psychological symptoms of temporal lobe epilepsies (TLEs). We examined the interaction of depressive mood and memory as a function of focus localization.

METHODS

One hundred fifty-two TLE patients with right mesial (n=37, RTLE-AHS), right lateral (n=31, RTLE-LAT), left mesial (n=42, LTLE-AHS), and left lateral (n=42, LTLE-LAT) lesions and epilepsies underwent comprehensive presurgical evaluation and neuropsychological assessment of mood and memory. Univariate and multivariate analyses of covariance (ANCOVAs) and partial correlation analyses were performed to reveal interactions of depression and memory as a function of focus localization.

RESULTS

No differences between the study groups were revealed for depression, indicating a general risk of 30% for depressed mood (BDI>12) in patients with TLE. ANCOVAs revealed significant main effects of focus side (left: verbal learning deficits; right: figural learning deficits) and site (mesial at disadvantage) on learning and memory scores. Correlation analyses revealed interactions between memory and mood only in LTLE-LAT patients.

CONCLUSIONS

Although the data provide evidence that side and site of the epileptogenic region differentially affect material specific memory performance, there was no evidence of a specific temporal target region for depressive mood. In the majority of the patients, depressed mood and memory impairment appeared as independent rather than as related symptoms of TLE. In LTLE-LAT, however, mood was significantly related to verbal and figural memory performance. Epilepsy-driven pathological left temporofrontal circuits are discussed as a prerequisite for the coupling of mood disorders and memory impairment in this specific patient subgroup which is also known from the psychiatric major depression syndrome.

Persistent severe amnesia due to seizure recurrence after unilateral temporal lobectomy

Anterograde amnesia is a severely disabling state which has been reported as a consequence of bilateral mesiotemporal lesions in humans. In the present paper, recurrent epileptic seizures after temporal lobectomy are described as a rare cause of severe amnesia in two patients. Diffusion-weighted MRI in one patient showed cytotoxic edema during a nonconvulsive status epilepticus and subsequent progressive hippocampal atrophy within the following month. In the other patient, repeated conventional MRI revealed no structural abnormalities in the contralateral temporal lobe.

Mesial Temporal Lobe Epilepsy with Hippocampal Sclerosis

Experts discussed the definition, natural history, pathologic features, pathogenesis, electroclinical, neurophysiological, neuropsychological, structural and functional imaging findings, as well as surgical outcome in mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS). After a long-lasting consensus process the ILAE Commission Neurosurgery of epilepsy accepted the resulting conclusions as state-of-the art report on MTLE-HS. The majority of contributors considered MTLE-HS to represent a sufficient cluster of signs and symptoms to make up a syndromic diagnostic entity.

Encoding of emotional memories depends on amygdala and hippocampus and their interactions

We have studied patients with variable degrees of left hippocampal and amygdala pathology who performed a verbal encoding task during functional magnetic resonance imaging (fMRI) to assess the impact of pathology on emotional-memory performance and encoding-evoked activity. The severity of left hippocampal pathology predicted memory performance for neutral and emotional items alike, whereas the severity of amygdala pathology predicted memory performance for emotional items alone. Encoding-related hippocampal activity for successfully remembered emotional items correlated with the degree of left amygdala pathology. Conversely, amygdala-evoked activity with respect to subsequently remembered emotional items correlated with the degree of left hippocampal pathology. Our data indicate a reciprocal dependence between amygdala and hippocampus during the encoding of emotional memories.

Face and facial expression memory in temporal lobe epilepsy patients: preliminary results

Right temporal lobe structures are involved in face and facial expression processing and in mnestic functions. Face and facial expression memory was investigated in 15 patients with left (LTLE) and 18 patients with right (RTLE) temporal lobe epilepsy as well as 13 healthy controls. Pairs of pictures combining four faces and four emotions had to be matched according to face identity or facial expression. In the memory tasks, the two pictures of a pair were divided by a memory interval of 2000 milliseconds, whereas in the perception tasks (control condition) both pictures were presented simultaneously. RTLE patients had significantly lower scores than healthy controls in face memory. LTLE patients had significantly lower scores than healthy controls in face and facial expression memory. The data confirm impaired face memory in RTLE patients and show that LTLE patients display deficits in face as well as in facial expression memory. Results are discussed according to functional reorganization, memory strategies, perception performance, naming problems, and group characteristics.

