Disproportion in the distribution of gray and white matter: neuropsychological correlates

Epilepsy as a Disease of White Matter

White Matter Abnormalities Across Different Epilepsy Syndromes in Adults: An ENIGMA-Epilepsy Study

Hatton SN, Huynh KH, Bonilha L, et al. Brain. 2020;143(8):2454-2473. doi:10.1093/brain/awaa200. PMID: 32814957

The epilepsies are commonly accompanied by widespread abnormalities in cerebral white matter. ENIGMA-Epilepsy is a large quantitative brain imaging consortium, aggregating data to investigate patterns of neuroimaging abnormalities in common epilepsy syndromes, including temporal lobe epilepsy, extratemporal epilepsy, and genetic generalized epilepsy. Our goal was to rank the most robust white matter microstructural differences across and within syndromes in a multicenter sample of adult epilepsy patients. Diffusion-weighted magnetic resonance imaging (MRI) data were analyzed from 1069 healthy controls and 1249 patients: temporal lobe epilepsy with hippocampal sclerosis (n = 599), temporal lobe epilepsy with normal MRI (n = 275), genetic generalized epilepsy (n = 182), and nonlesional extratemporal epilepsy (n = 193). A harmonized protocol using tract-based spatial statistics was used to derive skeletonized maps of fractional anisotropy and mean diffusivity for each participant, and fiber tracts were segmented using a diffusion MRI atlas. Data were harmonized to correct for scanner-specific variations in diffusion measures using a batch-effect correction tool (ComBat). Analyses of covariance, adjusting for age and sex, examined differences between each epilepsy syndrome and controls for each white matter tract (Bonferroni corrected at P < .001). Across “all epilepsies” lower fractional anisotropy was observed in most fiber tracts with small to medium effect sizes, especially in the corpus callosum, cingulum, and external capsule. There were also less robust increases in mean diffusivity. Syndrome-specific fractional anisotropy and mean diffusivity differences were most pronounced in patients with hippocampal sclerosis in the ipsilateral parahippocampal cingulum and external capsule, with smaller effects across most other tracts. Individuals with temporal lobe epilepsy and normal MRI showed a similar pattern of greater ipsilateral than contralateral abnormalities, but less marked than those in patients with hippocampal sclerosis. Patients with generalized and extratemporal epilepsies had pronounced reductions in fractional anisotropy in the corpus callosum, corona radiate, and external capsule, and increased mean diffusivity of the anterior corona radiata. Earlier age of seizure onset and longer disease duration were associated with a greater extent of diffusion abnormalities in patients with hippocampal sclerosis. We demonstrate microstructural abnormalities across major association, commissural, and projection fibers in a large multicenter study of epilepsy. Overall, patients with epilepsy showed white matter abnormalities in the corpus callosum, cingulum, and external capsule, with differing severity across epilepsy syndromes. These data further define the spectrum of white matter abnormalities in common epilepsy syndromes, yielding more detailed insights into pathological substrates that may explain cognitive and psychiatric comorbidities and be used to guide biomarker studies of treatment outcomes and/or genetic research.

Paradigm Shifts in the Neuropsychology of Epilepsy

This article reviews the major paradigm shifts that have occurred in the area of the application of clinical and experimental neuropsychology to epilepsy and epilepsy surgery since the founding of the International Neuropsychological Society. The five paradigm shifts discussed include: 1) The neurobiology of cognitive disorders in epilepsy - expanding the landscape of syndrome-specific neuropsychological impairment; 2) pathways to comorbidities: bidirectional relationships and their clinical implications; 3) discovering quality of life: The concept, its quantification and applicability; 4) outcomes of epilepsy surgery: challenging conventional wisdom; and 5) Iatrogenic effects of treatment: cognitive and behavioral effects of antiepilepsy drugs. For each area we characterize the status of knowledge, the key developments that have occurred, and how they have altered our understanding of the epilepsies and their management. We conclude with a brief overview of where we believe the field will be headed in the next decade which includes changes in assessment paradigms, moving from characterization of comorbidities to interventions; increasing development of new measures, terminology and classification; increasing interest in neurodegenerative proteins; transitioning from clinical seizure features to modifiable risk factors; and neurobehavioral phenotypes. Overall, enormous progress has been made over the lifespan of the INS with promise of ongoing improvements in understanding of the cognitive and behavioral complications of the epilepsies and their treatment. (JINS, 2017, 23, 791-805).

