Alzheimer's May Be a Biological Disease, Not Just Memory Loss

The Disease Before the Symptoms

For decades, we have treated Alzheimer's disease the way a fire department treats a house that is already engulfed in flames. By the time a patient walks into a neurologist's office unable to remember what they had for breakfast, the disease has been burning inside their brain for years. Sometimes decades.

That mismatch between when the disease starts and when we notice it has been a quiet catastrophe. It has skewed our understanding of what Alzheimer's actually is. We thought it was a disease of memory loss. But a landmark 2018 framework from Clifford Jack and colleagues at the National Institute on Aging and the Alzheimer's Association proposed something that still makes many clinicians uncomfortable: Alzheimer's is not a clinical syndrome. It is a biological disease. And you can have it without any symptoms at all.

The paper, published in Alzheimer's & Dementia and now cited over 10,000 times, does not just tweak the diagnostic criteria. It redefines the entire disease. "In the National Institute on Aging and Alzheimer's Association Research Framework, Alzheimer's disease (AD) is defined by its underlying pathologic processes that can be documented by postmortem examination or in vivo by biomarkers" (Jack et al., 2018). The diagnosis is no longer based on symptoms. It is based on biology.

This is not a semantic game. It is a fundamental shift in what we think we are treating.

What Actually Happens in the Brain

The paper introduces a classification system called AT(N). It sounds like alphabet soup, but it is actually a remarkably clean way to think about what goes wrong.

A stands for beta amyloid. These are sticky protein fragments that clump into plaques between neurons. For years, amyloid was the celebrity villain of Alzheimer's research. Drug companies spent billions trying to clear it from brains, often with disappointing results.

T stands for tau. These are proteins inside neurons that, when they go bad, twist into tangles that strangle the cell from within. Tau pathology correlates more closely with cognitive decline than amyloid does. But tau alone does not define Alzheimer's. The combination does.

N stands for neurodegeneration. This is the final consequence. Neurons die. Brain tissue shrinks. The damage becomes visible on scans and measurable in spinal fluid.

The framework groups biomarkers by which of these three processes they measure (Jack et al., 2018). A patient can be A+ (amyloid positive) but T negative and N negative. That person has the biological seeds of Alzheimer's in their brain but may be cognitively normal. They are not "at risk" for Alzheimer's. According to this framework, they have Alzheimer's. They just do not know it yet.

This is where the framework gets uncomfortable. And where it gets interesting.

Why Symptoms Are a Late Signal

The clinical staging of Alzheimer's has traditionally followed a familiar arc. Preclinical means no symptoms. Mild cognitive impairment means the patient notices something is off but can still function. Dementia means the disease has taken the wheel.

But the Jack framework argues that these stages describe the consequences of the disease, not the disease itself. You would not diagnose heart disease only after a heart attack. You would measure cholesterol, blood pressure, plaque buildup. You would intervene early.

Alzheimer's has been stuck in the heart attack era.

The paper explicitly states that "the diagnosis is not based on the clinical consequences of the disease (i.e., symptoms/signs) in this research framework, which shifts the definition of AD in living people from a syndromal to a biological construct" (Jack et al., 2018). That is a direct inversion of how most clinicians have been trained to think. You do not wait for the patient to complain. You look at the biology.

This matters because the window for intervention may be very narrow. Clinical trials that enrolled patients who already had symptoms largely failed. They were trying to put out a fire with a garden hose while the building was collapsing. Trials that enrolled patients based on biomarkers, before significant cognitive decline, have shown more promise. The Jack framework provides the scaffolding for those trials.

How the Research Was Structured

The authors did not run a new experiment. They synthesized existing evidence and created a consensus framework. The paper is a product of the National Institute on Aging and the Alzheimer's Association, involving experts like Clifford R. Jack (a neuroradiologist), David A. Bennett (a neuropathologist), Kaj Blennow (a biomarker specialist), and María C. Carrillo (the chief science officer of the Alzheimer's Association).

The framework is built on decades of biomarker research. Amyloid PET scans. Tau PET scans. CSF measures of amyloid and tau. MRI measurements of brain volume. The authors grouped these biomarkers into the AT(N) system and proposed that Alzheimer's be defined by the presence of abnormal amyloid and tau, regardless of whether the patient has cognitive symptoms (Jack et al., 2018).

They also made the framework flexible. "The AT(N) system is flexible in that new biomarkers can be added to the three existing AT(N) groups, and new biomarker groups beyond AT(N) can be added when they become available" (Jack et al., 2018). This is a living document, not a tombstone.