Preserved verbal memory function in left medial temporal pathology involves reorganisation of function to right medial temporal lobe

The left hippocampus and related structures mediate verbal memory function. The mechanism underlying preserved verbal memory function in patients with left hippocampal damage is unknown. Temporal lobe epilepsy, a common disease, is frequently the consequence of a characteristic hippocampal pathology termed hippocampal sclerosis, which may also affect the amygdala. In this setting, mapping the sites of memory function is a vital component of planning for surgical treatment for epilepsy. Using event-related functional magnetic resonance imaging, we studied 24 right-handed nonamnesic patients with left hippocampal sclerosis and 12 normal controls, performing a verbal encoding task. The patients were subdivided into two groups according to presence or absence of additional left amygdala pathology. Analysis of the data employed a two-level random-effects design, examining the main effects of subsequent memory in each group, as well as the differences between the groups. Additional effects of emotionality of the remembered words were also examined. Verbal memory encoding involved activation of left hippocampus in normals, but was associated with reorganisation to right hippocampus and parahippocampal gyrus in the patients. The additional presence of left amygdala sclerosis resulted in reorganisation for encoding of emotional verbal material to right amygdala. Retained verbal memory function in the presence of left medial temporal lobe pathology is mediated by recruitment of a parallel system in the right hemisphere consistent with adaptive functional reorganisation. The findings indicate a high degree of plasticity in medial temporal lobe structures.

Memory function in childhood epilepsy syndromes

OBJECTIVE

Children with epilepsy are at risk of specific cognitive deficits. We aimed to compare and characterize the memory function of children with childhood absence epilepsy (CAE), frontal lobe epilepsy (FLE) and temporal lobe epilepsy (TLE).

METHODS

Epilepsy syndrome was identified by clinical data, seizure semiology, interictal and ictal electroencephalogram (EEG). Seventy children aged 6-18 years with CAE, FLE or TLE had neuropsychological assessment including memory function. After adjusting for epilepsy variables, neuropsychological results of the syndrome groups and normative data were compared.

RESULTS

Children from all three syndrome groups were at risk of memory difficulties. The duration of epilepsy correlated negatively with memory function. Children with TLE had the worst memory function, significantly lower in verbal memory tasks than children with CAE (P = 0.02) and children with FLE (P = 0.01). The performance of children with TLE was significantly below the normed mean across all verbal and most visual tasks. Compared to the normed means, children with FLE had results that were statistically lower in some verbal and visual tasks, and children with CAE were lower in two visual tasks only.

CONCLUSIONS

This study demonstrates memory dysfunction in three common childhood epilepsy syndromes. Children with TLE had the greatest impairment, children with FLE had memory difficulties not previously reported, and children with CAE had subtle memory deficits. Qualitative differences were also evident. Longer duration of intractable epilepsy was associated with reduced memory ability. Memory function and its potential impact on academic achievement are vital considerations when managing children with epilepsy.

