Cognitive dysfunction following subcortical infarction

Image Findings as Predictors of Fall Risk in Patients with Cerebrovascular Disease

This study examined computed tomography findings in patients with cerebrovascular disease and determined predictors for falls. Images of the head were divided into 13 regions, and the relationships between computed tomography findings and the presence or absence of falls were investigated. A total of 138 patients with cerebrovascular disease (66% men, aged 73.8 ± 9.6 years) were included. A comparison between the fall and non-fall groups revealed a significant difference in the total functional independence measure scores and imaging findings at admission. Logistic regression analysis showed that the thalamus (p < 0.001), periventricular lucency (p < 0.001), lateral hemisphere room enlargement (p < 0.05), and age (p < 0.05) were related to the presence or absence of falls. For the 42 patients with cerebral hemorrhage, the thalamus (p < 0.01), periventricular lucency (p < 0.05), lateral ventricle vicinity (p < 0.05), and posterior limb of the internal capsule (p < 0.05) were extracted as factors related to the presence or absence of falls. For the 96 patients with cerebral infarction, the thalamus (p < 0.001), periventricular lucency (p < 0.01), and anterior limb of the internal capsule (p < 0.05) were extracted as factors related to the presence or absence of falls. This study found a relationship between the thalamus, lateral ventricle enlargement, periventricular lucency, and falls. Fall prognosis can potentially be predicted from computed tomography findings at admission.

Validation of cerebral blood flow connectivity as imaging prognostic biomarker on subcortical stroke

Stroke is a major cause of vascular cognitive dysfunction, such as memory impairment. We aimed to explore the neural substrates underlying verbal memory impairment in subcortical stroke patients by the methods of voxel‐wise cerebral blood flow (CBF) and the functional covariance network (FCN). Sixty patients with chronic subcortical stroke and 60 normal controls (NCs) were recruited into this study. We used a three‐dimensional pseudo‐continuous arterial spin‐labeling imaging to measure alterations in CBF and FCNs. We mapped the overall CBF alterations in a voxel‐wise manner and compared CBF measurements using a two‐sample t test. Correlations between CBF and verbal memory were also investigated. Subsequently, we constructed FCNs by calculating the correlation between specific regions and all other voxels of a whole brain, separately within the two groups. Thereafter, by comparing differences of the FCN patterns between the patient and NC groups, we investigated the connection alterations within the FCN maps. The stroke patients showed verbal short‐term memory (VSTM) deficits compared to NCs. The patients exhibited decreased CBF in the ipsilesional insula and ventral sensorimotor network, and increased CBF in contralesional frontal cortical and subcortical regions (putamen and thalamus). Meanwhile, the CBF in the ipsilesional insula was positively correlated, and the contralesional frontal cortical was negativity correlated, with VSTM scores. Moreover we found that stroke patients exhibited disordered connection within FCNs compared to NCs. The study suggests that the underlying imaging biomarker of VSTM impairment in patients with subcortical stroke was associated with disconnection of the frontal lobe network.

Subcortical contributions to higher cognitive function in tumour patients undergoing awake craniotomy

Primary brain tumours often occur near eloquent regions, affecting language, motor and memory capacity, with awake mapping and tailored resection designed to preserve higher cognitive functioning. The effects of such tumours on subcortical structures, including the thalamus and basal ganglia, have been largely unexplored, in spite of the known importance of such structures to higher cognitive functioning. We sought to explore the effects of volume changes of subcortical structures on cognition, in 62 consecutive patients diagnosed with primary brain tumour and cavernous malformations, referred to our neurosurgical practice. We found right caudate to be highly predictive of intelligence, left pallidum of total neuropsychological function and right hippocampus of mood. Our study is the largest of its kind in exploring subcortical substrates of higher cognition in consecutive patients with brain tumours. This research supports prior literature, showing subcortical structures to be related to higher cognitive functioning, particularly measures of memory and executive functioning implicated in fronto-subcortical circuits. Furthermore, involvement of right mesial temporal structures in mood, further strengthens the central role of Papez circuit in emotional quality of cognition. Attention to subcortical integrity is likely to be important in discussing postsurgical cognitive outcome with patients and their families.

Regional Shape Abnormalities in Thalamus and Verbal Memory Impairment After Subcortical Infarction

Background. Subcortical infarcts can result in verbal memory impairment, but the potential underlying mechanisms remain unknown. Objective. We investigated the spatiotemporal deterioration patterns of brain structures in patients with subcortical infarction and identified the regions that contributed to verbal memory impairment. Methods. Cognitive assessment and structural magnetic resonance imaging were performed 1, 4, and 12 weeks after stroke onset in 28 left-hemisphere and 22 right-hemisphere stroke patients with subcortical infarction. Whole-brain volumetric analysis combined with a further-refined shape analysis was conducted to analyze longitudinal morphometric changes in brain structures and their relationship to verbal memory performance. Results. Between weeks 1 and 12, significant volume decreases in the ipsilesional basal ganglia, inferior white matter, and thalamus were found in the left-hemisphere stroke group. Among those 3 structures, only the change rate of the thalamus volume was significantly correlated with that in immediate recall. For the right-hemisphere stroke group, only the ipsilesional basal ganglia survived the week 1 to week 12 group comparison, but its change rate was not significantly correlated with the verbal memory change rate. Shape analysis of the thalamus revealed atrophies of the ipsilesional thalamic subregions connected to the prefrontal, temporal, and premotor cortices in the left-hemisphere stroke group and positive correlations between the rates of those atrophies and the change rate in immediate recall. Conclusions. Secondary damage to the thalamus, especially to the left subregions connected to specific cortices, may be associated with early verbal memory impairment following an acute subcortical infarct.

Regional structural impairments outside lesions are associated with verbal short-term memory deficits in chronic subcortical stroke

Background and Purpose

We aimed to explore the neural mechanisms of verbal short-term memory (VSTM) impairment in subcortical stroke by evaluating the contributions of lesion and remote grey matter volume (GMV) reduction.

Results

There was no significant correlation between lesions and VSTM. In stroke patients with left lesions, GMV reductions in the right middle frontal gyrus and in the left inferior frontal gyrus were positively correlated with VSTM impairment. In patients with right lesions, GMV reduction in the right dorsal posterior cingulate cortex was positively correlated with VSTM impairment.

