The push to extend hypothetical diagnostic criteria for AD to preclinical and prodromal phases may be premature whilst the assumptions underlying the criteria remain unproven
In 2011 recommendations were published for identifying prodromal (‘mild cognitive impairment (MCI) due to AD [
2]’) and preclinical (‘AD-pathophysiological process [
255]’) stages of AD. Subsequently, proposals that these stages could be identified using in vivo ‘biomarker’ evidence of Aβ and tau pathology, irrespective of cognitive changes, have emerged.
The recent NIA-AA commissioned workgroup consolidated these proposals, and spoke of an ‘Alzheimer’s continuum’ [
119]. This was defined as:
“…individuals with biomarker designation of either AD or Alzheimer’s pathologic change
”, wherein “biomarker designation of AD
” refers to in vivo evidence of Aβ and tau pathology and “Alzheimer’s pathologic change
” refers to in vivo evidence of Aβ pathology alone (with normal tau biomarkers). In this system, evidence of abnormal tau and/or neurodegeneration biomarkers, in the absence of Aβ pathology, are defined as "non-AD pathologic change". Importantly, the IWG have also proposed guidelines for the identification of these states with the use of biomarkers [
69], but are less definitive than the NIA-AA workgroup on the primacy of Aβ measurements. They still allow for the possibility that those with evidence of tau pathology, in the absence of amyloid, are also at-risk for AD dementia [
69]. As noted [
119], the two advisory bodies are, however, harmonised on the concept of using the label ‘Alzheimer’s’ when Aβ and tau pathology are found, irrespective of the cognitive diagnosis.
Collectively therefore, these proposals appear to argue for the terms ‘Alzheimer’s continuum’, ‘Alzheimer’s Disease’ or ‘at-risk for Alzheimer’s’ being used only when Aβ (and, in the case of the IWG, tau) pathology is detected, irrespective of a clinical diagnosis or age. In turn, without such evidence, it is proposed the label ‘Alzheimer’s’ should not be applied.
Although these recommendations could be seen as merely a subtle extension of the neuropathological diagnostic principles that Aβ and tau pathology are the definitive neuropathological ‘proof’ of AD, they are in fact a radical departure from the traditional use of the label ‘Alzheimer’s’ only when the AD cognitive phenotype is identified. Indeed, the publication by the workgroup runs the risk of formalising the idea that AD cannot be hypothetically explained without accounting for the presence of Aβ and tau pathology, as encapsulated in the following statement:
We emphasise though that A and T proteinopathies define AD as a unique disease among the many that can lead to dementia. As a consequence, disease models where A and T are not in the primary causal pathway must provide a mechanistic explanation for the development of both of these diagnostic proteinopathies, as well as neurodegeneration and clinical symptoms. [
119]
This statement presents two major problems. First, it ignores contradictory literature on the absolute validity of Assumptions 1 and 2 (Box
1), which we discuss below in Questions 1–5. Second, it ignores equally valid alternate hypotheses of disease aetiology that do not require an Aβ (and/or tau) basis. We therefore view the statement as an inaccurate representation of the current state of AD research.
We do not see the problem lying with the hypothesis that Aβ and tau biomarkers could predict disease—it is a valid and testable idea. Instead, the crux of the problem is that this statement perpetuates the idea “A and T proteinopathies” define AD dementia as a priori fact, when this remains uncertain. Potentially more worrying is that in labelling “A and T proteinopathies” under the Alzheimer’s name in the absence of the cognitive phenotype, the inference is made that effective treatments for AD might now be defined by their ability to treat “A and T proteinopathies”, whether or not there is proven clinical benefit of such treatments.
Granted, the NIA-AA workgroup acknowledged disease causation may be independent of Aβ and tau, and went to considerable lengths to stress many clinicians and researchers do not necessarily wish to adopt proposed nomenclature (noting also that it is premature to extend these criteria to the clinic). However, busy clinicians and the public at large may not appreciate this subtle point. Instead, the take-away summary for most is likely to be that AD is to be defined and thus understood through an Aβ lens. Certainly, in the absence of other readily available biological markers of AD for clinicians, Aβ pathology may be a useful marker for AD dementia risk. However, as we discuss below, the literature suggests the causality of AD dementia is more complex than can be accounted for by the amyloid hypothesis.
Hence, these guidelines present a hypothetical idea, that in vivo measurements of Aβ (± tau) pathology can accurately predict disease, built upon another hypothetical idea, that Aβ and tau pathology mark a specific clinical disorder (AD dementia) and therefore provide clues to aetiology. In removing the safeguard of the clinical AD diagnosis, these recommendations can be viewed as an attempt to bypass many long-held concerns regarding the validity of these hypotheses. In the following we raise key questions testing Assumptions 1 and 2 (Box
1), illustrating there is still significant discrepant evidence to address before concluding the presence of Aβ pathology is definitive for AD or aetiologically significant. If these assumptions are ultimately proven incorrect, the push to defining disease on their bases presymptomatically is running the risk of sending research into aetiology down the wrong path.