Critical Comparison of Different Biomarkers for Alzheimer's Disease in a Clinical Setting

Cerebral amyloid load determination in a clinical setting: interpretation of amyloid biomarker discordances aided by tau and neurodegeneration measurements

BACKGROUND

Alzheimer's disease (AD) diagnosis can be hindered by amyloid biomarkers discordances.

OBJECTIVE

We aim to interpret discordances between amyloid positron emission tomography (Amy-PET) and cerebrospinal fluid (CSF) (Aβ₄₂ and Aβ_42/40), using Amy-PET semiquantitative analysis, [¹⁸F]fluorodeoxyglucose (FDG)-PET pattern, and CSF assays.

METHOD

Thirty-six subjects with dementia or mild cognitive impairment, assessed by neuropsychological tests, structural and functional imaging, and CSF assays (Aβ₄₂, Aβ_42/40, p-tau, t-tau), were retrospectively examined. Amy-PET and FDG-PET scans were analyzed by visual assessment and voxel-based analysis. SUVR were calculated on Amy-PET scans.

RESULTS

Groups were defined basing on the agreement among CSF Aβ₄₂ (A), CSF Aβ_42/40 Ratio (R), and Amy-PET (P) dichotomic results ( ±). In discordant groups, CSF assays, Amy-PET semiquantification, and FDG-PET patterns supported the diagnosis suggested by any two agreeing amyloid biomarkers. In groups with discordant CSF Aβ₄₂, the ratio always agrees with Amy-PET results, solving both false-negative and false-positive Aβ₄₂ results, with Aβ₄₂ levels close to the cut-off in A + R-P- subjects. The A + R + P- group presented high amyloid deposition in relevant areas, such as precuneus, posterior cingulate cortex (PCC) and dorsolateral frontal inferior cortex at semiquantitative analysis.

CONCLUSION

The amyloid discordant cases could be overcome by combining CSF Aβ₄₂, CSF ratio, and Amy-PET results. The concordance of any 2 out of the 3 biomarkers seems to reveal the remaining one as a false result. A cut-off point review could avoid CSF Aβ₄₂ false-negative results. The regional semiquantitative Amy-PET analysis in AD areas, such as precuneus and PCC, could increase the accuracy in AD diagnosis.

Challenges in Alzheimer's Disease Diagnostic Work-Up: Amyloid Biomarker Incongruences

BACKGROUND

Discordance among amyloid biomarkers is a challenge to overcome in order to increase diagnostic accuracy in dementia.

OBJECTIVES

1) To verify that cerebrospinal fluid (CSF) Aβ42/Aβ40 ratio (AβR) better agrees with Amyloid PET (Amy-PET) results compared to CSF Aβ42; 2) to detect differences among concordant positive, concordant negative, and discordant cases, basing the concordance definition on the agreement between CSF AβR and Amy-PET results; 3) to define the suspected underlying pathology of discordant cases using in vivo biomarkers.

METHOD

We retrospectively enrolled 39 cognitively impaired participants in which neuropsychological tests, apolipoprotein E genotype determination, TC/MRI, FDG-PET, Amy-PET, and CSF analysis had been performed. In all cases, CSF analysis was repeated using the automated Lumipulse method. In discordant cases, FDG-PET scans were evaluated visually and using automated classifiers.

RESULTS

CSF AβR better agreed with Amy-PET compared to CSF Aβ42 (Cohen's K 0.431 versus 0.05). Comparisons among groups did not show any difference in clinical characteristics except for age at symptoms onset that was higher in the 6 discordant cases with abnormal CSF AβR values and negative Amy-PET (CSF AβR+/AmyPET-). FDG-PET and all CSF markers (Aβ42, AβR, p-Tau, t-Tau) were suggestive of Alzheimer's disease (AD) in 5 of these 6 cases.