Materials and Methods

Ninety-seven patients with chronic subcortical ischemic stroke and seventy-nine healthy controls underwent VSTM and structural MRI examinations. Voxel-based lesion symptom mapping was used to identify correlations between lesions and VSTM. Voxel-wise comparisons were used to identify brain regions with significant GMV reduction in patients with left and right lesions. These regions were used in correlation analyses between GMV and VSTM in each patient subgroup.

Conclusions

These findings suggest that VSTM impairment in subcortical stroke is associated with secondary regional structural damage in non-lesion regions, rather than with the lesion itself. Moreover, different neural substrates may underlie VSTM impairment in stroke patients with left and right lesions.

Pathology and pathogenesis of vascular cognitive impairment-a critical update

Vascular cognitive impairment (VCI) [vascular cognitive disorder (VCD), vascular dementia] describes a continuum of cognitive disorders ranging from mild cognitive impairment (MCI) to dementia, in which vascular brain injury involving regions important for memory, cognition and behavior plays an important role. Clinical diagnostic criteria show moderate sensitivity (ca 50%) and variable specificity (range 64-98%). In Western clinical series, VaD is suggested in 8-10% of cognitively impaired elderly subjects. Its prevalence in autopsy series varies from 0.03 to 58%, with means of 8 to 15% (in Japan 22-35%). Major types of sporadic VaD are multi-infarct encephalopathy, small vessel and strategic infarct type dementias, subcortical arteriosclerotic leukoencephalopathy (SAE) (Binswanger), multilacunar state, mixed cortico-subcortical type, granular cortical atrophy (rare), postischemic encephalopathy, and a mixture of cerebrovascular lesions (CVLs). They result from systemic, cardiac and local large or small vessel disease (SVD); their pathogenesis is multifactorial. Hereditary forms of VaD caused by gene mutations are rare. Cognitive decline is commonly associated with widespread small ischemic vascular lesions involving subcortical brain areas (basal ganglia and hemispheral white matter). The lesions affect neuronal networks involved in cognition, memory, and behavior (thalamo-cortical, striato-subfrontal, cortico-subcortical, limbic systems). CVLs often coexist with Alzheimer-type lesions and other pathologies; 25-80% of elderly demented show mixed pathologies. The lesion pattern of "pure" VaD differs from that in mixed dementia (AD + CVLs) suggesting different pathogenesis of both phenotypes. Minor CVLs, except for severe amyloid angiopathy, appear not essential for cognitive impairment in full-blown AD, while both mild AD-type pathology and SVD may interact synergistically in promoting dementia. However, in a large percentage of non-demented elderly individuals, both AD-related and vascular brain pathologies have been reported. Despite recent suggestions for staging and grading CVLs in specific brain areas, due to the high variability of CVLs associated with cognitive impairment, no validated neuropathological criteria are currently available for VaD and mixed dementia. Further clinico-pathological studies and harmonization of neuropathological procedures are needed to validate the diagnostic criteria for VaD and mixed dementia in order to clarify the impact of CVLs and other coexistent pathologies on cognitive impairment as a basis for further successful therapeutic options.

Event-related desynchronization of sensorimotor EEG rhythms in hemiparetic patients with acute stroke

Previous neuroimaging studies based on neurovascular coupling have shown that stroke affects both, strength and spatial extent of brain activation during upper limb movements. Here, we investigated the sub-second amplitude dynamics of a direct neuronal measure, i.e., event-related desynchronization (ERD) of EEG oscillations during finger movements, in patients with acute cortical and subcortical stroke. Acute cortical strokes were found to decrease the ERD of alpha oscillations for the affected pericentral sensorimotor areas compared to a control group. Within the cortical stroke group, the affected hemisphere showed a smaller alpha-ERD compared to the unaffected hemisphere when each was contralateral to the acting hand. Furthermore, when cortical stroke patients moved their paretic hand, the ipsilateral (i.e., contralesional) alpha-ERD was stronger than the contralateral (ipsilesional) ERD. Interestingly, the alpha-ERD amplitude in a hemisphere with a cortical stroke was relatively well preserved for non-paretic hand movements compared to alpha-ERD amplitude for paretic hand movements. This finding provides a new perspective for assessing the rehabilitative potential, which could be utilized through training of the still responsive cortical network, e.g., via enforced use of the paretic hand.

Mild cognitive impairment in stroke patients with ischemic cerebral small-vessel disease: a forerunner of vascular dementia?

Ischemic cerebral small-vessel disease accounts for a third of acute cerebral ischemic events and contributes to the development of cognitive decline and dementia. Cerebral small-vessel disease can be visualized on MRI studies as lacunar infarcts, white matter lesions and cerebral microbleeds. In general, the short-term prognosis of ischemic cerebral small-vessel disease compared with other stroke subtypes is more favorable, with almost negligible early mortality, absence of neuropsychological impairment and excellent neurological recovery. However, it has been shown recently that the proportion of dementia caused by small-vessel disease ranges from 36 to 67%. On the other hand, patients with a first-ever lacunar infarction present with cognitive impairment (mainly executive dysfunction) in more than half of cases and more than 55% of patients fulfil the criteria of mild cognitive impairment of the vascular type. Moreover, patients with small-vessel disease have an increase in the mid- and long-term risk of death, stroke recurrence and dementia. Neuropsychological abnormalities in small-vessel disease occurred more frequently than previously recognized. Ischemic cerebral small-vessel disease should be regarded as a potentially severe condition prodrome of subcortical vascular dementia rather than a relatively benign disorder.

Neuroimaging predictors of cognitive impairment in confluent white matter lesion: volumetric analyses of 99 brain regions

BACKGROUND

Although confluent white matter lesion (WML) is associated with cognitive impairment, the mechanism explaining this association is controversial. We aimed to investigate comprehensively the MRI predictors of cognitive impairment in confluent WML.

METHODS

Among 45 lacunar stroke patients who had confluent WML, we evaluated the association of executive function [Mattis Dementia Rating Scale - Initiation/Perseveration subscale (MDRS I/P)] and global cognition [Mini-Mental State Examination (MMSE)] with the volume of WML, measures of lacunes and microbleeds, and the volumes of 99 other specific brain regions.

RESULTS

Regression analyses showed that WML volume predicted performance on the MDRS I/P (beta = -0.34, p = 0.016) independent of age. Volumes of cortical gray matter (cGM; beta = 0.41, p = 0.003), the lateral fronto-orbital gyrus (beta = 0.38, p = 0.01), superior frontal gyrus (beta = 0.29, p = 0.04), lateral ventricle (beta = -0.30, p = 0.04), and posterior limb of the internal capsule (beta = 0.43, p = 0.002) predicted MDRS I/P performance independent of WML volume. Volumes of cGM, and the lateral fronto-orbital gyrus predicted MMSE performance as well.

