Rapidly progressive dementia, defined by cognitive decline that unfolds over weeks to months rather than years, presents neurologists with one of the most urgent diagnostic challenges in medicine. The two leading causes are prion diseases, most commonly sporadic Creutzfeldt-Jakob disease, and rapidly progressive forms of Alzheimer disease. In typical presentations, the two can be separated with reasonable confidence: prion disease usually announces itself on magnetic resonance imaging with characteristic diffusion-weighted abnormalities, and cerebrospinal fluid tests for prion seeding activity can confirm suspicion. But as the new research demonstrates, a subset of patients slips through these diagnostic nets entirely, presenting with clinical pictures that faithfully reproduce the amnestic syndromes of Alzheimer disease.

The Mayo Clinic team, led by Anas Elgenidi and Gregory S. Day, examined a prospective cohort of 204 patients with rapidly progressive dementia evaluated between February 2020 and June 2026. Among these, five patients, or 2.5 percent, had been diagnosed with clinically probable Alzheimer disease but were ultimately determined to have prion disease. That figure may seem small, but its significance lies in what it reveals about the limits of current diagnostic algorithms. These were not obscure cases buried in ambiguity; they were patients who met accepted criteria for Alzheimer disease and behaved, at least initially, exactly as that diagnosis predicted.

To understand how widespread this phenomenon is, the researchers turned to a systematic review of MEDLINE and Embase, unearthing ten additional published cases. Together with the Mayo cohort, the analysis encompassed fifteen patients with a median age at onset of 59 years, roughly two-thirds of whom were male. The clinical phenotypes these patients displayed mapped remarkably well onto recognized Alzheimer disease variants: just over half presented with an amnestic syndrome dominated by memory loss, about a quarter with a dysexecutive pattern affecting planning and organization, and smaller proportions with primary progressive aphasia, a language-led deterioration, or posterior cortical atrophy, which disrupts visual processing and spatial abilities. In other words, these prion cases were not merely Alzheimer-like in a vague sense; they reproduced specific, named Alzheimer phenotypes with precision.

The technical details of the diagnostic workup reveal where the traditional tools faltered. Diffusion-weighted magnetic resonance imaging, widely regarded as a cornerstone of prion disease diagnosis, was absent in all of the Mayo Clinic cases. In the published cases, the characteristic imaging abnormalities were either absent or equivocal in two patients and, perhaps more troublingly, overlooked in eight. This pattern of missed radiological findings echoes earlier literature showing that MRI changes in Creutzfeldt-Jakob disease are frequently misread or underappreciated, particularly early in the disease course when cortical and basal ganglia changes may be subtle or confined to regions that are not scrutinized routinely.

Cerebrospinal fluid biomarkers added another layer of complexity. In six of nine tested patients, the CSF profile was actually consistent with Alzheimer disease, meaning the amyloid and tau signature that clinicians rely upon to confirm the diagnosis pointed in the wrong direction. Elevated total tau levels, however, emerged as a recurring signal: eleven of thirteen patients showed raised total tau, and in five of nine patients the ratio of total tau to phosphorylated tau 181 was disproportionately elevated. This distinction matters because total tau reflects neuronal injury in a relatively nonspecific fashion, whereas phosphorylated tau is more closely tied to Alzheimer-type pathology. When total tau climbs far out of proportion to phosphorylated tau, the brain is being injured faster and more destructively than Alzheimer disease alone typically explains, and prion disease should enter the differential.

The most definitive diagnostic tool in the study was the real-time quaking-induced conversion assay, or RT-QuIC, a laboratory technique that exploits the prion protein’s infectious misfolding behavior. In the assay, normal recombinant prion protein is exposed to a patient’s cerebrospinal fluid and agitated; if misfolded prions are present, they template the conversion of the normal protein into amyloid fibrils that can be detected with a fluorescent dye, providing a sensitive and specific readout of prion seeding activity. RT-QuIC was positive in the cerebrospinal fluid of nine of twelve patients tested. Across the full cohort, prion disease was ultimately confirmed by neuropathological examination in seven cases, by genetic testing in two, and by RT-QuIC in six, illustrating that no single modality sufficed in every patient.

The median interval between the initial Alzheimer diagnosis and the consideration of prion disease was two months, a delay that, while seemingly brief, is meaningful in a disease measured in months. The stakes are considerable. Prion diseases are uniformly fatal, typically causing death within a year of onset, and there is no established cure, although the field is stirring: an early-phase clinical trial of intrathecally administered prion protein siRNA, a gene-silencing therapy delivered into the spinal fluid, is now enrolling symptomatic patients. Accurate diagnosis also matters for infection control, since prions resist conventional sterilization and demand specialized decontamination protocols in surgical settings, and for genetic counseling, since a proportion of prion disease cases arise from mutations in the prion protein gene with implications for family members.