CONCLUSION

1) CSF AβR is the CSF amyloid marker that shows the better level of agreement with Amy-PET results; 2) The use of FDG-PET and CSF-Tau markers in CSFAβR+/Amy-PET-discordant cases can support AD diagnosis; 3) Disagreement between positive CSF AβR and negative Amy-PET in symptomatic aged AD patients could be due to the variability in plaques conformation and a negative Amy-PET scan cannot be always sufficient to rule out AD.

Biomarker counseling, disclosure of diagnosis and follow-up in patients with mild cognitive impairment: A European Alzheimer's disease consortium survey

OBJECTIVES

Mild cognitive impairment (MCI) is associated with an increased risk of further cognitive decline, partly depending on demographics and biomarker status. The aim of the present study was to survey the clinical practices of physicians in terms of biomarker counseling, management, and follow-up in European expert centers diagnosing patients with MCI.

METHODS

An online email survey was distributed to physicians affiliated with European Alzheimer's disease Consortium centers (Northern Europe: 10 centers; Eastern and Central Europe: 9 centers; and Southern Europe: 15 centers) with questions on attitudes toward biomarkers and biomarker counseling in MCI and dementia. This included postbiomarker counseling and the process of diagnostic disclosure of MCI, as well as treatment and follow-up in MCI.

RESULTS

The response rate for the survey was 80.9% (34 of 42 centers) across 20 countries. A large majority of physicians had access to biomarkers and found them useful. Pre- and postbiomarker counseling varied across centers, as did practices for referral to support groups and advice on preventive strategies. Less than half reported discussing driving and advance care planning with patients with MCI.

CONCLUSIONS

The variability in clinical practices across centers calls for better biomarker counseling and better training to improve communication skills. Future initiatives should address the importance of communicating preventive strategies and advance planning.

[Memory clinics in Germany-structural requirements and areas of responsibility]

Hintergrund

Gedächtnisambulanzen (GA) sind auf (Differenzial‑)Diagnostik, Therapie, Aufklärung, Management und Beratung von kognitiven Störungen im Alter und deren Risikostadien spezialisierte Einrichtungen. In der Praxis haben sie sehr unterschiedliche Organisationsformen. Aufgrund der wachsenden diagnostischen Möglichkeiten bei neurodegenerativen Erkrankungen, dem steigenden Bedarf an Früherkennung und Prädiktion sowie absehbaren neuen diagnostischen Verfahren und krankheitsmodifizierenden Therapien ist eine Vereinheitlichung der strukturellen Voraussetzungen und Aufgabenbereiche für GA sinnvoll.

Ziel der Arbeit

Der Artikel macht Vorschläge für strukturelle und organisatorische Voraussetzungen, Aufgaben sowie einheitliche Arbeitsweisen von GA in Deutschland.

Methoden

Expertenkonsens von Psychiatern, Neurologen und Geriatern aus universitären und außeruniversitären Einrichtungen.

Ergebnisse

Gedächtnisambulanzen sollen den Facharztstandard für Psychiatrie und/oder Neurologie vorhalten und sich in ihrer Arbeitsweise bez. (Differenzial‑)Diagnostik und Therapie von Demenzen eng an der S3-Leitlinie (S3LL-)Demenz orientieren. In Bezug auf Früherkennung und Prädiktion neurodegenerativer Erkrankungen gehen sie über die S3LL-Demenz hinaus. So werden leichte kognitive Störungen (MCI) als Risiko- oder auch Prodromalstadien neurodegenerativer Demenzen verstanden und Biomarker regelhaft für eine ätiologische (Früh- und Differenzial‑)Diagnostik eingesetzt. Es soll eine enge Vernetzung mit den diagnostischen Fachdisziplinen bestehen. Ferner sollen sie Beratung zu sozialen und rechtlichen Fragen sowie Angehörigenberatung anbieten. Aktuelle Erkenntnisse aus der Forschung sollen durch sie frühzeitig in die Versorgung integriert werden. GA sind damit regionale Expertenzentren.