CONCLUSION

Atrophy along the frontosubcortical pathways and cGM predict cognition in confluent WML independent of WML volume.

Japanese version of the Frontal Assessment Battery for dementia

The Frontal Assessment Battery (FAB) was developed as a short bedside cognitive and behavioral battery to assess frontal lobe functions. The purpose of this study was to evaluate the validity and reliability of a Japanese version of the FAB to measure cognitive dysfunction in patients with dementia. We studied 25 normal subjects and 105 patients with Alzheimer's disease, n=58, vascular dementia, n=24, and frontotemporal dementia, n=23. The neuropsychological test battery included the FAB, the Mini Mental State Examination (MMSE), a memory test, and the Wisconsin Card Sorting Test (Keio version: KWCST). Patients with dementia performed significantly more poorly than controls on all tests. The FAB showed a good correlation with other cognitive measures: MMSE (r=0.725), KWCST number of categories completed (r=0.654), KWCST number of perseveration errors (r=-0.484), and memory test (r=0.643). Patients with more severe Clinical Dementia Rating scores showed lower scores on the FAB. There was good inter-rater reliability (r=0.972), test-retest reliability (r=0.769), and internal consistency (Cronbach's coefficient alpha=0.715). The FAB is a valid and reliable screening test to evaluate cognitive dysfunction among patients with dementia.

Neuropathological evaluation of mixed dementia

Mixed dementia (MD) refers to a combination of definite Alzheimer disease (AD) and vascular encephalopathy, but the distinction between both disorders is controversial. For the diagnosis of MD the clinical/neuroimaging criteria of possible AD plus cerebrovascular disease (CVD) as separate entities are used, but causal relations between vascular brain lesions and dementia are unclear. We proposed the combination of autopsy-proven AD with multiple vascular or ischemic lesions with about 30-50 ml of infarcted/damaged brain tissue. The population-based prevalence of MD is unknown. In retrospective and prospective autopsy studies, it ranges from 2 to 58% with reasonable means of 6-12%. In a consecutive autopsy series of 1500 demented elderly subjects, 830 of which with clinically probable AD, in Vienna, Austria, 41.5 to 52.0% showed "pure" AD, 7% atypical AD, 16-20% AD plus cerebrovascular lesions, and 9% AD plus Lewy body pathology; MD was diagnosed in 4.6 and 2.4%, and "pure" vascular dementia (VaD) in 11 and 2.0%, respectively, while 16.3/6.1% were other dementing disorders, and 1% showed no specific pathology. Like the MRC-CFAS and other studies, this indicates frequent coexistence of AD with multiple cerebrovascular lesions in cognitively impaired patients. In both AD and VaD, vascular lesions frequently involved subcortical regions (basal ganglia, thalamus, hippocampus, and white matter) or were multiple microinfarcts, whereas in MD, large/hemispheral infarcts and multiple microinfarcts were more frequent, suggesting different pathogenic mechanisms. In early/mild AD, critically located small vascular lesions may induce/promote cognitive decline, but in full-blown AD they appear of minor importance. Discussion of the major pathogenic factors inducing AD, VaD and MD suggests synergistic relations between these disorders. However, currently available morphological criteria for AD and VaD are of limited value for the diagnosis of MD and generally accepted and validated histopathological criteria for the diagnosis of VaD and MD are currently not available. Therefore, more distinct and critically evaluated clinico-pathological criteria are warranted.

The enigma of vascular cognitive disorder and vascular dementia

The prevalence, morphology and pathogenesis of vascular dementia (VaD), recently termed vascular cognitive impairment, are a matter of discussion, and currently used clinical diagnostic criteria show moderate sensitivity (average 50%) and variable specificity (range 64-98%). In Western clinic-based series, VaD is suggested in 8-10% of cognitively impaired aged subjects. Its prevalence in autopsy series varies from 0.03 to 58%, with reasonable values of 8-15%, while in Japan it is seen in 22-35%. Neuropathologic changes associated with cognitive impairment include multifocal and/or diffuse disease and focal lesions: multi-infarct encephalopathy, white matter lesions or arteriosclerotic subcortical (leuko)encephalopathy, multilacunar state, mixed cortico-subcortical type, borderline/watershed lesions, rare granular cortical atrophy, post-ischemic encephalopathy and hippocampal sclerosis. They result from systemic, cardiac and local large or small vessel disease. Recent data indicate that cognitive decline is commonly associated with widespread small ischemic/vascular lesions (microinfarcts, lacunes) throughout the brain with predominant involvement of subcortical and functionally important brain areas. Their pathogenesis is multifactorial, and their pathophysiology affects neuronal networks involved in cognition, memory, behavior and executive functioning. Vascular lesions often coexist with Alzheimer disease (AD) and other pathologies. Minor cerebrovascular lesions, except for severe amyloid angiopathy, appear not essential for cognitive decline in full-blown AD, while both mild Alzheimer pathology and small vessel disease may interact synergistically. The lesion pattern of "pure" VaD, related to arteriosclerosis and microangiopathies, differs from that in mixed-type dementia (AD with vascular encephalopathy), more often showing large infarcts, which suggests different pathogenesis of both types of lesions. Due to the high variability of cerebrovascular pathology and its causative factors, no validated neuropathologic criteria for VaD are available, and a large variability across laboratories still exists in the procedures for morphologic examination and histology techniques.

Mild cognitive impairment after lacunar infarction: voxel-based morphometry and neuropsychological assessment

BACKGROUND

The aim of the present study was to investigate whether there were differences in neuroradiological features, including white-matter lesions and gray-matter volumes, between patients with lacunar infarction with and without mild cognitive impairment of the vascular type (MCI-V).

METHODS

A total of 40 patients with lacunar infarction were studied within 1 month after stroke.

RESULTS

MCI-V was found in 22 patients, who in comparison with patients without cognitive impairment were significantly older and had fewer years of formal education. MRI subcortical hyperintensities especially in the basal ganglia (putamen and thalamus) were significantly more frequent in the MCI-V group. In the voxel-based morphometric study, patients with MCI-V showed more atrophy bilaterally in the middle temporal gyrus, right and left frontal and posterior bilateral occipitoparietal regions including the posterior cingulate as well as in the cerebellum. A region of interest analysis restricted to the parahippocampi and hippocampi showed further reduced bilateral parahippocampal gyrus and right hippocampus volume reductions in this group of patients. Finally, the amount of white-matter lesions among MCI-V showed negative correlations with gray-matter volume in frontal and temporal areas as well as with the thalamus and mesencephalon.