Future directions in the neuropsychology of epilepsy

Two important themes for future clinical research in the neuropsychology of epilepsy are proposed: (1) the neurobiological abnormalities that underlie neuropsychological impairment in people with epilepsy, and (2) neuropsychological status of persons with new-onset epilepsy.

The neurobiology of cognitive disorders in temporal lobe epilepsy

Cognitive impairment, particularly memory disruption, is a major complicating feature of epilepsy. This Review will begin with a focus on the problem of memory impairment in temporal lobe epilepsy (TLE). We present a brief overview of anatomical substrates of memory disorders in TLE, followed by a discussion of how our understanding of these disorders has been improved by studying the outcomes of anterior temporal lobectomy. The clinical efforts made to predict which patients are at greatest risk of experiencing adverse cognitive outcomes following epilepsy surgery are also considered. Finally, we examine the vastly changing view of TLE, including findings demonstrating that anatomical abnormalities extend far outside the temporal lobe, and that cognitive impairments extend beyond memory function. Linkage between these distributed cognitive and anatomical abnormalities point to a new understanding of the anatomical architecture of cognitive impairment in epilepsy. Clarifying the origin of these cognitive and anatomical abnormalities, their progression over time and, most importantly, methods for protecting cognitive and brain health in epilepsy, present a challenge to neurologists.

Epilepsy and cognition

It is well known that neuropsychological impairment can be associated with chronic epilepsy. This review suggests that a broad lifespan perspective of cognition in epilepsy should include consideration of: a) neurobiological factors that antedate the first seizure and influence cognition, b) epilepsy-related factors that influence brain growth and cognitive development after epilepsy is diagnosed and treated, c) clinical epilepsy and other risk factors associated with poor cognitive prognosis in the context of chronic pharmacoresistant epilepsy, and d) the modifiable and non-modifiable risk factors that influence cognitive aging in the general population.

The contribution of neuropsychology to diagnostic assessment in epilepsy

Neuropsychology plays a vital role in the treatment of epilepsy, providing information on the effects of seizures on higher cortical functions through the measurement of behavioral abilities and disabilities. This is accomplished through the design, administration and interpretation of neuropsychological tests, including those used in functional neuroimaging or cortical mapping and in intracarotid anesthetic procedures. The objective of this paper is to define and summarize in some detail the role and methods of neuropsychologists in specialized epilepsy centers. Included are information and recommendations regarding basic ingredients of a thorough neuropsychological assessment in the epilepsy setting, as well as suggestions for an abbreviated alternative exam when needed, with emphasis on functions associated with specific brain regions. The paper is intended for novice and experienced neuropsychologists to enable them to develop or evaluate their current practices, and also for other clinicians, who seek a better understanding of the methodology underlying the neuropsychological input to their work.

The emerging architecture of neuropsychological impairment in epilepsy

A new literature is now under way, one linking cognitive abnormalities directly to indices of structural, functional, metabolic, and other neurobiologic markers of cerebral integrity, independent of their association with clinical epilepsy characteristics. These trends are reviewed in this article. The focus is on temporal lobe epilepsy (TLE) as a model with which to address the core points because this form of localization-related epilepsy has been very carefully studied from both a cognitive and imaging standpoint. Some pertinent historical issues are touched on first, followed by more detailed reviews of the cognitive and neuroimaging abnormalities that have been found in TLE, followed by an overview of studies examining direct structure-function relationships in TLE and other epilepsies.

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.

Correlation of cognitive functions with voxel-based morphometry in patients with hippocampal sclerosis

Material-specific memory dysfunction is a feature of mesial temporal lobe epilepsy (mTLE) and has lateralizing potential. We used voxel-based morphometry (VBM) and partial regression analysis of whole-brain tissue class volumes to test if there are correlations between material-specific cognitive dysfunction and localized gray matter loss. In a cohort of 89 patients with mTLE and hippocampal sclerosis (HS), we found correlations between global gray matter and cerebrospinal fluid volume and cognitive test scores in the group with left HS. These findings, however, were poorly anatomically localized; no global changes were detected in the group with right HS. Thus, correlations between gray matter loss and cognitive dysfunction were present and suggested the involvement of widespread neural networks.

