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We present a practical guide for the implementation of recently revised National Institute on Aging-Alzheimer’s Association guidelines for the neuropathologic assessment of Alzheimer’s disease (AD). Major revisions from previous consensus criteria are: (i) recognition that AD neuropathologic changes may occur in the apparent absence of cognitive impairment, (ii) an “ABC” score for AD neuropathologic change that incorporates histopathologic assessments of amyloid β deposits (A), staging of neurofibrillary tangles (B), and scoring of neuritic plaques (C), and (iii) more detailed approaches for assessing commonly co-morbid conditions such as Lewy body disease, vascular brain injury, hippocampal sclerosis, and TAR DNA binding protein (TDP)-43 immunoreactive inclusions. Recommendations also are made for the minimum sampling of brain, preferred staining methods with acceptable alternatives, reporting of results, and clinico-pathologic correlations.
A consensus panel from the United States and Europe was convened recently to update and revise the 1997  consensus guidelines for the neuropathologic evaluation of Alzheimer’s disease (AD). Here we summarize these consensus guidelines  and provide direction for their application.
“Alzheimer’s disease” refers to a constellation of cognitive and behavioral changes that are typical for patients who have substantial amounts of its hallmark lesions [6,39]. Similar neuropathologic changes, albeit usually at lower levels, also occur in individuals who did not show cognitive or behavioral impairments during life . We recommend that neuropathologists adopt the term “AD neuropathologic change” and report on the presence and extent of lesions observed at autopsy regardless of the individual’s cognitive state, or even if cognitive state is not known.
AD neuropathologic change should be assessed in all cases of dementia. There are many other neurodegenerative disorders that can cause dementia in addition to those discussed here, and any may be co-morbid with AD neuropathologic change, especially in the elderly. We recommend that all lesions be documented for type and extent of co-morbidity in brains of individuals with AD neuropathologic change.
Multiple diseases in cerebrum can conspire to worsen cognitive symptoms; however, it often is difficult to judge the extent to which each disease observed at autopsy may have contributed to a given patient’s cognitive state. It is essential that the extent of AD neuropathologic change, as well as neuropathologic findings for any other disease(s) that may have contributed to cognitive impairment or dementia, be correlated with clinical, neuropsychological, neuro-imaging, and other laboratory data as part of the neuropathology report.
Autopsy should follow best practices and local regulations. Gross inspection of brain should include assessment of regional atrophy and blood vessels for cerebrovascular disease (CVD). We recommend a minimum sampling of brain regions in Table 1. All gross lesions also should be sampled. Each of the following sections indicates preferred and acceptable alternative methods for detection of neuropathologic changes. While we do not propose specific protocols, our recommendations are compatible with conclusions by others who have optimized reproducibility of some neuropathologic assessments across multiple sites [2-5,41]. We anticipate further efforts for increased harmonization and molecular specification. Finally, although organized for brain autopsy, these neuropathologic assessments can be applied to specimens from surgery; however, regional evaluation will be limited in biopsy specimens.
Senile plaques, which are extracellular deposits of the β-amyloid (Aβ) peptides, and neurofibrillary degeneration, which is best exemplified by neurofibrillary tangles (NFTs), are considered essential neuropathologic features of AD. Accumulation of Aβ plaques and NFTs follows distinct regional progressions across brain regions as AD advances. We recommend continued use of the staging scheme for neurofibrillary degeneration as described originally by Braak and Braak [14,15], reduced to four stages (Table 2) that improves inter-rater reliability . We recommend a modified version of Thal phases of Aβ plaque accumulation , adapted to a four-point scale (Table 2). A subset of senile plaques called neuritic plaques appear closely associated with neuronal injury and are characterized by the occurrence of dystrophic neurites that frequently have phospho-tau immunoreactivity [19,36,37,55]. We recommend continued use of the Consortium to Establish a Registry for AD (CERAD) protocol for neuritic plaque scoring  (Table 2).
Preferred method for β-amyloid (Aβ) plaques is immunohistochemistry for Aβ, and for neurofibrillary tangles (NFTs) is immunohistochemistry for tau or phospho-tau . Other acceptable methods are Thioflavin S or sensitive silver histochemical stains . It is important to stress that neuritic plaques need to be distinguished from Aβ deposits by special stains. Preferred methods for detection of neuritic processes in senile plaques are Thioflavin S or modified Bielschowsky stain ; immunohistochemical stains for neuritic processes, such as amyloid precursor protein, ubiquitin, neurofilament or phospho-tau, will identify specific, and partially overlapping, subtypes of dystrophic neurites that may differ in disease relevance .