CONCLUSIONS

The present findings provide support for an anatomical substrate of the MCI entity in patients with lacunar infarction. Both gray- and white-matter changes seem to contribute to the cognitive impairment of such patients.

Poststroke memory function in nondemented patients: a systematic review on frequency and neuroimaging correlates

BACKGROUND AND PURPOSE

Poststroke memory dysfunction is a prerequisite for the diagnosis of poststroke dementia. This diagnosis is made within months after a stroke, apparently assuming a relatively stable course of the poststroke memory function. Clinical experience added to anecdotal evidence from the literature suggests that poststroke memory function may be reversible. The aim of the present study was to systematically review the available data on the time course of poststroke memory function in nondemented stroke survivors. In addition, we wanted to investigate the role of (pre-)stroke characteristics on poststroke memory function.

METHODS

We performed systematic literature search of PubMed with the following medical subject heading terms: memory and stroke. The search strategy yielded 798 articles of which 65 fulfilled our inclusion criteria and went on to the data extraction stage.

RESULTS

Five studies reported the prevalence of poststroke memory dysfunction at different poststroke intervals. The prevalence of poststroke memory dysfunction varied from 23% to 55% 3 months poststroke, which declined from 11% to 31% 1 year poststroke. Larger stroke volume, prestroke medial temporal lobe atrophy, and white matter lesions were related with decreased poststroke memory function.

CONCLUSIONS

Not all patients with poststroke memory dysfunction 3 months after a stroke had memory dysfunction 1 year poststroke. Consequently, not all criteria for the dementia diagnosis were fulfilled any more. This may indicate that poststroke dementia may be reversible in a substantial proportion of patients with stroke. Preferably, standardized reassessment of cognitive function should be performed in each patient diagnosed with poststroke dementia.

Vascular cognitive impairment

Vascular cognitive impairment encompasses a spectrum of clinically defined syndromes ranging from vascular cognitive impairment-no dementia, to vascular dementia. The underlying cerebrovascular pathology includes both overt infarction as well as rarefaction of gray and white matter. Alzheimer's pathology may coexist with vascular pathology. Diagnosis rests on identifying acquired cognitive impairment in the setting of documented cerebrovascular disease, based on clinical presentation and neuroimaging; MRI is more sensitive than CT. The course can be stepwise or gradually progressive. The clinical picture is typically dominated by deficits in executive function rather than the short-term memory deficit typical of Alzheimer's disease. No specific therapies exist, but treatment with anticholinesterase agents and N-methyl-d-aspartate antagonists may result in clinical improvement. Prevention remains paramount, with early recognition of populations at risk and early and aggressive management of risk factors, including hypertension, dyslipidemia, diabetes, and tobacco use as well as antithrombotic therapy, in appropriate populations.

Pre-existing hypertension and the impact of stroke on cognitive function

Hypertension has been associated with subclinical injury in the brain and may therefore increase the impact of an incident stroke on cognitive function. The Study of Osteoporotic Fractures (SOF) is a prospective, observational study of 9,704 women aged 65 years and older recruited from four U.S. metropolitan areas. Blood pressure was measured at study entry, and cognitive decline was defined by the change from prestroke to poststroke cognitive testing. During an average follow-up of 6.8 years, incident stroke occurred in 260 participants (3.1%) who had previously completed baseline cognitive testing. Among participants with stroke, 119 completed follow-up cognitive testing a median of 1.9 years after the stroke, 80 died before the next study visit, and 61 did not complete further cognitive testing. After adjustment for demographic factors and other confounders, pre-existing hypertension was a strong predictor of cognitive decline when a stroke occurred (odds ratio [OR], 4.07; 95% confidence interval [CI], 1.37-12.1). In contrast, hypertension was only weakly associated with cognitive decline in the absence of stroke (OR, 1.13; 95% CI, 1.04-1.22) (p for interaction = 0.032). Pre-existing hypertension in women is associated with a greater impact of stroke on cognitive function, possibly by influencing the ability to tolerate or recover from brain injury.

White matter lesions are associated with cortical atrophy more than entorhinal and hippocampal atrophy

The goal of this study was to examine the relationship between subcortical vascular disease and brain atrophy in patients with Alzheimer's disease (AD) and mixed dementia (i.e., AD and subcortical vascular disease together). MRI was performed on 77 cognitively normal (CN) subjects, 50 AD and 13 mixed dementia patients. Subcortical vascular disease was determined by white matter hyperintensities (WMH) volume and presence of subcortical lacunes. Brain atrophy was measured using total brain cortical gray matter (CGM), entorhinal cortex (ERC) and hippocampal volumes. CGM volume, but not ERC or hippocampal volume was inversely related to WMH volume in patients and controls. In contrast, no relationship was detected between CGM, ERC, or hippocampal volumes and subcortical lacunes. Furthermore, no interaction was found between WMH and diagnosis on cortical atrophy, implying that WMH affect cortical atrophy indifferently of group. These results suggest that subcortical vascular disease, manifested as WMH, may affect cortical atrophy more than ERC and hippocampal atrophy. Further, AD pathology and subcortical vascular disease may independently affect cortical atrophy.

Understanding the pathology of vascular cognitive impairment

The prevalence, morphology and pathogenesis of vascular dementia (VaD), recently termed vascular cognitive impairment (VCI), and of mixed dementia (Alzheimer disease+vascular encephalopathy) are a matter of discussion and no validated neuropathologic criteria for these disorders are currently available. In Western memory clinic-based series, VaD/CVI is suggested in 8-10% of cognitively impaired elderly; its prevalence in autopsy series ranges from 0.03% to 58% (mean 5-15%). Fairly unusual as an isolated nosological entity, CVI appears to correlate with focal, multifocal or diffuse cortical and/or subcortical microinfarcts and lacunes often affecting strategically important brain areas (thalamus, frontobasal, limbic system), hemispheric white matter and, less often, large brain areas. They result from systemic, cardiac or local large or small vessel disease. The lesion pattern in "pure" VCI with predominant multiple small (subcortical) lesions related to microangiopathies differs from that in "mixed dementia" (AD+VaD), more often associated with large infarcts, suggesting different pathogenesis. In very old subjects, selective hippocampal sclerosis may be accompanied by multiple other vascular pathologies. Minor cerebrovascular lesions (CVL), except for severe amyloid angiopathy, appear not essential for cognitive decline in full-blown AD, while both mild AD-type pathology and small vessel disease may interact synergistically in "unmasking" or promoting dementia. AD pathology is significantly less severe in the presence of cerebrovascular lesions. Further studies are needed to validate diagnostic criteria for VCI and to clarify the impact of vascular lesions on cognitive impairment.