What This Changes

The most immediate consequence is in clinical trial design. If you want to test a drug that targets amyloid, you need to enroll patients who actually have amyloid in their brains. That sounds obvious, but it was not standard practice. Before this framework, many trials enrolled patients based on clinical diagnosis alone, which is wrong about 20 to 30 percent of the time. Those misdiagnosed patients diluted the results and may have caused promising drugs to fail.

The framework also changes how researchers think about prevention. If Alzheimer's is a biological continuum, then preventing the pathology from ever starting becomes the goal. You do not wait for plaques to form. You try to stop them from forming in the first place.

The paper outlines two different categorical cognitive schemes for staging severity: a traditional three stage system (preclinical, MCI, dementia) and a more granular six stage numeric scheme (Jack et al., 2018). This allows researchers to track the relationship between biology and cognition with more precision.

What the Framework Does Not Prove

The authors are careful to state what they are not claiming. "It is premature and inappropriate to use this research framework in general medical practice" (Jack et al., 2018). This is not a guide for your doctor to order a spinal tap on a healthy 50 year old. It is a research tool.

The framework also does not prove that amyloid and tau are the cause of Alzheimer's. The authors acknowledge that "although it is possible that beta amyloid plaques and neurofibrillary tau deposits are not causal in AD pathogenesis, it is these abnormal protein deposits that define AD as a unique neurodegenerative disease" (Jack et al., 2018). The definition is descriptive, not necessarily causal. It is possible that amyloid and tau are bystanders or consequences of some deeper pathology. But they are what distinguishes Alzheimer's from other dementias.

There is also the question of what to tell a patient who has Alzheimer's pathology but no symptoms. The framework does not address this directly. It is one thing to define a disease biologically. It is another to tell someone they have a progressive neurodegenerative condition with no treatment and no timeline for when symptoms will appear. That is a conversation no one has figured out how to have.

The Open Questions

The biggest question is whether clearing amyloid and tau will actually prevent cognitive decline. The framework assumes that biology precedes symptoms and that intervening on biology will change the clinical trajectory. That assumption is being tested now, but the results are not yet in.

Another open question is what happens when people have Alzheimer's pathology but never develop dementia. Autopsy studies have found that some people die with full blown Alzheimer's pathology in their brains but were cognitively normal until the end. They have the biological disease but not the clinical syndrome. The framework calls this "asymptomatic Alzheimer's disease." But it raises the possibility that other factors, like cognitive reserve or resilience, can protect against the consequences of the pathology.

The framework also does not address the role of other pathologies. Many older adults have mixed dementia: Alzheimer's pathology plus vascular damage plus Lewy bodies. The AT(N) system focuses on Alzheimer's specifically, but the authors acknowledge that "this approach also will enable a more precise approach to interventional trials where specific pathways can be targeted in the disease process and in the appropriate people" (Jack et al., 2018). It is a tool for precision, not a complete map.

What This Actually Means

• ▸Alzheimer's is not a memory disease. It is a protein folding disease. The symptoms are downstream. If you want to stop the disease, you have to stop the proteins from misfolding, not wait for the memory loss to appear. This changes where research money goes and how trials are designed.

• ▸You can have Alzheimer's and not know it. The framework defines the disease by biomarkers, not by symptoms. This means millions of people may be walking around with Alzheimer's pathology in their brains, cognitively normal, unaware. This is not a scare tactic. It is a call to develop better screening tools and to have honest conversations about what we do with that information.

• ▸Clinical trials have been testing drugs on the wrong patients. If a trial enrolls people based on clinical diagnosis alone, a significant fraction will not have Alzheimer's pathology. That dilutes results and may have caused effective drugs to fail. The framework makes it possible to enroll the right patients from the start.

• ▸Prevention becomes the primary goal. If the disease starts years before symptoms, then the window for prevention is wide open. Lifestyle interventions, vaccines, and early stage drugs can be tested in people who have the biology but not the symptoms. This is already happening in trials like the A4 study, which enrolled amyloid positive older adults with normal cognition.

• ▸This framework is not ready for your doctor's office. The authors are explicit about this. Using biomarkers to diagnose Alzheimer's in asymptomatic people is not standard of care. It raises ethical questions about insurance, employment, and psychological harm. The framework is a research tool, not a clinical mandate. But it is a tool that will reshape how we think about the disease for the next decade.

The 2018 framework did not invent Alzheimer's biology. It just gave us the courage to name it. And naming things is the first step to understanding them.