Focal temporal lobe dysfunction in probable Alzheimer's disease predicts a slow rate of cognitive decline

Molecular subtyping of Alzheimer's disease with consensus non-negative matrix factorization

Alzheimer's disease (AD) is a heterogeneous disease and exhibits diverse clinical presentations and disease progression. Some pathological and anatomical subtypes have been proposed. However, these subtypes provide a limited mechanistic understanding for AD. Leveraging gene expression data of 222 AD patients from The Religious Orders Study and Memory and Aging Project (ROSMAP) Study, we identified two AD molecular subtypes (synaptic type and inflammatory type) using consensus non-negative matrix factorization (NMF). Synaptic type is characterized by disrupted synaptic vesicle priming and recycling and synaptic plasticity. Inflammatory type is characterized by disrupted IL2, interferon alpha and gamma pathways. The two AD molecular subtypes were validated using independent data from Gene Expression Omnibus. We further demonstrated that the two molecular subtypes are associated with APOE genotypes, with synaptic type more prevalent in AD patients with E3E4 genotype and inflammatory type more prevalent in AD patients with E3E3 genotype (p = 0.031). In addition, two molecular subtypes are differentially represented in male and female AD, with synaptic type more prevalent in male and inflammatory type in female patients (p = 0.051). Identification of AD molecular subtypes has potential in facilitating disease mechanism understanding, clinical trial design, drug discovery, and precision medicine for AD.

Profiling heterogeneity of Alzheimer's disease using white-matter impairment factors

The clinical presentation of Alzheimer's disease (AD) is not unitary as heterogeneity exists in the disease's clinical and anatomical characteristics. MRI studies have revealed that heterogeneous gray matter atrophy patterns are associated with specific traits of cognitive decline. Although white matter (WM) impairment also contributes to AD pathology, its heterogeneity remains unclear. The Latent Dirichlet Allocation (LDA) method is a suitable framework to study heterogeneity and allows to identify latent impairment factors of AD instead of simply mapping an overall disease effect. By exploring whole brain WM skeleton images by using LDA, three latent factors were revealed in AD: a temporal-frontal impairment factor (temporal and frontal lobes, especially hippocampus and para-hippocampus), a parietal factor (parietal lobe, especially precuneus), and a long fibre bundle factor (corpus callosum and superior longitudinal fasciculus). As revealed by longitudinal analysis, the latent factors have distinct impact on cognitive decline: for executive function (EF), the temporal-frontal factor was more strongly associated with baseline EF compared with the parietal factor, while the long-fibre bundle factor was most associated with decline rate of EF; for memory, the three factors showed almost equal effect on the baseline memory and decline rate. For each participant, LDA estimates his/her composition profile of latent impairment factors, which indicates disease subtype. We also found that the APOE genotype affects the AD subtype. Specifically, APOE ε4 was more associated with the long fibre bundle factor and APOE ε2 was more associated with temporal-frontal factor. By investigating heterogeneity and subtypes of AD through white matter impairment factors, our study could facilitate precision medicine.

•

LDA revealed three latent white matter impairment factors in Alzheimer’s disease.

•

Latent factors associate with executive function and memory decline differently.

•

Individual factor composition indicates disease subtype.

•

The APOE genotype is associated with the factor composition.

Cortical atrophy pattern-based subtyping predicts prognosis of amnestic MCI: an individual-level analysis