Chronic epilepsy and cognition: A longitudinal study in temporal lobe epilepsy

It remains unclear whether uncontrolled epilepsy causes mental decline. This longitudinal study contrasts change of memory and nonmemory functions in 147 surgically and 102 medically treated patients with temporal lobe epilepsy. All participants were evaluated at baseline (T1) and after 2 to 10 years (T3). Surgical patients underwent additional testing 1 year postoperatively (T2). Data were analyzed on an individual and group level. Sixty-three percent of the surgical and 12% of the medically treated patients were seizure-free at T3. Fifty percent of the medically treated and 60% of the surgical patients showed significant memory decline at T3 with little change in nonmemory functions (difference not significant). Surgery anticipated the decline seen in the medically treated group and exceeded it when surgery was performed on the left, or if seizures continued postoperatively. Seizure-free surgical patients showed recovery of nonmemory functions at T2 (p < 0.001) and of memory functions at T3 (T3, p = 0.03). Multiple regression indicated retest interval, seizure control, and mental reserve capacity as predictors of performance changes. In addition, psychosocial outcome was better when seizures were controlled. In conclusion, chronic temporal lobe epilepsy is associated with progressive memory impairment. Surgery, particularly if unsuccessful, accelerates this decline. However, memory decline may be stopped and even reversed if seizures are fully controlled.

Effects of chronic epilepsy on declarative memory systems

Memory is systematically affected by temporal lobe epilepsy. Since surgery is a promising alternative to pharmacological treatment the questions which memory system is affected and what the long-term prognosis of memory is are more relevant than ever. We address these issues by cross-sectional and longitudinal analysis of memory performance in large series of patients with temporal lobe epilepsy (TLE). The findings indicate that episodic memory rather than semantic memory is impaired in TLE, in particular in TLE with mesial temporal pathology. With the exception that mesial functions appear increasingly affected by chronic non-mesial TLE, memory decline in TLE is not different from that observed in healthy control subjects. However, since patients perform poorer than controls at any age, normal senescence brings patients to mnesic disability at a younger age. Semantic memory seems unaffected by this process but early cortical lesions appear to interfere with knowledge acquisition. Longitudinal data come to a different conclusion regarding the contribution of epilepsy/seizures to memory decline. Conservative treatment is associated with significant decline in figural memory and 37% of the patients experience some memory decline in the long run. Surgery partly anticipates the decline observed with conservative treatment, but losses are most marked after left temporal lobe surgery. After surgery, quite stable memory or even late recovery from surgery is indicated. Leaving aside the surgical intervention, the data provide evidence that the longitudinal memory outcome in TLE is determined by seizure control, seizure severity, mental reserve capacities, and the retest interval. Thus early and efficient seizure control and the prevention of any cerebral damage from the beginning of epilepsy are demanded.

Imaging the medial temporal lobe: exploring new dimensions

Cognitive scientists have used developments in functional imaging to explore the role of the medial temporal lobe (MTL) in memory formation. Lesion studies have suggested that separate MTL subregions make distinct contributions to memory. Functional imaging of these distinct contributions, however, remains a challenge, because the proximity of the MTL substructures tests the spatial resolution limits of current techniques. Recent findings using electrophysiological measures of neural activity highlight the importance of using information from other imaging modalities. Integrating the different modalities of neuroimaging with lesion studies, and, further, combining modalities within experiments, will provide new insights into the function of MTL subregions.

Childhood epilepsy in relation to mental handicap and behavioural disorders

Epidemiological studies indicate that there is a high rate of mental retardation and behavioural problems in children with epilepsy. In some cases both the epilepsy and the mental retardation will have a common cause, such as a metabolic disorder or brain trauma. However, in other children, the epilepsy itself may cause either temporary or permanent learning problems. When permanent learning disability can be prevented it is important to treat the epilepsy early and effectively. Children with specific learning difficulties and memory problems can benefit greatly from appropriate management. There are many causes of behavioural disturbance in children with epilepsy. These causes include the epilepsy itself, treatment of the epilepsy, reactions to the epilepsy, associated brain damage/dysfunction and causes that are equally applicable to children who do not have epilepsy. Identifying the cause or causes in each child allows rational management to be provided. Antiepileptic treatment with medication or surgery can either improve the situation or make matters worse. The treatment should be tailored to the needs of the individual child. If surgery is required, there is a strong argument for performing this early in life, both to allow the greatest opportunity for brain plasticity and also to allow the child full benefit from the important developmental and educational years, without the problems that can be associated with the epilepsy. Skilled management of children with epilepsy who have mental retardation and/or behavioural problems can be very rewarding both for the family and for the professionals involved.