Cerebral small vessel disease: how does it progress?

Small vessel disease (SVD), or microangiopathy, of the cerebral white and central grey matter is an important subtype of vascular dementia (VD). SVD-dementia is characterised by a "dysexecutive" type of cognitive impairment, neurological deficits including imbalance and voiding dysfunction, and emotional disturbances. SVD is also frequent among clinically healthy subjects and patients with mild cognitive impairment. It is easily visualised by imaging techniques, but difficult to distinguish from mixed SVD/Alzheimer Disease. SVD has an inherent tendency to progress, but data on its natural course are sparse, and there are almost no drug trials dedicated to it. This article reviews the evidence on the speed and predictors of progression of SVD in regard to cognitive deficits, functional decline and white matter lesions, as derived from epidemiological, clinical and imaging studies and the placebo branches of VD drug trials. Based on the available data, we make suggestions for future research and outcome measures.

Frontal lobe dementia and subcortical vascular dementia: a neuropsychological comparison

We compared the performance of 40 patients with frontal lobe dementia to that of 40 patients with subcortical vascular dementia (80 patients including, 46 men and 34 women) in a set of tasks assessing attentional, executive, and behavioural tasks. The frontal lobe dementia represents an important cause for degenerative disruption and is increasingly recognised as an important form (up to 25%) of degenerative dementia among individuals of late-middle-age. The main involvement is the frontal-subcortical pathway, which is the final target of impairment even in subcortical vascular dementia. A wider involvement of the cortical (decisional) layers in frontal dementia, in contrast with the prominent and widespread involvement of the subcortical pathways (refinement and corrections programs) creates the different profiles of the two groups. Frontal patients have more difficulties in abstract reasoning, focusing attention, and implementing strategies to solve problems. They exhibit more profound behavioural alterations in personality and social conduct and show only moderate depression, and a total lack of insight concerning their dinical condition. In contrast, the patients with subcortical vascular dementia have poor general cognitive functions, high insight, and important depression and apathy as the principal and most salient characteristic of their behavioral conduct.

Frontal lobe dysfunction caused by multiple lacunar infarction in community-dwelling elderly subjects

We examined the factors that influence frontal lobe function among 119 community-dwelling elderly subjects, based on magnetic resonance imaging (MRI) and clinical findings. We interviewed the subjects, and conducted a neurological examination, electrocardiogram, blood test, brain MRI examination, and cognitive function tests. The modified Stroop test and a personal computer version of the Wisconsin Card Sorting Test (WCST) were used to evaluate frontal lobe function. The subjects with impaired frontal lobe function defined by the modified Stroop test were significantly older, had more lacunar infarcts, and had lower HDL cholesterol values based on a logistic regression model. Among the aged who appear apparently normal, multiple lacunar infarcts are the cause of latent frontal lobe dysfunction.

The role of cerebrovascular disease in dementia

BACKGROUND

Improvements in health care over the last 50 years have lengthened average life expectancy significantly, resulting in considerable growth of the population over 65 years of age. With increased age, however, comes an increased risk for Alzheimer's disease (AD), and the prevalence of AD is predicted to reach epidemic proportions by the later half of the 21st century. The prevalence of cerebrovascular disease also increases with age, and recent evidence suggests that cerebrovascular risk factors such as hypertension and hypercholesterolemia also increase an individual's risk for AD, suggesting a potential interaction between these two very common disorders. The potential impact of cerebrovascular disease on general cognitive health is not yet well understood, but is now being actively explored and clarified.

REVIEW SUMMARY

Cerebrovascular disease may manifest itself in many ways, and this review begins by discussing the possible spectrum of brain injury associated with common cerebrovascular risk factors. The prominent role of brain imaging to detect clinically silent cerebrovascular disease is recognized and reviewed. The neuropsychological consequences of cerebrovascular disease across the cognitive spectrum is also reviewed, including potential mechanisms by which cerebrovascular disease may interact with AD to increase the expression or hasten the progression of dementia.

CONCLUSIONS

Cerebrovascular risk factors, common to the elderly, lead to pernicious brain injury and subtle cognitive impairment that most probably places the individual at greater lifetime risk for dementia. The cause of dementia among individuals with cerebrovascular disease, however, remains AD. Recognition of the potential role of cerebrovascular disease as an independent risk factor for AD offers the possibility of primary prevention through treatment of well-recognized risk factors and deserves further study. In the meantime, clinicians presented with an individual suffering from a slowly progressive dementia and findings of clinically silent cerebrovascular brain injury should recognize the potential role of cerebrovascular disease in the dementia process but not ignore the likely overwhelming effects of AD and treat appropriately.

Epidemiology: identifying vascular cognitive impairment

There has been a move in recent years to recognize that the most effective treatment for vascular dementia, and for the mixed component of mixed vascular dementia and Alzheimer's disease, lies not in treatment but in prevention. This requires that cases be identified before the onset of vascular damage (a stage termed "brain-at-risk") or, failing this, as soon as possible but certainly before dementia has developed. These early stages are termed vascular cognitive impairment (VCI). No criteria exist for this early stage of cognitive loss due to cerebrovascular disease and relatively little data exist to indicate how such cases might be identified. The data that do exist suggest that many of the traditional "vascular" features of sudden onset and stepwise progression, etc., are not common in VCI and new criteria will be needed to identify cases. This paper summarizes the data that describe the clinical, neuropsychological, and radiological features that are to be expected in VCI.

Subcortical vascular disease and functional decline: a 6-year predictor study

OBJECTIVES

To identify predictors of activity of daily living (ADL) and instrumental activity of daily living (IADL) decline in a population with subcortical vascular dementia (SVD) and to evaluate potential mechanism of decline.

DESIGN

Longitudinal.

SETTING

Hospital-based.

PARTICIPANTS

Computed tomographic (CT) scanning identified 77 participants as having subcortical infarction.