We categorized patients with amnestic mild cognitive impairment (aMCI) based on cortical atrophy patterns and evaluated whether the prognosis differed across the subtypes. Furthermore, we developed a classifier that learns the cortical atrophy pattern and predicts subtypes at an individual level. A total of 662 patients with aMCI were clustered into 3 subtypes based on cortical atrophy patterns. Of these, 467 patients were followed up for more than 12 months, and the median follow-up duration was 43 months. To predict individual-level subtype, we used a machine learning-based classifier with a 10-fold cross-validation scheme. Patients with aMCI were clustered into 3 subtypes: medial temporal atrophy, minimal atrophy (Min), and parietotemporal atrophy (PT) subtypes. The PT subtype had higher prevalence of APOE ε4 carriers, amyloid PET positivity, and greater risk of dementia conversion than the Min subtype. The accuracy for binary classification was 89.3% (MT vs. Rest), 92.6% (PT vs. Rest), and 86.6% (Min vs. Rest). When we used ensemble model of 3 binary classifiers, the accuracy for predicting the aMCI subtype at an individual level was 89.6%. Patients with aMCI with the PT subtype were more likely to have underlying Alzheimer's disease pathology and showed the worst prognosis. Our classifier may be useful for predicting the prognosis of individual aMCI patients.

Malignant progression in parietal-dominant atrophy subtype of Alzheimer's disease occurs independent of onset age

Recently, we reported that earlier stages of Alzheimer's disease (AD) can be categorized into 3 following anatomical subtypes using a hierarchical cluster analysis of cortical thickness across the entire brain: medial temporal-dominant (MT), parietal-dominant (P), and diffuse atrophy (D). The goal of this study was to investigate the rates of cognitive decline in these anatomical subtypes. Of the patients included in the prior study, 100 AD patients (MT, n = 36; P, n = 20; D, n = 44) who underwent follow-up neuropsychological assessments over a 3-year period were included. A linear mixed model analysis was performed to compare the longitudinal changes in neuropsychological test scores. The P subtype exhibited the most rapid cognitive decline in attention, language, visuospatial, memory, and frontal executive function, whereas MT and D subtypes did not differ in their longitudinal decline. When repeating the analyses with early-onset AD, which is known to progress faster than late-onset AD, only the P subtype showed such rapid progression. The P subtype appears to be a unique subtype of AD characterized by an aggressive rate of progression.

Anatomical heterogeneity of Alzheimer disease: based on cortical thickness on MRIs

OBJECTIVE

Because the signs associated with dementia due to Alzheimer disease (AD) can be heterogeneous, the goal of this study was to use 3-dimensional MRI to examine the various patterns of cortical atrophy that can be associated with dementia of AD type, and to investigate whether AD dementia can be categorized into anatomical subtypes.

METHODS

High-resolution T1-weighted volumetric MRIs were taken of 152 patients in their earlier stages of AD dementia. The images were processed to measure cortical thickness, and hierarchical agglomerative cluster analysis was performed using Ward's clustering linkage. The identified clusters of patients were compared with an age- and sex-matched control group using a general linear model.

RESULTS

There were several distinct patterns of cortical atrophy and the number of patterns varied according to the level of cluster analyses. At the 3-cluster level, patients were divided into (1) bilateral medial temporal-dominant atrophy subtype (n = 52, ∼ 34.2%), (2) parietal-dominant subtype (n = 28, ∼ 18.4%) in which the bilateral parietal lobes, the precuneus, along with bilateral dorsolateral frontal lobes, were atrophic, and (3) diffuse atrophy subtype (n = 72, ∼ 47.4%) in which nearly all association cortices revealed atrophy. These 3 subtypes also differed in their demographic and clinical features.

CONCLUSIONS

This cluster analysis of cortical thickness of the entire brain showed that AD dementia in the earlier stages can be categorized into various anatomical subtypes, with distinct clinical features.

Advancing research diagnostic criteria for Alzheimer's disease: the IWG-2 criteria

In the past 8 years, both the International Working Group (IWG) and the US National Institute on Aging-Alzheimer's Association have contributed criteria for the diagnosis of Alzheimer's disease (AD) that better define clinical phenotypes and integrate biomarkers into the diagnostic process, covering the full staging of the disease. This Position Paper considers the strengths and limitations of the IWG research diagnostic criteria and proposes advances to improve the diagnostic framework. On the basis of these refinements, the diagnosis of AD can be simplified, requiring the presence of an appropriate clinical AD phenotype (typical or atypical) and a pathophysiological biomarker consistent with the presence of Alzheimer's pathology. We propose that downstream topographical biomarkers of the disease, such as volumetric MRI and fluorodeoxyglucose PET, might better serve in the measurement and monitoring of the course of disease. This paper also elaborates on the specific diagnostic criteria for atypical forms of AD, for mixed AD, and for the preclinical states of AD.