AD neuropathologic change should be ranked along three parameters, Aβ plaque score (Figure 1) , Braak NFT stage (Figure 2; silver-based histochemistry  or phospho-tau immunohistochemistry ), and CERAD neuritic plaque score (Figure 3)  to obtain an “ABC score” (Table 2). Although cerebral amyloid angiopathy (CAA) is not considered in the “ABC” score, it is very commonly observed in cases with parenchymal Aβ plaques and should be evaluated and reported systematically as well as appreciated for its potential pathophysiological significance [56,61].
For all cases, regardless of clinical history, reporting should follow the format of these examples: “Alzheimer Disease Neuropathologic Changes: A1, B0, C0” or “Alzheimer Disease Neuropathologic Changes: A3, B3, C3”. Using the system shown in Table 3, the ABC scores are transformed into one of four levels of AD neuropathologic change: Not, Low, Intermediate or High.
Clinico-pathologic correlations for individuals without cognitive impairment should indicate that it is possible for AD neuropathologic change to predate onset of symptoms by years . For individuals with cognitive impairment at the time tissue was obtained, “Intermediate” or “High” level (Table 3 black background) of AD neuropathologic change should be considered adequate explanation of cognitive impairment or dementia, and should be reported with a final diagnosis of Alzheimer’s disease. “Low” level of AD neuropathologic change is not considered adequate explanation for cognitive impairment or dementia. In all cases with cognitive impairment, regardless of the extent of AD neuropathologic change, it is essential to determine the presence or absence, as well as extent, of other disease(s) that might have contributed to the clinical deficits. For cases with incomplete clinical history, higher levels of AD neuropathologic change typically are correlated with greater likelihood of cognitive impairment .
Lewy body disease (LBD), including Parkinson’s disease and dementia with Lewy bodies (DLB), shares abnormal accumulation of α-synuclein within inclusions called LBs, as well as α-synuclein-immunoreactive neurites (so called “Lewy neurites”) and diffuse cytoplasmic immunoreactivity. LBs are frequent in the setting of moderate-to-severe levels of AD neuropathologic change [27,59], including some early-onset familial AD cases [32,33]. Given our focus on cognitive impairment and dementia rather than movement disorders , we recommend a modification of previous consensus guidelines  to classify LBD in five stages.
LBs may be detected in neurons of medulla, pons and midbrain with hematoxylin and eosin (H&E)-stained sections; however, greater sensitivity can be achieved with immunohistochemistry for α-synuclein and this approach is strongly preferred [3,4,13]. Abnormal neuropil and neuronal cytoplasmic α-synuclein immunoreactivity are usually present with LBs but will not be apparent by H&E; in some instances, these changes occur in the absence of LBs.
Classification of all types of LBD should fall into a one of five categories following a modification of existing criteria (Figure 4) [16,38]: none, brainstem-predominant, limbic (transitional), neocortical (diffuse), or amygdala-predominant (Table 4). Although not part of this classification scheme, it is important to realize that LBD also occurs frequently in the olfactory bulb in older adults and should be sampled when possible .
For all cases, regardless of clinical history, reporting should follow the format of these examples: “Lewy body disease, limbic” or “Lewy body disease, amygdala-predominant”.
Clinico-pathologic correlation for individuals without cognitive impairment should indicate that, although much less common than AD neuropathologic change, LBD has been observed in individuals without apparent cognitive or motor deficit [1,49]; this may represent pre-clinical LBD [12,22,31]. For individuals with cognitive impairment, we recommend that “Neocortical LBD” be considered adequate explanation of cognitive impairment or dementia (Table 4 shaded gray); however, this does not preclude contribution from other diseases. “Brainstem-predominant LBD” in the setting of cognitive impairment should prompt consideration of other diseases. “Amygdala-predominant LBD” typically occurs in the context of advanced AD neuropathologic change . For cases with incomplete clinical history, we note that “Neocortical LBD” is correlated with greater likelihood of cognitive impairment [1,11-13,22,31-33,38,46,49,52].
CVD and vascular brain injury (VBI)  commonly are encountered in the brains with AD neuropathologic change . We recommend reporting all macroscopic VBI and enumerating microvascular lesions or MVLs (microscopic infarcts/hemorrhages) in standard screening sections [26,51,53,60]. Diffuse white matter injury is a form of VBI but it is more difficult to judge objectively and is not specific to VBI.
VBI, including MVLs (Figure 5), may occur in any region of brain but only those MVLs in the standardized sections (Table 1 shaded green) should be enumerated when correlating with cognitive impairment or dementia.
All infarcts and hemorrhages should be documented including location, size, and age.