The timing of this research is particularly consequential given the transformation of Alzheimer therapeutics. With disease-modifying monoclonal antibodies such as lecanemab and donanemab now approved and increasingly deployed, the pressure to confirm an Alzheimer diagnosis early and confidently has never been greater. Blood-based biomarkers are being promoted as accessible confirmatory tests, and CSF profiles are routinely used to anchor clinical decisions. Yet this study demonstrates that a positive Alzheimer biomarker signature does not exclude prion disease, and that patients with rapidly progressive courses deserve scrutiny that goes beyond a single biomarker panel. A patient whose dementia gallops forward while their biomarkers say Alzheimer may, in rare instances, be harboring a prion.

The authors’ central recommendation is pragmatic and easily actionable: disproportionate elevations in cerebrospinal fluid total tau, or an abnormally high ratio of total tau to phosphorylated tau 181, should prompt clinicians to consider prion disease and pursue RT-QuIC testing. Combined with careful re-review of diffusion-weighted MRI, this biomarker-driven vigilance offers a path to catching these mimics earlier. For the vast majority of patients with rapidly progressive dementia, the diagnosis will not be prion disease, and Alzheimer disease itself remains a common and legitimate explanation for swift decline. But for the roughly one in forty patients in this cohort whose illness wore Alzheimer’s mask, the difference between a routine diagnosis and a searching second look was the difference between two entirely different diseases, and the study stands as a reminder that in neurology, the most convincing diagnosis sometimes deserves to be tested against its own biomarkers.

Subject of Research: Prion disease presenting as rapidly progressive Alzheimer disease, diagnosed through CSF tau biomarkers and RT-QuIC assays

Article Title: Prion disease mimicking rapidly progressive Alzheimer disease: case series and systematic review

Article References: Elgenidi, A., Shir, D., Piura, Y. D., Sokol, L. L., Duara, R., Lachner, C., & Day, G. S. (2026). Prion disease mimicking rapidly progressive Alzheimer disease: case series and systematic review. Journal of Neurology, 273(10), Article 565. https://doi.org/10.1007/s00415-026-14087-3

Image Credits: AI Generated

DOI: 10.1007/s00415-026-14087-3

Keywords: prion disease, Creutzfeldt-Jakob disease, Alzheimer disease, rapidly progressive dementia, RT-QuIC, cerebrospinal fluid biomarkers, total tau, phosphorylated tau, diffusion-weighted MRI, differential diagnosis, neurodegeneration, Mayo Clinic

The research, published in European Geriatric Medicine, leveraged the A/T/N classification framework, a widely adopted scheme in which ‘A’ denotes amyloid-beta pathology, ‘T’ denotes phosphorylated tau, and ‘N’ denotes neurodegeneration, typically measured as atrophy of the medial temporal lobe on CT or MRI. In this study, cerebrospinal fluid levels of amyloid-beta 42 and phosphorylated tau defined the A and T markers, while radiologists rated medial temporal atrophy using the Scheltens visual scale, with age-adjusted cut-offs determining whether a score was abnormal. Episodic memory was assessed with the Rey Auditory Verbal Learning Test, a 15-item word-list task that measures both learning across five trials and free recall after a 30-minute delay.

The cohort was deliberately naturalistic rather than curated. Unlike highly selected research samples such as the Alzheimer’s Disease Neuroimaging Initiative, the MemClin project enrolled all patients referred for neuropsychological examination across participating clinics, capturing the messy heterogeneity of real clinical practice. The final sample comprised 141 patients with Alzheimer’s disease dementia, 403 with mild cognitive impairment, and 132 with subjective cognitive impairment, with ages ranging from roughly 36 to 94 years. Diagnoses were made through multidisciplinary consensus meetings in which clinical presentation remained primary and biomarkers played a supportive role, mirroring the way most memory clinics actually operate.

Because many patients scored zero on delayed recall—a floor effect expected in a memory-clinic population—the team employed weighted least-squares regression rather than ordinary linear models, assigning observation-specific weights to stabilize variance. Six regression models tested whether age moderated the relationship between each biomarker and each memory measure, controlling for sex and education, with a Bonferroni-corrected significance threshold of p less than 0.008. The results were unambiguous for two of the three biomarkers. Abnormal amyloid-beta 42 interacted significantly with age on delayed recall, with the detrimental effect of amyloid abnormality shrinking as age increased (β = 0.14, p < 0.001). Medial temporal atrophy showed a parallel interaction (β = 0.13, p = 0.002). Both models explained about 22 percent of the variance in delayed recall performance.