Impaired visual recognition memory predicts Alzheimer's disease in amnestic mild cognitive impairment

BACKGROUND

In the common form of Alzheimer's disease (AD), neurofibrillary tangles, which are associated with cognitive dysfunction, initially develop in the anterior subhippocampal (perirhinal/entorhinal) cortex before reaching the hippocampus. This area plays a key role in visual recognition memory (VRM). Impaired VRM could therefore be an early marker of AD.

METHODS

An extensive neuropsychological assessment including VRM tasks was performed in 26 patients with single-domain amnestic mild cognitive impairment at baseline. We evaluated the diagnostic accuracy of neuropsychological tests using ROC curve analyses in a prospective longitudinal study until conversion to probable AD or with a follow-up of at least 6 years.

RESULTS

VRM performance predicted conversion to AD with a sensitivity of 80% and a specificity of 90.9%. Combining the assessment of VRM with a verbal memory task increased diagnostic accuracy.

CONCLUSIONS

Cognitive 'biomarkers' evaluating the function of brain areas that are the target of degenerative change should be considered for the early diagnosis of AD.

Clinical, imaging, and pathological heterogeneity of the Alzheimer's disease syndrome

With increasing knowledge of clinical in vivo biomarkers and the pathological intricacies of Alzheimer's disease (AD), nosology is evolving. Harmonized consensus criteria that emphasize prototypic illness continue to develop to achieve diagnostic clarity for treatment decisions and clinical trials. However, it is clear that AD is clinically heterogeneous in presentation and progression, demonstrating variable topographic distributions of atrophy and hypometabolism/hypoperfusion. AD furthermore often keeps company with other conditions that may further nuance clinical expression, such as synucleinopathy exacerbating executive and visuospatial dysfunction and vascular pathologies (particularly small vessel disease that is increasingly ubiquitous with human aging) accentuating frontal-dysexecutive symptomatology. That some of these atypical clinical patterns recur may imply the existence of distinct AD variants. For example, focal temporal lobe dysfunction is associated with a pure amnestic syndrome, very slow decline, with atrophy and neurofibrillary tangles limited largely to the medial temporal region including the entorhinal cortex. Left parietal atrophy and/or hypometabolism/hypoperfusion are associated with language symptoms, younger age of onset, and faster rate of decline - a potential 'language variant' of AD. Conversely, the same pattern but predominantly affecting the right parietal lobe is associated with a similar syndrome but with visuospatial symptoms replacing impaired language function. Finally, the extremely rare frontal variant is associated with executive dysfunction out of keeping with degree of memory decline and may have prominent behavioural symptoms. Genotypic differences may underlie some of these subtypes; for example, absence of apolipoprotein E e4 is often associated with atypicality in younger onset AD. Understanding the mechanisms behind this variability merits further investigation, informed by recent advances in imaging techniques, biomarker assays, and quantitative pathological methods, in conjunction with standardized clinical, functional, neuropsychological and neurobehavioral evaluations. Such an understanding is needed to facilitate 'personalized AD medicine', and eventually allow for clinical trials targeting specific AD subtypes. Although the focus legitimately remains on prototypic illness, continuing efforts to develop disease-modifying therapies should not exclude the rarer AD subtypes and common comorbid presentations, as is currently often the case. Only by treating them as well can we address the full burden of this devastating dementia syndrome.

Pure progressive amnesia: An atypical amnestic syndrome?

