The phenotypical core of Alzheimer's disease-related and nonrelated variants of the corticobasal syndrome: A systematic clinical, neuropsychological, imaging, and biomarker study

INTRODUCTION

The corticobasal syndrome (CBS) constitutes a neurodegenerative disease spectrum with substantial phenotypical or biological heterogeneity, requiring large or multimodal studies to identify its clinico-biological signature while disentangling Alzheimer's disease (AD)-related from non-AD-related CBS.

METHODS

We analyzed a large (N = 45) monocenter expert-clinic CBS cohort, recruited in motor and/or cognitive units to avoid recruitment biases, assessed with standardized motor and/or cognitive-language tests, brain perfusion imaging, and cerebrospinal fluid biomarkers.

RESULTS

CBS mainly manifests as a motor and/or language disorder incorporating a "mixed progressive aphasia" phenotype, consistent with left-lateralized damage to frontal-parietal-temporal cortices. Biomarker expression indicates in 18% underlying AD causing predominant parietal-temporal damage and Gerstmann syndrome (sensitivity 75%; specificity 75%), whereas non-AD-CBS presented with predominant prefrontal and lexical-semantic impairment.

DISCUSSION

CBS is primarily a "motor-plus-aphasia" disease unfolding into AD-related and non-AD-related variants with distinctive cognitive-anatomic patterns. CBS, and notably its "Gerstmann variant", should be included in the new AD "lexicon" and categorized in the evolving diagnostic spectrum of "atypical AD"d.

The Brain Network of Naming: A Lesson from Primary Progressive Aphasia

OBJECTIVE

Word finding depends on the processing of semantic and lexical information, and it involves an intermediate level for mapping semantic-to-lexical information which also subserves lexical-to-semantic mapping during word comprehension. However, the brain regions implementing these components are still controversial and have not been clarified via a comprehensive lesion model encompassing the whole range of language-related cortices. Primary progressive aphasia (PPA), for which anomia is thought to be the most common sign, provides such a model, but the exploration of cortical areas impacting naming in its three main variants and the underlying processing mechanisms is still lacking.

METHODS

We addressed this double issue, related to language structure and PPA, with thirty patients (11 semantic, 12 logopenic, 7 agrammatic variant) using a picture-naming task and voxel-based morphometry for anatomo-functional correlation. First, we analyzed correlations for each of the three variants to identify the regions impacting naming in PPA and to disentangle the core regions of word finding. We then combined the three variants and correlation analyses for naming (semantic-to-lexical mapping) and single-word comprehension (lexical-to-semantic mapping), predicting an overlap zone corresponding to a bidirectional lexical-semantic hub.

RESULTS AND CONCLUSIONS

Our results showed that superior portions of the left temporal pole and left posterior temporal cortices impact semantic and lexical naming mechanisms in semantic and logopenic PPA, respectively. In agrammatic PPA naming deficits were rare, and did not correlate with any cortical region. Combined analyses revealed a cortical overlap zone in superior/middle mid-temporal cortices, distinct from the two former regions, impacting bidirectional binding of lexical and semantic information. Altogether, our findings indicate that lexical/semantic word processing depends on an anterior-posterior axis within lateral-temporal cortices, including an anatomically intermediate hub dedicated to lexical-semantic integration. Within this axis our data reveal the underpinnings of anomia in the PPA variants, which is of relevance for both diagnosis and future therapy strategies.

Parkinsonism, movement disorders and genetics in frontotemporal dementia

Frontotemporal dementia (FTD) refers to a group of clinically and genetically heterogeneous neurodegenerative disorders that are a common cause of adult-onset behavioural and cognitive impairment. FTD often presents in combination with various hyperkinetic or hypokinetic movement disorders, and evidence suggests that various genetic mutations underlie these different presentations. Here, we review the known syndromatic-genetic correlations in FTD. Although no direct genotype-phenotype correlations have been identified, mutations in multiple genes have been associated with various presentations. Mutations in the genes that encode microtubule-associated protein tau (MAPT) and progranulin (PGRN) can manifest as symmetrical parkinsonism, including the phenotypes of Richardson syndrome and corticobasal syndrome (CBS). Expansions in the C9orf72 gene are most frequently associated with familial FTD, typically combined with motor neuron disease, but other manifestations, such as symmetrical parkinsonism, CBS and multiple system atrophy-like presentations, have been described in patients with these mutations. Less common gene mutations, such as those in TARDBP, CHMP2B, VCP, FUS and TREM2, can also present as atypical parkinsonism. The most common hyperkinetic movement disorders in FTD are motor and vocal stereotypies, which have been observed in up to 78% of patients with autopsy-proven FTD. Other hyperkinetic movements, such as chorea, orofacial dyskinesias, myoclonus and dystonia, are also observed in some patients with FTD.