Diskussion

Gedächtnisambulanzen implementieren den evidenzbasierten Standard in Diagnostik und Therapie in die klinische Versorgung von Patienten mit kognitiven Störungen im Alter. Zusätzlich führen sie diagnostische und therapeutische Innovationen in die Versorgung dieser Patienten ein. Ihre Rolle in der Regelversorgung muss gestärkt werden, wobei auch Finanzierungsfragen geklärt werden müssen, da die derzeitigen Finanzierungsmodelle in der Regel nicht kostendeckend sind.

Structural magnetic resonance imaging for the early diagnosis of dementia due to Alzheimer's disease in people with mild cognitive impairment

BACKGROUND

Mild cognitive impairment (MCI) due to Alzheimer's disease is the symptomatic predementia phase of Alzheimer's disease dementia, characterised by cognitive and functional impairment not severe enough to fulfil the criteria for dementia. In clinical samples, people with amnestic MCI are at high risk of developing Alzheimer's disease dementia, with annual rates of progression from MCI to Alzheimer's disease estimated at approximately 10% to 15% compared with the base incidence rates of Alzheimer's disease dementia of 1% to 2% per year.

OBJECTIVES

To assess the diagnostic accuracy of structural magnetic resonance imaging (MRI) for the early diagnosis of dementia due to Alzheimer's disease in people with MCI versus the clinical follow-up diagnosis of Alzheimer's disease dementia as a reference standard (delayed verification). To investigate sources of heterogeneity in accuracy, such as the use of qualitative visual assessment or quantitative volumetric measurements, including manual or automatic (MRI) techniques, or the length of follow-up, and age of participants. MRI was evaluated as an add-on test in addition to clinical diagnosis of MCI to improve early diagnosis of dementia due to Alzheimer's disease in people with MCI.

SEARCH METHODS

On 29 January 2019 we searched Cochrane Dementia and Cognitive Improvement's Specialised Register and the databases, MEDLINE, Embase, BIOSIS Previews, Science Citation Index, PsycINFO, and LILACS. We also searched the reference lists of all eligible studies identified by the electronic searches.

SELECTION CRITERIA

We considered cohort studies of any size that included prospectively recruited people of any age with a diagnosis of MCI. We included studies that compared the diagnostic test accuracy of baseline structural MRI versus the clinical follow-up diagnosis of Alzheimer's disease dementia (delayed verification). We did not exclude studies on the basis of length of follow-up. We included studies that used either qualitative visual assessment or quantitative volumetric measurements of MRI to detect atrophy in the whole brain or in specific brain regions, such as the hippocampus, medial temporal lobe, lateral ventricles, entorhinal cortex, medial temporal gyrus, lateral temporal lobe, amygdala, and cortical grey matter.

DATA COLLECTION AND ANALYSIS

Four teams of two review authors each independently reviewed titles and abstracts of articles identified by the search strategy. Two teams of two review authors each independently assessed the selected full-text articles for eligibility, extracted data and solved disagreements by consensus. Two review authors independently assessed the quality of studies using the QUADAS-2 tool. We used the hierarchical summary receiver operating characteristic (HSROC) model to fit summary ROC curves and to obtain overall measures of relative accuracy in subgroup analyses. We also used these models to obtain pooled estimates of sensitivity and specificity when sufficient data sets were available.