We report on M.S., an 83-year-old patient with isolated pure progressive amnesia. This rare, recently identified, form of amnesia has been described in elderly patients. Neuropathological studies suggest that this syndrome is an atypical clinical presentation of Alzheimer's disease. The aim of our study was to characterize the neuropsychological pattern of pure progressive amnesia in comparison with other amnestic syndromes and memory dissociations reported in the literature. Our results indicate that pure progressive amnesia is characterized by a highly unusual dissociation in the realm of memory, with severe deficits on tests based on recognition and recall of verbal and visual single items, contrasting with relatively preserved anterograde autobiographical and spatial memory and normal recall of complex material such as stories. These findings suggest that memory for single items could depend on an independent system. One hypothesis is that M.S.'s unusual memory profile results from relative dysfunction of the ventral medial temporal lobe pathway. An alternative explanation implicates cognitive reserve. Further studies are required in order to progress on this matter. In any case, pure progressive amnesia is a clinical syndrome that may provide further insight into the organization of declarative memory.

Probable Alzheimer's disease patients presenting as "focal temporal lobe dysfunction" show a slow rate of cognitive decline

Several authors have recently shown that anterograde amnesia is often associated with semantic memory impairment in amnesic MCI patients. Similarly, after the MCI condition, some patients who convert to Alzheimer's disease (AD) show the classic onset (cAD) characterized by the impairment of memory and executive functions, whereas other AD patients show isolated defects of episodic and semantic memory without deficits in other cognitive domains. The latter have been considered an AD variant characterized by 'focal Temporal Lobe Dysfunction' (TLD). The aim of the present study was to assess the differences in disease progression between cAD and TLD. For this purpose a continuous series of newly diagnosed probable AD patients presenting as cAD (n = 30) and TLD (n = 25), matched for severity, and 65 healthy controls underwent a comprehensive neuropsychological evaluation at baseline; TLD and cAD were re-evaluated at a 24-month follow-up. At follow-up, TLD patients showed no significant worsening of cognitive functions, whereas cAD subjects displayed a significant worsening in all explored cognitive domains. In conclusion, our results confirm that probable AD presenting as TLD represents a specific onset of AD characterized by a slower rate of progression.

Variability in cognitive presentation of Alzheimer's disease

The aim of the present study was to explore the nature and prevalence of phenotypic variations in Alzheimer's disease (AD). Neuropsychological profiles of a large cross-sectional cohort of patients with a clinical diagnosis of the disease were examined. All tests distinguished the AD group from controls confirming their sensitivity to the presence of early AD. Factor analysis of test scores revealed five factors, reflecting the discrete cognitive domains of memory, language, perceptuospatial abilities, executive skills, and praxis. Cluster analysis revealed distinct performance profiles that could not be accounted for by disease severity. Some patients showed an accentuation of memory impairment relative to other domains, whereas others showed relative sparing. Cognitive deficits other than memory were the salient presenting feature in a relatively high proportion of patients. A subset of the cohort (22%) showed grossly disproportionate impairments in one cognitive domain. The findings emphasise variability in presentation and indicate that distinct phenotypic variations appear to lie on a continuum rather than representing discrete forms of disease.

New assessment of dependency in demented patients: impact on the quality of life in informal caregivers

A qualitative tool was recently developed for evaluation of dependency in a demented population. This tool assesses the impact of cognitive impairment on functional status, taking into account disability in both the basic and the instrumental activities of daily living. The purpose of the present paper was to study the impact of dependency on informal caregivers who assist demented patients at home, with this new useful tool.

METHODS

A cross-sectional analysis was undertaken of the subgroup of 145 demented patients of the National Dementia Economic Study, aged > or = 65 years, living in the community, with an available caregiver. A neuropsychological assessment of patients (Mini-Mental State Examination) and a comprehensive evaluation of caregivers (quality of life, Short Form Health Survey-36, depression, Sense of Competence) were recorded. A total of 32.4% were dependent, disabled in both basic and instrumental functions, 42.1% were non-dependent but with instrumental functional disabilities and 25.5% were non-dependent. Impact of dependency on the caregiver's experience was significant for different aspects (satisfaction with caregiving, subjective burden, quality of life, depression). Medical and non-medical costs increased with the severity of functional disability. Findings indicate that this tool is also useful to assess the impact of progression of functional disability in patients with dementia, on the caregiver issues. The consequences appeared both on personal feelings and on quality of life and financial involvement in management of the patient. Cognitive impairment appears to have more moderate repercussions in these areas.