MEASUREMENTS

Participants were neurologically, neuropsychologically, behaviorally, and functionally assessed four times over 5.82 years. Baseline data were grouped into four modules: basic demographic and risk factor, CT scan, neurological and other clinical, and neuropsychological and behavioral. Multivariate analysis determined predictors of decline in ADLs and IADLs.

RESULTS

Predictors of ADL decline were age, alcohol consumption, coordination, snout reflex, and performance on a neuropsychological test (Block Design). Predictors of IADL decline were predominantly cognitive and included the presence of paratonia and performance on the two neuropsychological tests (attention and memory tasks).

CONCLUSION

These findings suggest that cognitive impairments are most likely to have an effect on IADL function, because the skills involved are complex and involve integrative activity, whereas physical and cognitive impairments combined are likely to compromise ADL function, given the more basic and physical nature of the functions involved. These findings indicate that in people with SVD, both ADL and IADL status should be monitored, because, for many, decline in function over time is likely, and thus the provision of appropriate support required.

The concept of vascular cognitive impairment

Vascular dementia (VaD) is increasingly recognised to reflect an outmoded concept in that it identifies cases too late for preventive therapy to have an opportunity to prevent the development of dementia and uses a cognitive paradigm inappropriately based on Alzheimer's disease. A replacement is urgently required and a new concept, that of vascular cognitive impairment (VCI), has been proposed to meet this need. It is imperative that criteria for VCI are developed on the basis of knowledge and data rather than supposition and assumption, as was the case for VaD. This review details the state of knowledge that we have now reached concerning the fundamental points of severity and cognitive paradigm and also covers a number of other imaging-related essential points embracing atrophy, leukoaraiosis, infarct volume and infarct location. Finally, the increasingly important concept of mixed dementia (co-existent Alzheimer's disease and VCI) is discussed.

Preclinical syndromes predict dementia: the Sydney older persons study

OBJECTIVES

To identify if preclinical syndromes for Alzheimer's disease, vascular dementia, and Parkinson's disease and related dementias exist. Identification of dementia at early or even preclinical stages has important implications for treatment.

METHODS

A community dwelling sample of 647 subjects aged 75 and over at recruitment were followed up for a mean period of 3.19 years (range 2.61 to 4.51 years). Each subject was asked to participate in a medical assessment which included a standardised medical history examining both past and current health and medication usage; a neuropsychological battery (mini mental state examination, Reid memory test, verbal fluency, subsets of the Boston naming test and similarities, clock drawing and copied drawings) and physical examination. Preclinical syndromes for the three predominant dementias (Alzheimer's disease, vascular dementia and Parkinson's disease, and related dementias) and their combinations were defined using cognitive, motor, and vascular features. Their longitudinal outcome as defined by death and dementia incidence was examined.

RESULTS

Preclinical syndromes affected 55.7% (n=299) of subjects. Preclinical syndromes showed a trend for an increased odds of death (odds ratio 1.72, p=0.056) and a significantly increased odds of developing dementia (odds ratio 4.81, p<0.001). Preclinical syndromes were highly sensitive, detecting 52 of 58 (89.7%) incident dementias. Two hundred and sixteen of 268 (80.6%) preclinical subjects did not show dementia over the 3 year period (positive predictive value 19.4%). Subjects defined as having a combination of cognitive, extrapyramidal, and vascular features were at greatest risk of progressing to dementia.

CONCLUSIONS

Preclinical syndromes were sensitive and significant predictors of dementia. In view of their poor positive predictive value, the preclinical syndromes as defined in this study remain a research tool needing both definitional refinement and greater periods of observation. Multiple coexistent preclinical disorders resulted in a greater incidence of dementia, providing evidence for an additive role between multiple disorders.

Magnetic resonance imaging and magnetic resonance spectroscopy in dementias

This article reviews recent studies of magnetic resonance imaging and magnetic resonance spectroscopy in dementia, including Alzheimer's disease, frontotemporal dementia, dementia with Lewy bodies, idiopathic Parkinson's disease, Huntington's disease, and vascular dementia. Magnetic resonance imaging and magnetic resonance spectroscopy can detect structural alteration and biochemical abnormalities in the brain of demented subjects and may help in the differential diagnosis and early detection of affected individuals, monitoring disease progression, and evaluation of therapeutic effect.

Hippocampal and cortical atrophy predict dementia in subcortical ischemic vascular disease

BACKGROUND

The cause of dementia in subcortical ischemic vascular disease (SIVD) is controversial.

OBJECTIVES

To determine whether cognitive impairment in SIVD 1) correlates with measures of ischemic brain injury or brain atrophy, and/or 2) is due to concomitant AD.

METHODS

Volumetric MRI of the brain was performed in 1) elderly subjects with lacunes (L) and a spectrum of cognitive impairment-normal cognition (NC+L, n = 32), mild cognitive impairment (CI+L, n = 26), and dementia (D+L, n = 29); 2) a comparison group with probable AD (n = 28); and 3) a control group with normal cognition and no lacunes (NC). The authors examined the relationship between the severity of cognitive impairment and 1) volume, number, and location of lacunes; 2) volume of white matter signal hyperintensities (WMSH); and 3) measures of brain atrophy (i. e., hippocampal, cortical gray matter, and CSF volumes).

RESULTS

Among the three lacune groups, severity of cognitive impairment correlated with atrophy of the hippocampus and cortical gray matter, but not with any lacune measure. Although hippocampal atrophy was the best predictor of severity of cognitive impairment, there was evidence for a second, partially independent, atrophic process associated with ventricular dilation, cortical gray matter atrophy, and increase in WMSH. Eight autopsied SIVD cases showed variable severity of ischemic and neurofibrillary degeneration in the hippocampus, but no significant AD pathology in neocortex. The probable AD group gave evidence of only one atrophic process, reflected in the severity of hippocampal atrophy. Comparison of regional neocortical gray matter volumes showed sparing of the primary motor and visual cortices in the probable AD group, but relatively uniform atrophy in the D+L group.

CONCLUSIONS

Dementia in SIVD, as in AD, correlates best with hippocampal and cortical atrophy, rather than any measure of lacunes. In SIVD, unlike AD, there is evidence for partial independence between these two atrophic processes. Hippocampal atrophy may result from a mixture of ischemic and degenerative pathologies. The cause of diffuse cortical atrophy is not known, but may be partially indexed by the severity of WMSH.