MAIN RESULTS

We included 33 studies, published from 1999 to 2019, with 3935 participants of whom 1341 (34%) progressed to Alzheimer's disease dementia and 2594 (66%) did not. Of the participants who did not progress to Alzheimer's disease dementia, 2561 (99%) remained stable MCI and 33 (1%) progressed to other types of dementia. The median proportion of women was 53% and the mean age of participants ranged from 63 to 87 years (median 73 years). The mean length of clinical follow-up ranged from 1 to 7.6 years (median 2 years). Most studies were of poor methodological quality due to risk of bias for participant selection or the index test, or both. Most of the included studies reported data on the volume of the total hippocampus (pooled mean sensitivity 0.73 (95% confidence interval (CI) 0.64 to 0.80); pooled mean specificity 0.71 (95% CI 0.65 to 0.77); 22 studies, 2209 participants). This evidence was of low certainty due to risk of bias and inconsistency. Seven studies reported data on the atrophy of the medial temporal lobe (mean sensitivity 0.64 (95% CI 0.53 to 0.73); mean specificity 0.65 (95% CI 0.51 to 0.76); 1077 participants) and five studies on the volume of the lateral ventricles (mean sensitivity 0.57 (95% CI 0.49 to 0.65); mean specificity 0.64 (95% CI 0.59 to 0.70); 1077 participants). This evidence was of moderate certainty due to risk of bias. Four studies with 529 participants analysed the volume of the total entorhinal cortex and four studies with 424 participants analysed the volume of the whole brain. We did not estimate pooled sensitivity and specificity for the volume of these two regions because available data were sparse and heterogeneous. We could not statistically evaluate the volumes of the lateral temporal lobe, amygdala, medial temporal gyrus, or cortical grey matter assessed in small individual studies. We found no evidence of a difference between studies in the accuracy of the total hippocampal volume with regards to duration of follow-up or age of participants, but the manual MRI technique was superior to automatic techniques in mixed (mostly indirect) comparisons. We did not assess the relative accuracy of the volumes of different brain regions measured by MRI because only indirect comparisons were available, studies were heterogeneous, and the overall accuracy of all regions was moderate.

AUTHORS' CONCLUSIONS

The volume of hippocampus or medial temporal lobe, the most studied brain regions, showed low sensitivity and specificity and did not qualify structural MRI as a stand-alone add-on test for an early diagnosis of dementia due to Alzheimer's disease in people with MCI. This is consistent with international guidelines, which recommend imaging to exclude non-degenerative or surgical causes of cognitive impairment and not to diagnose dementia due to Alzheimer's disease. In view of the low quality of most of the included studies, the findings of this review should be interpreted with caution. Future research should not focus on a single biomarker, but rather on combinations of biomarkers to improve an early diagnosis of Alzheimer's disease dementia.

Aβ and the dementia syndrome: Simple versus complex perspectives

BACKGROUND

The amyloid cascade hypothesis (ACH) has dominated strategy in dementia research for decades despite evidence of its limitations including known heterogeneity of the dementia syndrome in the population and the narrow focus on a single molecule - the amyloid beta protein (Aβ) as causal for all Alzheimer-type dementia. Other hypotheses relevant to Aβ are the presenilin (PS) hypothesis (PSH) relating to the involvement of PS in the generation of Aβ, and the amyloid precursor protein (APP) matrix approach (AMA), relating to the complex and dynamic breakdown of APP, from which Aβ derives.

MATERIALS AND METHODS

In this article we explore perspectives relating to complex disorders occurring mainly in older populations through a detailed case study of the role of Aβ in AD.

RESULTS

Scrutiny of the evidence generated so far reveals and a lack of understanding of the wider APP proteolytic system and how narrow research into the dementia syndrome has been to date. Confounding factors add significant limitations to the understanding of the current evidence base.

CONCLUSIONS

A better characterisation of the entire APP proteolytic system in the human brain is urgently required to place Aβ in its complex physiological context. From a molecular perspective, a combination of the alternative hypotheses, the PSH and the AMA may better describe the complexity of the APP proteolytic system leading to new therapeutic approaches. The reductionist approach is widespread throughout biomedical research and this example highlights how neglect of complexity can undermine investigations of complex disorders, particularly those arising in the oldest in our populations.

Biomarkers study in atypical dementia: proof of a diagnostic work-up

BACKGROUND

An early differentiation between Alzheimer's Disease (AD) and other dementias is crucial for an adequate patients' management, albeit it may result difficult for the occurrence of "atypical presentations." Current diagnostic criteria recognize the importance of biomarkers for AD diagnosis, but still an optimal diagnostic work-up isn't available.