Étude de la planification de l’action au moyen du test du plan du zoo dans la maladie d’Alzheimer

INTRODUCTION

Executive dysfunction is regularly reported in patients with Alzheimer's disease. Nevertheless few studies have focused on planning ability in this neurodegenerative disease.

OBJECTIVES

This study aimed to investigate the formulation and the execution of plans in Alzheimer's disease using an ecological planning subtask derived from the Behavioural Assessment of the Dysexecutive Syndrome test battery, the "Zoo Map Test". There are two trials. The first trial consists of a "high demand" version of the subtask in which the subjects must plan in advance the order in which they will visit designated locations in a zoo (formulation level). In the second, or "low demand" version, the subject is simply required to follow a concrete externally imposed strategy to reach the locations to visit (execution level). The test was given to 16 patients with Alzheimer's disease and 13 normal elderly subjects.

RESULTS

The two way ANOVAs mainly showed more difficulties in patients with Alzheimer's disease than in healthy elderly in both conditions. The difference between formulation and execution was greater in patients with Alzheimer's disease than in healthy elderly. Planning impairments mainly correlated with behavioural changes (in particular motivational changes) observed by patient's relatives.

CONCLUSION

These results suggest that patients with Alzheimer's disease have some problems to mentally develop logical strategies and to execute complex predetermined plans, which are partially related to behavioural changes.

Hemispheric asymmetries of hypometabolism associated with semantic memory impairment in Alzheimer's disease: a study using positron emission tomography with fluorodeoxyglucose-F18

Considerable disagreement exists about the neuroanatomical basis of conceptual-semantic impairments observed in a subgroup of patients with Alzheimer's disease (AD) at mild to moderate stages of the disease. Several studies of groups of patients have shown correlations between focal hypometabolism or hypoperfusion in left hemispheric areas and measures of verbal semantic memory impairment in AD patients. The question remains, however, whether left hemispheric hypometabolism is sufficient to produce such impairment in the single case and whether nonverbal semantic knowledge is also affected. We used positron emission tomography (PET) with fluorodeoxyglucose-F18 (FDG), statistical parametric mapping (SPM), and tests of verbal and nonverbal semantic memory in 11 AD patients with a mean score on the Mini-Mental State Examination of 22.6 (+/-2.8). Naming impairment was significantly associated with left hemispheric asymmetry of hypometabolism on a single-case basis. Our correlation analysis showed that metabolism in left anterior temporal, posterior inferior temporal, inferior parietal and medial occipital areas (Brodmann areas: 21/38, 37, 40 and 19) correlated with both verbal and nonverbal semantic performance. We conclude that left hemispheric synaptic dysfunction, as measured by regional glucose hypometabolism, was sufficient to produce semantic impairments in our patients. The majority of areas affected in our patients with semantic impairments were involved in multimodal or supramodal (verbal and nonverbal) semantic knowledge.

Relational integration and executive function in Alzheimer's disease

Executive functions depend on the ability to represent relations between objects and events, and the prefrontal cortex provides the neural substrate for this capacity. Patients with probable Alzheimer's disease (AD) and control participants were administered measures of working memory and reasoning that varied systematically in their relational complexity. AD patients showed impairment on reasoning measures that required the online integration of relations but performed as well as control participants on nonrelational items and items requiring the processing of only single relations. When AD patients were divided into subgroups based on their performance on relational reasoning measures, the subgroup that showed significant impairment on relational integration measures exhibited a neuropsychological profile consistent with prefrontal cortical dysfunction.