Spatial neglect in a patient with logopenic progressive aphasia

Spatial neglect and extinction are induced by posterior superior temporal and inferior parietal dysfunction. In patients with logopenic progressive aphasia (LPA) these structures are often degenerated, but there are no reports of these disorders being associated. A 53-year-old man with the signs of LPA revealed right-sided spatial neglect on line bisection and drawing tests as well as multimodal extinction. MRI showed left hemispheric posterior temporoparietal atrophy. Since injury to the core structures for these aphasic and attentional syndromes overlaps, patients with LPA should be screened for spatial neglect and extinction.

The Wernicke area: Modern evidence and a reinterpretation

The term "Wernicke's area" is most often used as an anatomical label for the gyri forming the lower posterior left sylvian fissure. Although traditionally this region was held to support language comprehension, modern imaging and neuropsychological studies converge on the conclusion that this region plays a much larger role in speech production. This evidence is briefly reviewed, and a simple schematic model of posterior cortical language processing is described.

Progressive transcortical sensory aphasia and progressive ideational apraxia owing to temporoparietal cortical atrophy

Background

In contrast to frontotemporal lobar degeneration, atrophy of the focal posterior lateral cortex has not been thoroughly studied. Three clinical types of focal cortical atrophy have been described: 1) logopenic variant of primary progressive aphasia, which presents with impaired repetition despite normal articulation; 2) posterior cortical atrophy, which presents with prominent visuospatial deficits; and 3) primary progressive apraxia. All three clinical types are characterized by specific patterns of hypometabolism/hypoperfusion: the left posterior perisylvian area in the logopenic variant of primary progressive aphasia, bilateral parietooccipital areas in posterior cortical atrophy, and the parietal cortex in primary progressive apraxia. However, not every patient clearly fits into one of these categories.

Case presentation

Here we describe two patients with atypical focal cortical presentations. They presented with a history of a few years of progressive transcortical sensory aphasia characterized by fluent output with normal grammar and syntax, normal repetition, sentence comprehension deficits, and anomia without loss of word meaning. They also presented with progressive apraxia that began at the initial stages. Some forms of posterior symptoms including acalculia, agraphia, and visuospatial deficits were also observed. Hypoperfusion was noted mainly in the left temporoparietal region, which is slightly posterior to the perisylvian area.

Conclusions

Although our cases lack in CSF findings and PIB scan, these two cases and previous reports might suggest the existence of a subgroup of patients presenting with transcortical sensory aphasia, apraxia, and posterior symptoms (acalculia, agraphia, and visuospatial deficits) in the setting of Alzheimer’s disease. This subgroup may reflect the spectrum of clinical manifestations between logopenic variant of primary progressive aphasia and posterior cortical atrophy.

Automated lesion detection on MRI scans using combined unsupervised and supervised methods

BACKGROUND

Accurate and precise detection of brain lesions on MR images (MRI) is paramount for accurately relating lesion location to impaired behavior. In this paper, we present a novel method to automatically detect brain lesions from a T1-weighted 3D MRI. The proposed method combines the advantages of both unsupervised and supervised methods.

METHODS

First, unsupervised methods perform a unified segmentation normalization to warp images from the native space into a standard space and to generate probability maps for different tissue types, e.g., gray matter, white matter and fluid. This allows us to construct an initial lesion probability map by comparing the normalized MRI to healthy control subjects. Then, we perform non-rigid and reversible atlas-based registration to refine the probability maps of gray matter, white matter, external CSF, ventricle, and lesions. These probability maps are combined with the normalized MRI to construct three types of features, with which we use supervised methods to train three support vector machine (SVM) classifiers for a combined classifier. Finally, the combined classifier is used to accomplish lesion detection.