OBJECTIVE

Evaluate the utility and reproducibility of biomarkers and propose an "optimal" diagnostic work-up in atypical dementia.

METHODS

(1) a retrospective selection of "atypical dementia cases"; (2) a repetition of diagnostic assessment by two neurologists following two different diagnostic work-ups, each consisting of multiple steps; (3) a comparison between diagnostic accuracy and confidence reached at each step by both neurologists and evaluation of the inter-rater agreement.

RESULTS

In AD, regardless of the undertaken diagnostic work-up, a significant gain in accuracy was reached by both neurologists after the second step, whereas in frontotemporal dementia (FTD), adding subsequent steps was not always sufficient to increase significantly the baseline accuracy. A relevant increment in diagnostic confidence was detectable after studying pathophysiological markers in AD, and after assessing brain metabolism in FTD. The inter-rater agreement was higher at the second step for the AD group when the pathophysiological markers were available and for the FTD group when the results of FDG-PET were accessible.

CONCLUSIONS

In atypical cases of dementia, biomarkers significantly raise diagnostic accuracy, confidence, and agreement. This study introduces a proof of diagnostic work-up that combines imaging and CSF biomarkers and suggests distinct ways to proceed on the basis of a greater diagnostic likelihood.

Combination of Structural MRI and FDG-PET of the Brain Improves Diagnostic Accuracy in Newly Manifested Cognitive Impairment in Geriatric Inpatients

BACKGROUND

The cause of cognitive impairment in acutely hospitalized geriatric patients is often unclear. The diagnostic process is challenging but important in order to treat potentially life-threatening etiologies or identify underlying neurodegenerative disease.

OBJECTIVE

To evaluate the add-on diagnostic value of structural and metabolic neuroimaging in newly manifested cognitive impairment in elderly geriatric inpatients.

METHODS

Eighty-one inpatients (55 females, 81.6±5.5 y) without history of cognitive complaints prior to hospitalization were recruited in 10 acute geriatrics clinics. Primary inclusion criterion was a clinical hypothesis of Alzheimer's disease (AD), cerebrovascular disease (CVD), or mixed AD+CVD etiology (MD), which remained uncertain after standard diagnostic workup. Additional procedures performed after enrollment included detailed neuropsychological testing and structural MRI and FDG-PET of the brain. An interdisciplinary expert team established the most probable etiologic diagnosis (non-neurodegenerative, AD, CVD, or MD) integrating all available data. Automatic multimodal classification based on Random Undersampling Boosting was used for rater-independent assessment of the complementary contribution of the additional diagnostic procedures to the etiologic diagnosis.

RESULTS

Automatic 4-class classification based on all diagnostic routine standard procedures combined reproduced the etiologic expert diagnosis in 31% of the patients (p = 0.100, chance level 25%). Highest accuracy by a single modality was achieved by MRI or FDG-PET (both 45%, p≤0.001). Integration of all modalities resulted in 76% accuracy (p≤0.001).

CONCLUSION

These results indicate substantial improvement of diagnostic accuracy in uncertain de novo cognitive impairment in acutely hospitalized geriatric patients with the integration of structural MRI and brain FDG-PET into the diagnostic process.

Understanding the roles of mutations in the amyloid precursor protein in Alzheimer disease

Many models of disease progression in Alzheimer's disease (AD) have been proposed to help guide experimental design and aid the interpretation of results. Models focussing on the genetic evidence include the amyloid cascade (ACH) and presenilin (PSH) hypotheses and the amyloid precursor protein (APP) matrix approach (AMA), of which the ACH has held a dominant position for over two decades. However, the ACH has never been fully accepted and has not yet delivered on its therapeutic promise. We review the ACH, PSH and AMA in relation to levels of APP proteolytic fragments reported from AD-associated mutations in APP. Different APP mutations have diverse effects on the levels of APP proteolytic fragments. This evidence is consistent with at least three disease pathways that can differ between familial and sporadic AD and two pathways associated with cerebral amyloid angiopathy. We cannot fully evaluate the ACH, PSH and AMA in relation to the effects of mutations in APP as the APP proteolytic system has not been investigated systematically. The confounding effects of sequence homology, complexity of competing cleavages and antibody cross reactivities all illustrate limitations in our understanding of the roles these fragments and the APP proteolytic system as a whole in normal aging and disease play. Current experimental design should be refined to generate clearer evidence, addressing both aging and complex disorders with standardised reporting formats. A more flexible theoretical framework capable of accommodating the complexity of the APP proteolytic system is required to integrate available evidence.