Selection Ability in Alzheimer's Disease: Investigation of a Component of Semantic Processing

Selection ability (selecting a response from several competing semantic and/or lexical representations) was tested in 21 participants with Alzheimer's disease (AD) and 28 control participants to help clarify the nature of semantic impairments in AD. Selection demands were manipulated in 3 tasks (lexical fluency, comparison, and verb generation). In each, high-selection conditions required response selection from competing alternatives, whereas low-selection conditions had a reduced need for selection. Patients with AD were disproportionately impaired on the high-selection conditions of all tasks, even when this condition was easier. Selection deficits on verb generation were evident only relative to nonspeeded controls. Overall results indicate impaired semantic selection abilities in AD, which may contribute to poor performance on some semantic tasks.

A new approach to the qualitative evaluation of functional disability in dementia

BACKGROUND

Dementia patients suffer from the progressive deterioration of cognitive and functional abilities. Instrumental disabilities usually appear in the earlier stages of the disease while basic disabilities appear in the more advanced stages. In order to differentiate between mild, moderate and severe patients both instrumental and basic functional disabilities should be taken into account simultaneously.

OBJECTIVES

The objective of this study was to find a new method for classifying dementia patients based on their disabilities by using a basic and an instrumental Activities of Daily Living (ADL) scale.

METHODS

Functional disability was assessed in a Belgian cohort of dementia patients using the Katz and Lawton Instrumental Activities of Daily Living (IADL) scales. A k-means derived clustering method allocated patients to disability clusters according to their Katz and Lawton scores. In order to validate the classification, we compared socio-demographic, clinical and costs parameters between the groups.

RESULTS

The clustering method allocated patients between three clusters: dependent, non-dependent with instrumental functional disability (ND-IFD) and non-dependent. Dependence, as defined by these clusters, significantly correlates with age, residential setting, MMSE, patient's quality of life and costs.

CONCLUSION

This new classification of patients suffering from dementia will provide better understanding of functional disabilities and will complement the evaluation of disease severity based on cognitive function.

Decline in learning ability best predicts future dementia type: the Freedom House Study

The authors studied longitudinal change in learning efficiency as a predictor of future dementia type among healthy, well-educated, noninstitutionalized elderly retirees. Serial assessments of memory were obtained using the California Verbal Learning Test (CVLT). Latent growth (LG) models were developed from the slopes of the subjects' performance over the first five CVLT learning trials at each of three serial administrations (e.g., cohort inception [i.e., baseline] [CVLT1], 18 months [CVLT2] and 36 months [CVLT3]). The resulting growth curves were incorporated into a higher order LG model representing the dynamic change in learning efficiency over time (DeltaCVLT). DeltaCVLT was used to predict each subject's "dementia type" (i.e., clinical state) at 36 months (e.g., no dementia, Type 1 [Alzheimer type] dementia or Type 2 [non-Alzheimer type] dementia), after adjusting for CVLT1, baseline age, and baseline dementia type. Nonlinear (logarithmic) LG models of CVLT1-CVLT3 and DeltaCVLT best fit the data. There was significant variability about both CVLT1 and DeltaCVLT, suggesting subgroups in the sample with significantly different baseline memory function, and different rates of deterioration in learning efficiency. Age, baseline dementia type, and DeltaCVLT made significant independent contributions to final dementia type. CVLT1 did not predict final dementia type independently of the other covariates. These data suggest that baseline memory performance in noninstitutionalized elderly retirees does not predict future dementia type independently of the dynamic rate of change in memory measures. Serial administrations of memory tests may help identify nondemented persons at greater or lesser risk for conversion to frank dementia in the near-term.

Value of clinical data and neuropsychological measures in probable Alzheimer's disease

We examined retrospectively 60 probable Alzheimer's disease (AD) outpatients, 30 with early onset (EOP) and 30 with late onset (LOP), divided into two groups on the basis of illness duration (within 2 years (P<2) and over 2 years (P>2)), compared with 60 normal controls (NC). We employed a battery of neuropsychological tests including the mini mental state examination (MMSE) and our brief mental deterioration battery (BMDB), computerized psychomotor performance tests and staging of functional impairment. EOP were worse than LOP in verbal fluency and in functional impairment, being better only in Rey's long-term verbal memory (RLT). P>2 were more compromised than P<2 in functional impairment, MMSE, personal and temporal orientation and RLT. Our BMDB showed the highest accuracy in classifying probably AD patients, whereas, MMSE had a high specificity but poor sensitivity as well as psychomotor performance tasks. In conclusion, AD patients with early onset, having a worse functional impairment, appear to be an eligible group to evaluate possible changes in response to antidementia treatment.