Reporting should follow the format of these examples: “Cerebrovascular disease: Atherosclerosis, moderate, non-occlusive, affecting basilar artery, left internal carotid artery and middle cerebral artery; Arteriolosclerosis, severe, widespread involvement of hemispheric white matter” or “Vascular brain injury: Infarct in the territory of the left middle cerebral artery, remote, measuring 3 × 3 × 2 cm; Lacunar infarct, right anterior caudate, remote, measuring 0.5 × 0.3 × 0.2 cm; Microvascular lesions: 2 remote lesions detected on standard sections (right middle frontal gyrus and right inferior parietal lobule)”
Clinico-pathologic correlations for grossly visible infarcts or hemorrhages should follow classic neuropathologic approaches. Although there are some differences in approach, guidelines have emerged for clinico-pathologic correlation of MVLs: one MVL identified in standard sections of brain (Table 1 regions shaded green) is of unclear relationship to cognitive function, while multiple MVLs in these regions are associated with increased likelihood of cognitive impairment or dementia [24,52,62].
Hippocampal sclerosis (HS) is defined by pyramidal cell loss and gliosis in CA1 and subiculum of the hippocampal formation that is out of proportion to AD neuropathologic change in the same structures . HS can be observed in the context of AD neuropathologic change, frontotemporal lobar degeneration (FTLD), and VBI, likely reflecting a heterogeneous etiology. We recommend that HS be reported as present or absent.
TDP-43 immunoreactive inclusions are observed in the majority of cases of HS (Figure 6) [8,28,40,64], about one-half of cases with FTLD and ubiquitin inclusions with or without motor neuron disease, most sporadic cases of amyotrophic lateral sclerosis, and commonly in cases with AD neuropathologic change [8,9] or with LBD , as well as other neurodegenerative diseases . With the exception of individuals with mutations in TARDBP, GRN, VCP or c9ORF72 [10,18], it is not clear whether TDP-43 proteinopathy in these other neurodegenerative diseases is a primary, secondary, or coincidental event .
HS should be evaluated by hematoxylin and eosin (H&E)-stained sections together with neurofibrillary tangle (NFT) stains. HS can be focal, thus its absence in the recommended screening section does not rule out the possibility of HS elsewhere in the hippocampal formation. If HS is present, further evaluation is indicated, including TDP-43 immunohistochemistry. If work up is negative for TDP-43 but there is other evidence to suggest FTLD, consider immunohistochemistry for phospho-tau, ubiquitin, or “fused in sarcoma” (FUS). In the absence of HS, screening for TDP-43 inclusions as part of evaluating AD neuropathologic change is of unclear value.
HS should be reported as present or absent with a description of immunohistochemistry results.
Clinico-pathological correlations need to recognize that HS can occur in several different diseases and has varying clinical implications in different settings. Relatively isolated HS may occur in very old individuals, and in this context it is associated with TDP-43-immunoreactive inclusions and with cognitive impairment [45,47].
Both of these classes of neurodegenerative diseases are complex and beyond the scope of this summary. Nevertheless, each must be distinguished from AD neuropathologic change. We refer the reader to recent consensus guidelines for the neuropathologic evaluation of FTLD and its subtypes [17,34,35], and issues important in the distinction of AD neuropathologic change from those of some forms of FTLD . Finally, not only can the neuropathologic changes of prion disease be co-morbid with AD, but some forms of prion disease can overlap with AD and need to be distinguished with special stains .
The new consensus criteria recognize the continuum of AD neuropathologic change that underlies the progression of this disease from preclinical to dementia stage. The new consensus criteria also amplify methods for evaluating Aβ plaques, better define intermediate levels of AD pathologic change, and emphasize a structured approach to commonly co-morbid diseases.
Support was provided by the National Institute on Aging (AG05134, AG05136, AG03991, AG05681, AG24904, AG28383, AG10161, AG17917, AG10124, AG13854, AG19610, NS62684, AG16570, AG15819, AG12435, AG05131, AG016976, AG16570, AG15866 and AG18840) as well as the Alzheimer’s Association, Arizona Department of Health Services (contract 211002, Arizona Alzheimer’s Consortium), Deutsche Forschungsgemeinschaft (DFG TH-624-4-1) and Alzheimer Forschung Initiative (AFI #10810), the Nancy and Buster Alvord Endowment, and the Charles and Joanne Knight Alzheimer Research Initiative. We thank Dr. Joshua Sonnen for providing some photomicrographs. The authors extend their deepest thanks to Dr. Heiko Braak, Dr. Kelly Del Tredici, Dr. Nina Silverberg, Dr. Walter Kukull, Dr. Kathleen Montine, Dr. Cerise Elliott, Dr. Bill Thies, Dr. Maria C. Carrillo, and Ms. Sarah Monsell for their valuable input.