Biomarkers for Alzheimer Disease: Classical and Novel Candidates' Review

The biomarkers may be useful for predictive diagnosis of Alzheimer's disease (AD). The current challenge is to diagnose it in its preclinical phase. The combination of cerebrospinal fluid (CSF) biomarkers and imaging has been investigated extensively for a number of years. It can provide an increased diagnostic accuracy. This review discusses the contribution of classical biomarkers to predict AD and highlights novel candidates identified as potential markers for AD. We referred to the electronic databases PubMed/Medline and Web of Science to search for articles that were published until February 2016. Sixty-two records were included in qualitative synthesis. In the first section, the results show the contribution of biomarkers to predict and track AD considered as classical biomarkers. In the second section, the results highlight the involvement of novel candidates that should be considered for future evaluation in the characterization of the AD progression. Reported findings open prospect to define noninvasive biomarkers to predict AD before symptoms onset.

Feasibility and acceptance of simultaneous amyloid PET/MRI

PURPOSE

Established Alzheimer's disease (AD) biomarker concepts classify into amyloid pathology and neuronal injury biomarkers, while recent alternative concepts classify into diagnostic and progression AD biomarkers. However, combined amyloid positron emission tomography/magnetic resonance imaging (PET/MRI) offers the chance to obtain both biomarker category read-outs within one imaging session, with increased patient as well as referrer convenience. The aim of this pilot study was to investigate this matter for the first time.

METHODS

100 subjects (age 70 ± 10 yrs, 46 female), n = 51 with clinically defined mild cognitive impairment (MCI), n = 44 with possible/probable AD dementia, and n = 5 with frontotemporal lobe degeneration, underwent simultaneous [¹⁸F]florbetaben or [¹¹C]PIB PET/MRI (3 Tesla Siemens mMR). Brain amyloid load, mesial temporal lobe atrophy (MTLA) by means of the Scheltens scale, and other morphological brain pathologies were scored by respective experts. The patients/caregivers as well as the referrers were asked to assess on a five-point scale the convenience related to the one-stop-shop PET and MRI approach.

RESULTS

In three subjects, MRI revealed temporal lobe abnormalities other than MTLA. According to the National Institute on Aging-Alzheimer's Association classification, the combined amyloid-beta PET/MRI evaluation resulted in 31 %, 45 %, and 24 % of the MCI subjects being categorized as "MCI-unlikely due to AD", "MCI due to AD-intermediate likelihood", and "MCI due to AD-high likelihood", respectively. 50 % of the probable AD dementia patients were categorized as "High level of evidence of AD pathophysiological process", and 56 % of the possible AD dementia patients as "Possible AD dementia - with evidence of AD pathophysiological process". With regard to the International Working Group 2 classification, 36 subjects had both positive diagnostic and progression biomarkers. The patient/caregiver survey revealed a gain of convenience in 88 % of responders as compared to a theoretically separate PET and MR imaging. In the referrer survey, an influence of the combined amyloid-beta PET/MRI on the final diagnosis was reported by 82 % of responders, with a referrer acceptance score of 3.7 ± 1.0 on a 5-point scale.

CONCLUSION

Simultaneous amyloid PET/MRI is feasible and provides imaging biomarkers of all categories which are able to supplement the clinical diagnosis of MCI due to AD and that of AD dementia. Further, patient and referrer convenience is improved by this one-stop-shop imaging approach.