The paper by Dominic Holland et al. reporting that AD-associated decline slows with advancing age is interesting. Although no autopsy confirmation was available in this study, these data agree with neuropathologic studies in the oldest-old (Haroutunian et al., 2008).

However, there are some limitations of this study, which only in part have been mentioned by the authors:

1. The importance of confounding pathologies, in particular, cerebrovascular lesions including the importance of CAA (the authors only mentioned microvascular pathology), Lewy body pathology, argyrophilic grain disease, hippocampal atrophy, etc., which are frequent in aged human brains (see "mixed dementia," e.g., Kovacs et al., 2008; Jellinger and Attems, 2011). It should be emphasized that up to two-thirds of aged human brains contain non-AD type pathologies (Nelson et al., 2007, and others).

2. A high percentage of demented persons aged 80+ do not meet the morphological criteria of AD and are classified "dementia of unknown etiology" (Crystal et al., 1988; Jellinger, 2001; Corrada et al., 2012),...  Read more

The paper by Dominic Holland et al. reporting that AD-associated decline slows with advancing age is interesting. Although no autopsy confirmation was available in this study, these data agree with neuropathologic studies in the oldest-old (Haroutunian et al., 2008).

However, there are some limitations of this study, which only in part have been mentioned by the authors:

1. The importance of confounding pathologies, in particular, cerebrovascular lesions including the importance of CAA (the authors only mentioned microvascular pathology), Lewy body pathology, argyrophilic grain disease, hippocampal atrophy, etc., which are frequent in aged human brains (see "mixed dementia," e.g., Kovacs et al., 2008; Jellinger and Attems, 2011). It should be emphasized that up to two-thirds of aged human brains contain non-AD type pathologies (Nelson et al., 2007, and others).

2. A high percentage of demented persons aged 80+ do not meet the morphological criteria of AD and are classified "dementia of unknown etiology" (Crystal et al., 1988; Jellinger, 2001; Corrada et al., 2012), which cannot be detected without morphological verification.

In general, density and pattern of neurofibrillary tangles (NFTs) show significant correlations with the severity of cognitive decline across old age including 90+ patients, at least in those without other pathologies superimposed (Nelson et al., 2007; Nelson et al., 2010; Nelson et al., 2012).

It should further be considered that there are several morphological subtypes of AD, which differ in age, duration, and dementia severity (Murray et al., 2011; Jellinger, 2012).

All together, these and other data indicate that clinical detection of AD and its distinction from normal (and "pathological") aging—the latter featured by generalized Aβ deposition with only little tau pathology limited to the limbic areas—are more difficult in the oldest old, as stated by the authors.

References:
Corrada MM, Berlau DJ, Kawas CH (2012) A population-based clinicopathological study in the oldest-old: the 90+ study. Curr Alzheimer Res 9:709-717. Abstract

Crystal H, Dickson D, Fuld P, Masur D, Scott R, Mehler M, Masdeu J, Kawas C, Aronson M, Wolfson L (1988) Clinico-pathologic studies in dementia: nondemented subjects with pathologically confirmed Alzheimer's disease. Neurology 38:1682-1687. Abstract

Haroutunian V, Schnaider-Beeri M, Schmeidler J, Wysocki M, Purohit DP, Perl DP, Libow LS, Lesser GT, Maroukian M, Grossman HT (2008) Role of the neuropathology of Alzheimer disease in dementia in the oldest-old. Arch Neurol 65:1211-1217. Abstract

Jellinger K (2001) Frequency of "dementia of unknown etiology" increases with age. Arch Neurol 58:1498-1499. Abstract

Jellinger KA (2012) Neuropathological subtypes of Alzheimer's disease. Acta Neuropathol 123:153-154. Abstract

Jellinger KA, Attems J (2011) Prevalence and pathology of dementia with Lewy bodies in the oldest old: a comparison with other dementing disorders. Dement Geriatr Cogn Disord 31:309-316. Abstract

Kovacs GG, Alafuzoff I, Al-Sarraj S, Arzberger T, Bogdanovic N, Capellari S, Ferrer I, Gelpi E, Kovari V, Kretzschmar H, Nagy Z, Parchi P, Seilhean D, Soininen H, Troakes C, Budka H (2008) Mixed brain pathologies in dementia: the BrainNet Europe consortium experience. Dement Geriatr Cogn Disord 26:343-350. Abstract

Murray ME, Graff-Radford NR, Ross OA, Petersen RC, Duara R, Dickson DW (2011) Neuropathologically defined subtypes of Alzheimer's disease with distinct clinical characteristics: a retrospective study. Lancet Neurol 10:785-796. Abstract

Nelson PT, Abner EL, Schmitt FA, Kryscio RJ, Jicha GA, Smith CD, Davis DG, Poduska JW, Patel E, Mendiondo MS, Markesbery WR (2010) Modeling the association between 43 different clinical and pathological variables and the severity of cognitive impairment in a large autopsy cohort of elderly persons. Brain Pathol 20:66-79. Abstract

Nelson PT, Alafuzoff I, Bigio EH, Bouras C, Braak H, Cairns NJ, Castellani RJ, Crain BJ, Davies P, Del Tredici K, Duyckaerts C, Frosch MP, Haroutunian V, Hof PR, Hulette CM, Hyman BT, Iwatsubo T, Jellinger KA, Jicha GA, Kovari E, Kukull WA, Leverenz JB, Love S, Mackenzie IR, Mann DM, Masliah E, McKee AC, Montine TJ, Morris JC, Schneider JA, Sonnen JA, Thal DR, Trojanowski JQ, Troncoso JC, Wisniewski T, Woltjer RL, Beach TG (2012) Correlation of Alzheimer disease neuropathologic changes with cognitive status: a review of the literature. J Neuropathol Exp Neurol 71:362-381. Abstract

Nelson PT, Jicha GA, Schmitt FA, Liu H, Davis DG, Mendiondo MS, Abner EL, Markesbery WR (2007) Clinicopathologic correlations in a large Alzheimer disease center autopsy cohort: neuritic plaques and neurofibrillary tangles "do count" when staging disease severity. J Neuropathol Exp Neurol 66:1136-1146. Abstract

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