Regional neocortical thinning in mesial temporal lobe epilepsy

PURPOSE

To determine the nature and extent of regional cortical thinning in patients with mesial temporal lobe epilepsy (MTLE).

METHODS

High-resolution volumetric MRIs were obtained on 21 patients with MTLE and 21 controls. Mean cortical thickness was measured within regions of interest and point-by-point across the neocortex using cortical reconstruction and parcellation software.

RESULTS

Bilateral thinning was observed within frontal and lateral temporal regions in MTLE patients relative to controls. The most striking finding was bilateral cortical thinning in the precentral gyrus and immediately adjacent paracentral region and pars opercularis of the inferior frontal gyrus, extending to the orbital region. Within the temporal lobe, bilateral thinning was observed in Heschl's gyrus only. Ipsilateral only thinning was observed in the superior and middle temporal gyri, as well as in the medial orbital cortex. Greater asymmetries in cortical thickness were observed in medial temporal cortex in patients relative to controls. Individual subject analyses revealed that this asymmetry reflected significant ipsilateral thinning of medial temporal cortex in 33% of patients, whereas it reflected ipsilateral thickening in 20% of MTLEs.

DISCUSSION

Patients with MTLE show widespread, bilateral pathology in neocortical regions that is not appreciated on standard imaging. Future studies are needed that elucidate the clinical implications of neocortical thinning in MTLE.

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.

Predicting memory decline following epilepsy surgery: a multivariate approach

BACKGROUND

While some patients experience a decline in memory function following an anterior temporal lobe resection, there is considerable individual variation in the extent, nature, and direction of postoperative memory change. Patients with surgically remediable temporal lobe epilepsy differ in etiology, the extent and type of underlying pathology, and on demographic and epilepsy-related variables, all of which may have an impact on their pre- and postoperative neuropsychological functioning. This study examined the relationship between these variables and postoperative memory decline.

METHODS

Logistic regression was used to examine the effects of age, laterality of surgery, age of onset of epilepsy, underlying pathology and preoperative level of memory function on postoperative verbal learning in 288 patients who had undergone an anterior temporal lobe resection. One hundred twenty-five patients underwent a right temporal lobe resection (RTL), 163 patients underwent a left temporal lobe resection (LTL).

RESULTS

In the group as a whole, 25% of the patients demonstrated a significant postoperative deterioration in verbal learning. Postoperative deterioration in verbal learning was significantly associated with higher levels of preoperative function in both the RTL and LTL groups. Older age at the time of the operation and a lower verbal IQ were additional significant predictors for the RTL group. The presence of cortical dysgenesis was a significant predictor of postoperative decline in the LTL group. The logistic regression models accurately identified 3/4 of those who experienced a postoperative decline in memory, using a cutoff of 0.25 or above to identify high risk.

CONCLUSIONS

Our analyses suggest that the majority of patients with a high risk of significant postoperative memory decline can be reliably identified preoperatively. These models are valuable tools helping patients make an informed decision regarding surgery.

Quantitative MRI biomarkers of cognitive morbidity in temporal lobe epilepsy

PURPOSE

To determine the relation between neuropsychological morbidity, quantitative magnetic resonance imaging (MRI) measures of whole brain structure, and clinical seizure factors reflecting epilepsy cause, course, and treatment.

METHODS

Quantitative MRI measurements of total (whole brain) cerebrospinal fluid (CSF) and gray- and white-matter volumes and clinical seizure features were examined in relation to summary indices of cognitive morbidity in 96 patients with temporal lobe epilepsy. MRI volumes were adjusted for intracranial volume (ICV), and cognitive scores were adjusted for age, education, and gender, based on a sample of 82 healthy controls.

RESULTS

Whole-brain volumes (gray, white, and CSF) were abnormal in chronic temporal lobe epilepsy patients compared with controls and were related significantly to neuropsychological morbidity, especially total CSF. Statistical modeling demonstrated that markers of total atrophy (CSF) was the primary mediator of the relation between clinical seizure variables and neuropsychological morbidity.

CONCLUSIONS

Quantitative measurements of overall brain abnormality (atrophy) in temporal lobe epilepsy are clinically meaningful markers that are associated with increased cognitive morbidity. These biomarkers appear to mediate the adverse effects of some clinical seizure variables on cognition.