Brain perfusion abnormalities in Alzheimer's disease: comparison between patients with focal temporal lobe dysfunction and patients with diffuse cognitive impairment

OBJECTIVES

Patients with Alzheimer's disease (AD) showing a selective impairment of episodic and semantic memory have recently been classified as affected by focal temporal lobe dysfunction (FTLD) and considered as a distinct subgroup of patients affected by a particular form of AD. The aim was to compare the cerebral perfusion of patients with AD with FTLD and patients with AD with the more typical profile of diffuse cognitive impairment (dAD).

METHODS

Ten patients with AD with FTLD, 14 patients with AD with dAD, and 12 normal controls were studied. All the 24 patients with AD underwent a complete neuropsychological assessment. SPECT examination with [(99m)Tc]-HMPAO, using a four head brain dedicated tomograph, was performed in patients and controls. Tracer uptake was quantified in 27 regions of interest (ROIs), including lateral and mesial temporal areas. Mean counts in the 27 ROIs of controls, patients with FTLD and those with dAD were compared using an ANOVA for repeated measures with Bonferroni's correction. A logistic regression analysis, followed by a receiver operating characteristic (ROC) analysis, was also applied to select SPECT patterns which significantly differentiated patients with FTLD and those with dAD.

RESULTS

Two scintigraphic patterns of abnormalities, shaping a double dissociation between the FTLD and dAD groups, emerged: a bilateral mesial temporal hypoperfusion, characteristic of FTLD and a posterior parietal (and temporal parietal) hypoperfusion characteristic of patients with dAD.

CONCLUSIONS

These scintigraphic findings provide further support to the hypothesis that FTLD is not a mere stage but a distinct anatomoclinical form of AD. The combination of neuropsychological tests and [(99m)Tc]-HMPAO SPECT may be very useful in identifying patients with FTLD from the wider group of patients with dAD. This issue is particularly worthwhile, as there is increasing evidence that patients with FTLD have a slower rate of cognitive decline.

Cholinergic modulation, visual function and Alzheimer's dementia

Electrophysiological evidence at a cellular level and in vivo macroelectrode recordings converge in indicating a degree of specificity of acetylcholine action in vision. Acetylcholine (ACh) function is also thought to play a significant role in memory, learning and other cognitive processes. In this respect, ACh action is suggested to serve in both sensory and cognitive processes. The pharmacological blocking of brain muscarinic transmission has been proposed as a model of geriatric memory impairment and Alzheimer's dementia. Visual electrophysiological testing is deemed of diagnostic specificity for this disease. ACh brain neurotransmission, however, mostly contributes to the modulation of nonspecific aspects of cognition, such as arousal or attention. Alzheimer's dementia results from complex neuron alterations [which also affect muscarinic receptors among other (sub)cellular structures] rather than simply reflecting ACh impoverishment. A substantial loss of retinal ganglion cells is documented in patients with Alzheimer's disease and is consistent with electrophysiological observations. However, it is unclear to what extent the dysfunction of the visual system observable in Alzheimer's dementia is qualitatively different from that occurring spontaneously during aging. The dissimilarities between the effect of acute muscarinic blocking (e.g. by scopolamine) and dementia outnumber the similarities. Accordingly, the conventional ACh agonist-antagonist model of dementia now appears questionable, and replacement treatment with compounds enhancing ACh function proved disappointing. It is suggested that (nonspecific) ACh action becomes function-specific, as determined by the architecture of local brain circuits in which it is involved.