Neuronal metabolic changes in the cortical region after subcortical infarction: a proton MR spectroscopy study

OBJECTIVES

To investigate whether proton magnetic resonance spectroscopy ((1)H MRS) can detect neuronal metabolic changes in the cortical region in patients with cortical dysfunction after subcortical infarction.

METHODS

Fifteen patients with subcortical large (diameter>/=20 mm) infarcts were studied; nine patients with cortical dysfunction (group A), and six without (group B). Seven patients with lacunar infarction served as controls. Infarct volume was measured on T2 weighted images with an image analyser. (1)H MRS data were obtained in three regions; high signal intensity area on T2 weighted image, overlying cortex with a normal T2 MRI appearance, and contralateral homologous cortical region.

RESULTS

Infarct volume was not different between groups A and B. N-acetylaspartate (NAA)/creatine+phosphocreatine (Cr) ratios in the cortical region overlying subcortical infarcts in group A were significantly lower than those in group B and the control group (p=0.002). The NAA/Cr ratios in the overlying cortex were significantly lower than those in the contralateral normal cortex in group A on the initial (p=0. 015) and follow up (p=0.028) (1)H MRS, but these differences were not found in group B and the control group.

CONCLUSIONS

The results support the idea that the cerebral cortex is a responsible location for cortical dysfunction after subcortical infarctions. (1)H MRS can be used as a sensitive method for the detection of a neuronal metabolic damage, which is not demonstrated by conventional MRI.

Memory in patients with subcortical infarction--an auditory event-related potential study

Event-related potentials (ERPs) were recorded from 21 patients with subcortical infarction (mean age, 62.1 years) and 14 normal control subjects (mean age, 62.7 years) as they listened to lists of words or pronounceable non-words. Some words were repeated immediately after initial presentation (lag 0), while others were repeated after five intervening words (lag 5), or after 2 to 4 min (lag 11-77). The subjects were asked to push a button upon hearing the occasional non-words. The Auditory-Verbal Learning Test (AVLT) also was administered to the patients to examine explicit memory. The mean N400 amplitude, appearing between 300 and 800 ms after the stimulus, was smaller in patients with subcortical infarction than in control subjects. The N400 in response to repeated words at lags 0, 5 and 11-77 was attenuated for both the patient and control groups. On the AVLT the total number of recalled words and number of words after interference were significantly decreased in patients relative to controls, while recognition was relatively preserved. The results suggest that lexical processing and retrieval mechanism of explicit memory are disturbed in patients with subcortical infarction, but implicit memory measured by this N400 paradigm is relatively preserved.

Apolipoprotein E phenotypes in demented and cognitively impaired patients with and without cerebrovascular disease

Controversy exists regarding the apolipoprotein E (ApoE) epsilon4 allele association with vascular dementia (VaD), ranging from increased epsilon4 frequency, similar to that found for Alzheimer's disease (AD), to no association between the epsilon4 allele and VaD. To clarify further the relationship between ApoE alleles polymorphism and cerebrovascular disease (CVD) in demented and cognitively impaired patients, we examined the ApoE phenotypes in a sample of 280 patients: 155 with AD, 21 with VaD, 32 with mixed dementia (MD), 45 with mild cognitive impairment (MCI) but without CVD, and 27 in which vascular disease was the most probable cause of cognitive decline [vascular mild cognitive impairment (VMCI)]. Our results show that the frequency of the ApoE epsilon4 allele in patients over 70 years old with clinically diagnosed VaD and VMCI does not differ significantly from that of controls. In contrast, ApoE epsilon4 allele-bearing individuals had greater risk of having late-onset AD (OR = 8.8; 95% CI 3.7-21.0), or non-vascular cognitive impairment (OR = 7.0; 95% CI 2.5-19.0).

Screening of vascular cognitive impairment on a Hungarian cohort

Cerebrovascular disease is a major public health problem in Eastern European countries. A Hungarian post-stroke population was examined to estimate the rate of dementia, the risk factors for cognitive impairment, and the applicability of a recently established Canadian diagnostic checklist in this cohort. Chronic cerebrovascular outpatients were screened for cognitive impairment with a combined checklist: the Diagnostic Checklist for Vascular Dementia established by the Consortium of Canadian Centres for Clinical Cognitive Research using the Mini Mental State Examination instead of the detailed neuropsychological part of the Checklist. Of the 247 consecutive patients at a cerebrovascular outpatient unit, 176 had cerebrovascular disorder diagnosed either by computed tomography (CT; n=126) or by the clinical signs. Of these, 15% were cognitively impaired and 5% fulfilled the criteria of dementia. The mean age of the patients with cognitive impairment was significantly higher than that of patients with normal cognition (68.2+/-10.2 and 60.5+/-10.5 years, P<0.001). The Barthel index was significantly lower in the cognitively affected group than in non-affected patients (92.4+/-16.0 and 97.1+/-8.7, P=0.027). Diabetes and more than two subcortical infarcts on CT or magnetic resonance imaging were more frequent in patients with cognitive loss (P=0.043 and P=0.013, respectively). Cognitive performance was also influenced by the level of education. Higher age, diabetes, motor deficits, and multiple subcortical infarcts are risk factors for cognitive impairment after stroke. The combined checklist appears to be a practical screening test for cognitive impairment in patients with chronic cerebrovascular diseases.

Aging and cerebrovascular disease

Aging influences cerebrovascular disease expression by a variety of mechanisms. Age-related changes in cerebral autoregulation, cellular metabolism, the blood-brain barrier, and autonomic function may leave the cerebrovascular system vulnerable to injury. Certain cerebrovascular disease, such as atrial fibrillation, watershed infarctions, carotid artery atherosclerosis, cerebral hemorrhages, subdural hematomas, and transient global amnesia manifest in the elderly. Vascular dementia and white matter disease are better understood with newer neuroimaging studies, careful neuropsychological and histopathologic examinations. Atherosclerosis and cerebral amyloid angiopathy may have larger roles than previously understood in Alzheimer's disease.

Fallacies in the pathological confirmation of the diagnosis of Alzheimer's disease

OBJECTIVE

Necropsy confirmed clinical diagnostic accuracy for Alzheimer's disease is claimed to exceed 90%. This figure contains two fallacies; it includes cases in which Alzheimer's disease exists with other diseases affecting cognition and the studies that report these figures excluded cases without necropsy (verification bias). The effect of these errors is estimated.