RESULTS

We tested this method using T1-weighted MRIs from 60 in-house stroke patients. Using leave-one-out cross validation, the proposed method can achieve an average Dice coefficient of 73.1% when compared to lesion maps hand-delineated by trained neurologists. Furthermore, we tested the proposed method on the T1-weighted MRIs in the MICCAI BRATS 2012 dataset. The proposed method can achieve an average Dice coefficient of 66.5% in comparison to the expert annotated tumor maps provided in MICCAI BRATS 2012 dataset. In addition, on these two test datasets, the proposed method shows competitive performance to three state-of-the-art methods, including Stamatakis et al., Seghier et al., and Sanjuan et al.

CONCLUSIONS

In this paper, we introduced a novel automated procedure for lesion detection from T1-weighted MRIs by combining both an unsupervised and a supervised component. In the unsupervised component, we proposed a method to identify lesioned hemisphere to help normalize the patient MRI with lesions and initialize/refine a lesion probability map. In the supervised component, we extracted three different-order statistical features from both the tissue/lesion probability maps obtained from the unsupervised component and the original MRI intensity. Three support vector machine classifiers are then trained for the three features respectively and combined for final voxel-based lesion classification.

What do pauses in narrative production reveal about the nature of word retrieval deficits in PPA?

Naming and word-retrieval deficits, which are common characteristics of primary progressive aphasia (PPA), differentially affect production across word classes (e.g., nouns, verbs) in some patients. Individuals with the agrammatic variant (PPA-G) often show greater difficulty producing verbs whereas those with the semantic variant (PPA-S) show greater noun deficits and those with logopenic PPA (PPA-L) evince no clear-cut differences in production of the two word classes. To determine the source of these production patterns, the present study examined word-finding pauses as conditioned by lexical variables (i.e., word class, frequency, length) in narrative speech samples of individuals with PPA-S (n=12), PPA-G (n=12), PPA-L (n=11), and cognitively healthy controls (n=12). We also examined the relation between pause distribution and cortical atrophy (i.e., cortical thickness) in nine left hemisphere regions of interest (ROIs) linked to word production. Results showed higher overall pause rates for PPA compared to unimpaired controls; however, greater naming severity was not associated with increased pause rate. Across all groups, more pauses were produced before lower vs. higher frequency words, with no independent effects of word length after controlling for frequency. With regard to word class, the PPA-L group showed a higher rate of pauses prior to production of nouns compared to verbs, consistent with noun-retrieval deficits arising at the lemma level of word production. Those with PPA-G and PPA-S, like controls, produced similar pause rates across word classes; however, lexical simplification (i.e., production of higher-frequency and/or shorter words) was evident in the more-impaired word class: nouns for PPA-S and verbs for PPA-G. These patterns are consistent with conceptual and/or lemma-level impairments for PPA-S, predominantly affecting objects/nouns, and a lemma-level verb-retrieval deficit for PPA-G, with a concomitant impairment in phonological encoding and articulation affecting overall pause rates. The greater tendency to pause before nouns was correlated with atrophy in the left precentral gyrus, inferior frontal gyrus and inferior parietal lobule, whereas the greater tendency to pause before less frequent and longer words was associated with atrophy in left precentral and inferior parietal regions.

Frontotemporal dementia: diagnosis, deficits and management

Frontotemporal dementia (FTD) is a progressive neurologic syndrome with diverse clinical presentations and attendant underlying pathologies. Psychiatric prodrome, neuropsychiatric symptoms and language difficulties are common in FTD, but the diversity of presentation raises unique diagnostic challenges that can significantly impact patient care and counsel for caregivers regarding clinical status and prognosis. While neuropsychiatric symptom measures are helpful, more sensitive assessments delineating the specific behavioral and linguistic deficits accompanying FTD are needed. Comprehensive clinical assessment in combination with evaluation of language, socio-emotional functioning, cognition and neuroimaging aid in accurate and early diagnosis and treatment planning. In what follows, we review each of the FTD syndromes, highlight current research investigating the cognitive, behavioral and socio-emotional deficits observed with this disease, address common diagnostic challenges and summarize best practices associated with management of FTD.