Contralateral medial temporal lobe damage in right but not left temporal lobe epilepsy: a (1)H magnetic resonance spectroscopy study

BACKGROUND

Proton magnetic resonance spectroscopy (MRS) of the hippocampus is useful in lateralising the epileptic focus in temporal lobe epilepsy for subsequent surgical resection. Previous studies have reported abnormal contralateral MRS values in up to 50% of the patients.

OBJECTIVE

To identify the contributing factors to contralateral damage, as determined by MRS, and its extension in patients with temporal lobe epilepsy.

METHODS

Single voxel MRS was carried out in the hippocampus and lateral temporal neocortex of both hemispheres in 13 patients with left temporal lobe epilepsy (LTLE) and 16 patients with right temporal lobe epilepsy (RTLE). All patients had mesial temporal lobe epilepsy with hippocampal sclerosis. Controls were 21 healthy volunteers of comparable age.

RESULTS

Consistent with previous studies, the NAA/(Cho+Cr) ratio was abnormally low in the hippocampus ipsilateral to the focus (p < 0.0001), and there were lower values in both patient groups in the ipsilateral temporal neocortex (p < 0.0001). Patients with RTLE had left hippocampal MRS anomalies (p = 0.0018), whereas the right hippocampus seemed to be undamaged in LTLE patients.

CONCLUSIONS

Unilateral mesial temporal lobe epilepsy is associated with widespread metabolic abnormalities which involve contralateral mesial and neocortical temporal lobe structures. These abnormalities appear to be more pronounced in patients with RTLE.

Efficacy of topiramate in migraine prophylaxis: a retrospective chart analysis

OBJECTIVE

Topiramate, a broad-spectrum anticonvulsant with multiple mechanisms of action, may be effective in preventing migraine headaches. We report the results of a retrospective chart review of patients treated with topiramate for prophylaxis of migraine.

METHODS

Patients with a diagnosis of migraine who had at least one follow-up visit after > or =4 weeks on topiramate were eligible; 69 patients (56 women and 13 men) aged 18 to 68 years met these criteria. Charts were reviewed for frequency of mild or moderate/severe headaches at the start of topiramate (baseline) and at all subsequent visits up to 24 weeks.

RESULTS

The 28-day frequency of moderate/severe migraines declined significantly from baseline to end of treatment (10.6 +/- 8.4 to 7.4 +/- 7.7, respectively; P = 0.0004), whereas that of mild headaches did not demonstrate a significant change (from 11.8 +/- 8.9 to 11.0 +/- 10.0). Among the 38 patients who had not responded previously to >or =9 preventive medications, the monthly frequency of moderate/severe (but not mild) headaches decreased significantly from baseline to end of treatment. Patients who previously had not responded to <9 prior medications experienced significant declines in both mild and moderate/severe headaches. The most common adverse events were paresthesias, drowsiness, diarrhea, decreased appetite, and weight loss. Twenty-seven patients discontinued topiramate therapy, 20 as a result of adverse events and 7 due to lack of response.

CONCLUSION

Topiramate may be effective in reducing the frequency of both mild and moderate/severe migraine headaches. In particular, topiramate may offer relief to patients with moderate/severe migraines who do not respond to other treatments.

Topiramate monotherapy in adults with newly diagnosed epilepsy

This 6-month open-label study evaluated the efficacy and safety of topiramate monotherapy for partial-onset or primarily generalized seizures in 22 adults with newly diagnosed epilepsy. The initial topiramate dose of 25 mg could be titrated to a maximum effective or maximum tolerated dose of 200 mg/d. Eighteen patients completed the study. The mean topiramate dose was 200 mg/d (range, 75-200 mg/d). Compared with baseline, monthly seizure frequency decreased 89% (median), with 17 patients (94%) achieving complete control. Five patients dropped out of the study. No changes in hematologic, renal, and hepatic function evaluations were noted, and only 1 patient experienced clinical signs significant enough to warrant discontinuation of treatment. In this small study, topiramate in doses ranging from 75 to 200 mg/d was effective and well tolerated as monotherapy in adults with newly diagnosed epilepsy.