METHODS

Data were taken from the University of Western Ontario Dementia Study, a registry of dementia cases with clinical and psychometric follow up to necropsy based in a university memory disorders clinic with secondary and tertiary referrals. Data were available on 307 patients; 200 (65%) had clinically diagnosed Alzheimer's disease, 12 (4%) vascular dementia, 47 (15%) mixed dementia, and 48 (16%) had other diagnoses. One hundred and ninety two of 307 cases (63%) died and 122 of 192 fatalities (64%) had necropsies. The pathological material was interpreted in two ways, allowing and disallowing coexistent disease in making a diagnosis of Alzheimer's disease. In cases without necropsy, progressive cognitive loss was used as a marker for degenerative dementia. The outcome measures of interest were the positive predictive value of a clinical diagnosis of Alzheimer's disease allowing and disallowing coexistent diseases and with and without correction for cases that were not necropsied.

RESULTS

The clinical diagnoses differed significantly between the population who died and those who did not. In cases without necropsy, 22% had no dementia on follow up, concentrated in early cases and men, showing considerable scope for verification bias. The positive predictive value of a diagnosis of Alzheimer's disease was 81% including coexistent diseases, falling to 44% when limited to pure cases. Combined, these factors reduce the positive predictive value to 38% for pure Alzheimer's disease.

CONCLUSIONS

Correction for dual pathology and verification bias halves the positive predictive value of the clinical diagnosis of Alzheimer's disease. Data derived from necropsy studies cannot be extrapolated to the whole population. This has important implications including uncertainty about diagnosis and prognosis and a dilution effect in therapeutic trials in Alzheimer's disease.

Memory processes in depressed geriatric patients with and without subcortical hyperintensities on MRI

In this study, 12 patients over age 60 with depression with moderate to severe subcortical hyperintensities (SH) localized to the periventricular white matter were identified by quantitative MRI. Using the California Verbal Learning Test, they were compared with 12 age-, education-, and severity-matched patients with depression with minimal white matter changes on specific aspects of memory performance. Patients with cortical lesions, neurologic or systemic illness affecting cognition, and history of substance abuse were excluded. Patients in the group with high SH showed reduced use of semantic encoding strategies (p < 0.05), reduced learning efficiency (p < 0.05), and a greater discrepancy between free recall and recognition discriminability (p < 0.05) than their low SH counterparts. This pattern of performance on memory tasks is similar to that found in previous studies to be associated with subcortical degenerative disorders such as Huntington's and Parkinson's diseases. Geriatric patients with depression with SH may represent a subgroup with greater subcortical involvement, with associated cognitive and functional decline.

How can be best explain retrograde amnesia in human memory disorder?

I firstly consider general issues relating to our attempts to understand retrograde amnesia. Three main hypotheses are reviewed that have been proposed to account for retrograde amnesia. A theory is outlined to explain the dense autobiographical amnesia that is a focal phenomenon in some cases of severe head injury. This theory postulates that autobiographical retrieval requires the activation of a distributed network of cognitive operations, and that autobiographical amnesia results from the occurrence of multiple areas of pathology, distributed over both space and time.

Slowly progressive anarthria with late anterior opercular syndrome: a variant form of frontal cortical atrophy syndromes

We describe eight patients with slowly progressive speech production deficit combining speech apraxia, dysarthria, dysprosody and orofacial apraxia, and initially no other deficit in other language and non-language neuropsychological domains. Long-term follow-up (6-10 years) in 4 cases showed an evolution to muteness, bilateral suprabulbar paresis with automatic-voluntary dissociation and frontal lobe cognitive slowing without generalised intellectual deterioration. Most disabled patients presented with an anterior opercular syndrome (Foix-Chavany-Marie syndrome), and pyramidal or extrapyramidal signs. CT and MRI findings disclosed asymmetric (left > right) progressive cortical atrophy of the frontal lobes predominating in the posterior inferior frontal region, notably the operculum. SPECT and PET revealed a decreased cerebral blood flow and metabolism, prominent in the left posterior-inferior frontal gyrus and premotor cortex, extending bilaterally in the most advanced cases. Pathological study of two cases showed non-specific neuronal loss, gliosis, and spongiosis of superficial cortical layers, mainly confined to the frontal lobes, with no significant abnormalities in the basal ganglia, thalamus, cerebellum, brain stem (except severe neuronal loss in the substantia nigra in one case), and spinal cord. We propose to call this peculiar syndrome Slowly Progressive Anarthria (SPA), based on its specific clinical presentation, and its metabolic and pathological correlates. SPA represents another clinical expression of focal cortical degeneration syndromes, that may overlap with other similar syndromes, specially primary progressive aphasia and the various frontal lobe dementias.

The first Italian report on "Binswanger's disease"

The eponym "Binswanger's disease" is frequently used to indicate a form of vascular dementia characterized by white matter rarefaction and lacunar infarcts; however, it is difficult to find any consistency between this picture and the single case reported by Binswanger in 1894, which was vaguely defined both clinically and pathologically, and was probably a case of neurosyphilis. The first Italian report of a case of "Binswanger's disease" was published in 1958 by Donegani and Grattarola, who described a patient with a history of manic episodes followed by progressive mental deterioration. Pathological examination revealed changes mainly located in the hemispheric white matter, and the authors defined the patient as being affected by Binswanger's encephalopathy. Re-evaluation of this report indicates that the term "Binswanger's disease" has been applied to cases with non-specific clinical and pathological characteristics in the past. In this paper, we briefly compare Binswanger's original and the 1958 Italian case report, and discuss the current use of the term "Binswanger's disease".

Patterns of functional damage in neural network models of associative memory

Current understanding of the effects of damage on neural networks is rudimentary, even though such understanding could lead to important insights concerning neurological and psychiatric disorders. Motivated by this consideration, we present a simple analytical framework for estimating the functional damage resulting from focal structural lesions to a neural network model. The effects of focal lesions of varying area, shape, and number on the retrieval capacities of a spatially organized associative memory are quantified, leading to specific scaling laws that may be further examined experimentally. It is predicted that multiple focal lesions will impair performance more than a single lesion of the same size, that slit like lesions are more damaging than rounder lesions, and that the same fraction of damage (relative to the total network size) will result in significantly less performance decrease in larger networks. Our study is clinically motivated by the observation that in multi-infarct dementia, the size of metabolically impaired tissue correlates with the level of cognitive impairment more than the size of structural damage. Our results account for the detrimental effect of the number of infarcts rather than their overall size or structural damage, and for the "multiplicative" interaction between Alzheimer's disease and multi-infarct dementia.