Atrophy patterns in early clinical stages across distinct phenotypes of Alzheimer's disease

Alzheimer's disease (AD) can present with distinct clinical variants. Identifying the earliest neurodegenerative changes associated with each variant has implications for early diagnosis, and for understanding the mechanisms that underlie regional vulnerability and disease progression in AD. We performed voxel-based morphometry to detect atrophy patterns in early clinical stages of four AD phenotypes: Posterior cortical atrophy (PCA, "visual variant," n=93), logopenic variant primary progressive aphasia (lvPPA, "language variant," n=74), and memory-predominant AD categorized as early age-of-onset (EOAD, <65 years, n=114) and late age-of-onset (LOAD, >65 years, n=114). Patients with each syndrome were stratified based on: (1) degree of functional impairment, as measured by the clinical dementia rating (CDR) scale, and (2) overall extent of brain atrophy, as measured by a neuroimaging approach that sums the number of brain voxels showing significantly lower gray matter volume than cognitively normal controls (n=80). Even at the earliest clinical stage (CDR=0.5 or bottom quartile of overall atrophy), patients with each syndrome showed both common and variant-specific atrophy. Common atrophy across variants was found in temporoparietal regions that comprise the posterior default mode network (DMN). Early syndrome-specific atrophy mirrored functional brain networks underlying functions that are uniquely affected in each variant: Language network in lvPPA, posterior cingulate cortex-hippocampal circuit in amnestic EOAD and LOAD, and visual networks in PCA. At more advanced stages, atrophy patterns largely converged across AD variants. These findings support a model in which neurodegeneration selectively targets both the DMN and syndrome-specific vulnerable networks at the earliest clinical stages of AD.

Is descriptive writing useful in the differential diagnosis of logopenic variant of primary progressive aphasia, Alzheimer's disease and mild cognitive impairment?

Current classification of primary progressive aphasia (PPA) encompasses three variants: non-fluent (nfvPPA), semantic (svPPA) and logopenic (lvPPA). Previously lvPPA was regarded as aphasic form of Alzheimer's disease (AD). However, not all patients with lvPPA phenotype present with AD pathology. Despite abundant literature on differentiation of lvPPA from svPPA and nfvPPA, studies comparing lvPPA with AD and mild cognitive impairment (MCI) are scarce. This study aimed at analyzing written descriptive output in lvPPA, AD and MCI. Thirty-five patients participated in the study: 9 with lvPPA, 13 with AD and 13 with MCI. Most aspects of writing performance were comparable in three groups. However, letter insertion errors appeared in 44% patients with lvPPA, while they were absent in AD and MCI. Patients with lvPPA used more verbs than patients with AD. Writing profile may complement other neuropsychological assessment results in the differential diagnosis of lvPPA. Letter insertion errors and frequent verb use may raise a query of lvPPA.

Phonological short-term memory in logopenic variant primary progressive aphasia and mild Alzheimer's disease

It has been argued that individuals with logopenic variant primary progressive aphasia (lvPPA) have an impairment of the phonological loop, which is a component of the short-term memory (STM) system. In contrast, this type of impairment is not thought to be present in mild typical Alzheimer's disease (AD). Thus, one would predict that people with lvPPA would score significantly lower than a matched AD group on tasks that require phonological STM. In the current study, an lvPPA group was compared with a mild AD group that was matched on age, education, and general cognitive functioning. For a subset of the tasks that involved pseudowords, the AD and lvPPA groups were compared to a healthy control group that was matched on age and education. The lvPPA group was more impaired than the AD group on all of the tasks that required phonological STM, including the pseudoword tasks, but there were no significant differences between these groups on tasks that required visuospatial STM. Compared to the healthy controls, the lvPPA group performed significantly worse on the repetition and reading of pseudowords, while the AD group did not differ significantly from the controls on these tasks. These findings are consistent with the hypothesis that phonological STM is impaired in lvPPA.