Pediatric partial and generalized seizures

Eight new anticonvulsant medications have been approved in the United States since 1993, offering physicians a greater range of options for treating children with partial and generalized seizures. However, pediatric neurologists have been faced with limited pediatric pharmacokinetic and pharmacodynamic information. This article reviews the newer antiepilepsy drugs-gabapentin, felbamate, lamotrigine, topiramate, oxcarbazepine, levetiracetam, and zonisamide-and summarizes what is currently known about the safety and efficacy of these drugs in treating partial and generalized seizures in the pediatric population. Further studies are needed, however, to thoroughly evaluate their efficacy and safety in children.

Object naming and semantic knowledge in temporal lobe epilepsy

Object-naming impairment is common among temporal lobe epilepsy (TLE) patients, but other aspects of semantic memory have received limited attention in this population. This study examined object-naming ability and depth of semantic knowledge in healthy controls (n = 29) and patients with early onset TLE (n = 21). After administration of the Boston Naming Test (BNT), the authors asked participants to provide detailed definitions of 6 BNT objects. The TLE group demonstrated a significant deficit relative to controls in both object-naming ability and semantic knowledge for the target objects, even after controlling for IQ. In a multiple regression analysis that included other neuropsychological test scores as independent variables, the semantic knowledge score was the only significant predictor of patients' object-naming performance. Thus, at the group level, early onset TLE patients have a semantic knowledge deficit that contributes to dysnomia.

Surgery for malformations of cortical development causing epilepsy

Malformations of cortical development (MCD) are responsible for many cases of refractory epilepsy in adults and children. The results of surgical treatment are difficult to assess from the published literature. Judging from the limited number of adequately reported cases, approximately 40% of all cases of MCD treated surgically may be rendered seizure-free over a minimum 2-year follow-up period. This figure is the same for focal cortical dysplasia (FCD), the most common variety of MCD in surgical reports. In comparison with outcome for epilepsy associated with hippocampal sclerosis, this figure is low. Part of the difference may be artificial and related to limited reporting. Much of the difference is likely to relate to the complex underlying biology of MCD. Analysis of epileptogenesis in MCD has been undertaken. Different types of MCD have different sequelae. Some varieties are intrinsically epileptogenic; these include FCD and heterotopia. Although in most cases, the visualized MCD lies within the region of brain responsible for generating seizures (the epileptogenic zone), it may not constitute the entire epileptogenic zone in all cases. For polymicrogyria and schizencephaly in particular, the visualized abnormalities are probably not the most important component of the epileptogenic zone. There is evidence that the epileptogenic zone is spatially distributed and also, in some cases, temporally distributed. These findings may explain poor surgical outcome and the inadequacy of current presurgical evaluative methods. New preoperative techniques offer the opportunity of improved presurgical planning and selection of cases more likely to be rendered seizure-free by current surgical techniques. Of paramount importance is improved reporting. The establishment of a central registry may facilitate this aim. Specific recommendations are made for surgical strategies based on current experience and understanding.

Reduction of frontal neocortical grey matter associated with affective aggression in patients with temporal lobe epilepsy: an objective voxel by voxel analysis of automatically segmented MRI

BACKGROUND

Interictal episodes of aggression are often reported in patients with epilepsy. Some have characteristics of what has been referred to as episodic dyscontrol or intermittent explosive disorder (IED). Although structural brain abnormalities are thought to play a part in the pathophysiology of aggression, there are few in vivo studies of structural cerebral changes in patients with epilepsy and aggression. Using quantitative MRI, subtle structural brain abnormalities can be investigated in subgroups of patients with both epilepsy and episodes of affective aggression.

METHODS

After automated segmentation of cerebral grey matter from T1 weighted MRI, the objective technique of statistical parametric mapping (SPM) was applied to the analysis of 35 control subjects, 24 patients with temporal lobe epilepsy (TLE) with a history of repeated, interictal episodes of aggression, and 24 patients with TLE without episodes of aggression. Both TLE patient groups were compared with each other and with the control subjects on a voxel by voxel basis for increases and decreases of grey matter.

RESULTS

The patients with TLE with aggressive episodes had a decrease of grey matter, most markedly in the left frontal lobe, compared with the control group and with patients with TLE without aggressive episodes.

CONCLUSION

These findings suggest that a reduction of frontal neocortical grey matter might underly the pathophysiology of aggression in TLE. These voxel by voxel comparisons can guide further in vivo studies into aggression.