Phosphorylated tau, by contrast, did not survive the statistical correction, though its interaction pattern was borderline significant and trended in the same direction. The authors suggest this may reflect the comparatively stronger specificity of phosphorylated tau as an Alzheimer-specific marker, one whose relationship to cognition may be less entangled with age than amyloid or atrophy. Previous work has indicated that phosphorylated tau levels are less strongly related to age than amyloid-beta 42 or total tau, lending plausibility to that interpretation, although the researchers caution that a non-significant interaction should not be read as proof that tau is entirely age-independent.

To pinpoint where the biomarker-cognition link begins to fail, the team stratified the sample into two-year age bands and re-ran the association between abnormal amyloid status and memory performance repeatedly across those strata. The attenuation accelerated sharply at the upper end of the age distribution: for participants aged 80 to 82 and older, abnormal amyloid-beta 42 no longer showed a statistically meaningful association with episodic memory performance, with p-values exceeding 0.36, while the association remained robust in younger bands. Medial temporal atrophy followed the same trajectory. In other words, the diagnostic sensitivity of these markers appears to erode earlier than the traditional ‘oldest old’ threshold of 85 years, a finding the authors describe as unexpected from a clinical standpoint.

The biological explanation likely lies in the sheer prevalence of Alzheimer pathology in advanced age. Autopsy and imaging studies have shown that abnormal amyloid can be detected in up to 40 percent of cognitively healthy elderly individuals, and that by the time symptoms emerge, amyloid burden has largely saturated. Neuropathological research has also demonstrated that the correlation between Alzheimer-type pathology and dementia weakens with advancing age, as vascular disease, hippocampal sclerosis, TDP-43 proteinopathy, inflammatory processes, and individual differences in cognitive reserve increasingly shape clinical outcomes. The landmark 90+ Study illustrated this vividly: roughly half of its participants without dementia nonetheless met criteria for Alzheimer pathology at autopsy. In the oldest old, medial temporal atrophy may similarly reflect a mixture of age-related processes rather than Alzheimer-specific neurodegeneration, diluting its predictive value.

The clinical implications are provocative. The authors raise the question of whether lumbar puncture and cerebrospinal fluid assessment are justified in patients older than 80, given the weak association between the biomarkers and core clinical measures such as learning and free recall. They are careful, however, to draw boundaries around that claim. The finding should not be interpreted as questioning the broader utility of CSF biomarkers, which may remain important for diagnostic evaluation, prognosis, and determining eligibility for emerging disease-modifying therapies, including anti-amyloid immunotherapies. Nor should the exploratory age-stratified analyses be treated as confirmatory; small subgroup sizes, the cross-sectional design, and the risk of type 1 error all temper the conclusions, and the authors frame these results as hypothesis-generating pending large-scale longitudinal validation.

The study also carries methodological caveats that the researchers confront directly. Participants excluded for missing data differed in age from those included—excluded dementia patients were older, while excluded MCI and SCI patients were younger—raising the possibility of selection effects, although the pattern of CSF testing being more common in younger, diagnostically challenging patients arguably makes the sample representative of real practice. Visual atrophy ratings were based on CT in 60 percent of cases and MRI in 40 percent, a combination supported by evidence of comparable inter-rater reliability. Biomarkers were evaluated individually rather than in combination, and only verbal learning and free recall were examined, leaving recognition memory, cued recall, and executive functions for future study.

What emerges is a nuanced portrait of biomarker diagnostics at the frontier of human longevity. In a naturalistic cohort spanning the full cognitive-impairment continuum, the two biomarkers most proximal to memory circuitry—amyloid and medial temporal atrophy—lost traction against advancing age, while phosphorylated tau held its pattern more steadily. If replicated longitudinally, these findings could reshape diagnostic algorithms for the fastest-growing segment of the dementia population, prompting clinicians to weigh clinical presentation more heavily and biomarkers more cautiously once patients cross their ninth decade. For now, the message is one of calibrated skepticism: the molecular signature of Alzheimer’s disease does not translate into memory impairment with equal fidelity at every age, and medicine’s most trusted biomarkers may need an age-adjusted interpretation of their own.

Subject of Research: Age-related weakening of the association between Alzheimer's disease ATN biomarkers and episodic memory in memory clinic patients

Article Title: The associations between ATN biomarkers and episodic memory diminish as age increases

Article References: Ekman, U., Gyllenhammar, M., Almkvist, O., Ocklind, A., Hallerstam, L., Ferreira, D., Eriksdotter, M., Wahlund, L.-O., Rennie, A., & Westman, E. (2026). The associations between ATN biomarkers and episodic memory diminish as age increases. European Geriatric Medicine. https://doi.org/10.1007/s41999-026-01606-8

Image Credits: AI Generated

DOI: 10.1007/s41999-026-01606-8

Keywords: Alzheimer's disease, ATN biomarkers, amyloid-beta 42, phosphorylated tau, medial temporal atrophy, episodic memory, cerebrospinal fluid, cognitive aging, memory clinics, RAVLT, mild cognitive impairment, diagnostic biomarkers