Clinical and neuroimaging biomarkers of amyloid-negative logopenic primary progressive aphasia

Logopenic primary progressive aphasia (lvPPA) is a progressive language disorder characterized by anomia, difficulty repeating complex sentences, and phonological errors. The majority, although not all, lvPPA patients have underlying Alzheimer's disease. We aimed to determine whether clinical or neuroimaging features differ according to the deposition of Aβ on Pittsburgh-compound B PET in lvPPA. Clinical features, patterns of atrophy on MRI, hypometabolism on FDG-PET, and white matter tract degeneration were compared between six PiB-negative and 20 PiB-positive lvPPA patients. PiB-negative patients showed more asymmetric left-sided patterns of atrophy, hypometabolism and white matter tract degeneration, with greater left anteromedial temporal and medial prefrontal involvement, than PiB-positive patients. PiB-positive patients showed greater involvement of right temporoparietal and frontal lobes. There was very little evidence for clinical differences between the groups. Strikingly asymmetric neuroimaging findings with relatively preserved right hemisphere may provide clues that AD pathology is absent in lvPPA.

Pathology in primary progressive aphasia syndromes

'Primary progressive aphasia' (PPA) refers to core linguistic disorders caused by neurodegenerative disease. Three main PPA variants are recognized: nonfluent/agrammatic, semantic and logopenic. Correctly classifying patients during life according to the underlying histopathology will become increasingly important as cause-specific treatments become available. This article reviews clinical and histopathological studies of PPA, with particular reference to updated PPA classifications. Currently, one-to-one relationships do not exist within PPA subtypes. The semantic variant has the best correspondence between the clinical syndrome and the underlying pathological cause and the logopenic variant the worst correspondence. The use of future biomarkers should facilitate accurate clinicopathological correlation of patients during life.

Understanding the cause of sporadic Alzheimer's disease

The increasing prevalence of Alzheimer's disease (AD) and a lack of effective prevention or disease-modifying therapies are global challenges with devastating personal, social and economic consequences. The amyloid β (Aβ) hypothesis posits that cerebral β-amyloidosis is a critical early event in AD pathogenesis. However, failed clinical trials of Aβ-centric drug candidates have called this hypothesis into question. Whereas we acknowledge that the Aβ hypothesis is far from disproven, we here re-visit the links between Aβ, tau and neurodegeneration. We review the genetics, epidemiology and pathology of sporadic AD and give an updated account of what is currently known about the molecular pathogenesis of the disease.

Progranulin-associated PiB-negative logopenic primary progressive aphasia

The logopenic variant of primary progressive aphasia (lvPPA) strongly associates with Alzheimer's disease, but can also associate with frontotemporal lobar degeneration. We aimed to assess the frequency of lvPPA in patients with speech and language disorders without β-amyloid deposition, and to perform detailed neuroimaging and genetic testing in such lvPPA patients. Seventy-six patients with a neurodegenerative speech and language disorder and Pittsburgh compound B (PiB) PET imaging demonstrating no β-amyloid deposition were analyzed. Six lvPPA patients (8 %) were identified. All six underwent progranulin (GRN) gene testing. Structural abnormality index maps and Cortex ID analysis were utilized to assess individual patterns of grey matter atrophy on MRI and hypometabolism on 18-F fluorodeoxyglucose (FDG) PET. Statistical parametric mapping was used to perform MRI and FDG-PET group comparisons between those with (GRN-positive) and without (GRN-negative) progranulin mutations. All six lvPPA patients showed left temporoparietal atrophy and hypometabolism. Three patients (50 %) were GRN-positive. Speech, language, and neurological and neuropsychological profiles did not differ between GRN-positive and negative patients, although GRN-positive patients had family histories, were on average 8 years younger, and had lower PiB-PET ratios. All six patients showed similar patterns of atrophy and hypometabolism, although, as a group, GRN-positive patients had more severe abnormalities, particularly in anteromedial temporal lobes. Logopenic PPA accounts for a small minority of neurodegenerative speech and language disorders not associated with β-amyloid deposition. Identification of such patients, however, should prompt testing for GRN mutations, since GRN-positive patients do not have distinctive features, yet account for 50 % of this patient population.